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Table of Content Topic Sub-Topic Page No

Movements of Earth

1. Interior of Earth 5

2. Endogenetic Movements 11

3. Exogenetic Earth Movements 17

4. Classification of Mountains 31

5. Fold Mountains 37

6. Block Mountains 51

7. Rocks and types of Rock 55

Plate Movement Theories

8. Continental Drift Theory 68

9. See Floor Spreading – Paleomagnetism 75

10. Plate Tectonics 82

11. Divergent Boundary 89

12. Earthquakes 93

13. Tsunami 105

Volcanism

14. Volcanism 116

15. Volcanic Landforms – Extrusive, Intrusive 128

16. Volcanism Types Based on Out Flow of Lava 136

17. Pseudo Volcanic features 144

18. Hotspot Volcanism 148

Landforms

19. Fluvial Depositional Landforms 157

20. Fluvial Erosional Landforms 170

21. Glacial Landforms 186

22. Karst Landforms 199

23. Marine Landforms 208

24. Arid Landforms 220

25. Lake – Classification of Lakes 241


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1. Interior of Earth

Structure and Composition of the Earth

The interior structure of the Earth is made up of three main shells: the very thin and brittle crust, the mantle, and the core. Furthermore, the mantle and core are each divided into two parts. The core and mantle are equal in thickness but, the core of the earth only occupies 15 percent of Earth’s volume whereas the mantel occupies 84 percent and the crust occupies remaining 1 percent.

Sources of Information about Interior of the Earth

The earth’s radius is 6,370 km. It is rather difficult to make observations or collect samples of the material from inside of the earth due to its huge size and the changing nature of its internal composition. Only a part of the information is obtained through direct observations and analysis of materials. Most of our understanding about the interior of the earth is based on estimates and inferences.


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Direct Sources of Information

Surface rock – Surface rocks are the most readily available solid earth material to make direct observations. Laboratory experiments on surface rocks and minerals provide important information about the interior of the earth.

Mining – rocks that we get from mining areas are another source that gives us information about Earth’s interior. Through mining and drilling operations we have been able to observe the earth’s interior directly only up to a depth of few kilometres. World’s deepest mining is limited only to the depth of fewer than 5 kilometres. Going beyond this depth is not possible due to excessive heat at this depth.

Deep Ocean Drilling Projects – Scientists have undertaken some major projects to penetrate the surface of oceans to assess the conditions in crustal portions. The deepest drill at Kola, in the Arctic Ocean, has so far reached a depth of 12 km. This and many deep drilling projects have provided a large volume of information through the analysis of materials collected at different depths.

Volcanic Eruptions – Volcanic eruption forms an important source of obtaining direct information through laboratory analysis of the molten material (magma) that is thrown onto the surface of the earth, during a volcanic eruption. However, it is difficult to find out about the depth of the source of such magma.

Indirect Sources of Information

Meteors – Meteors are bits of interplanetary material falling through Earth’s atmosphere and heated to incandescence by friction. Meteors that at times reach the earth are an important source of information about the interior structure of the Earth. Although the material that becomes available for analysis from meteors do not from the part of the interior of the earth, they provide valuable information as the structure observed in meteors are similar to that of the earth.

Gravitation – The reading of the gravity at different places is influenced by many factors viz. distribution of mass, distance from the centre of the Earth. Such a difference is called gravity anomaly. Gravity anomaly gives us information about the distribution of mass of the material in the crust of the earth.


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Magnetic Field – Magnetic surveys provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.

Seismic Activity – Seismic activity is one of the most important sources of information about the interior of the earth. Body waves, generated by an earthquake, especially S-waves, which travel only through solid material, have helped in understanding the interior structure of the Earth.

The Layers of the Earth

Earth’s interior is divided into basically three layers – Crust, Mantle and Core, which we shall discuss in detail as below:

The Crust

Due to its accessibility, its geology has been widely studied So, we have good understanding of the structure and composition of the

crust. The crust is the outermost layer of the earth. It is brittle in nature.


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The crust of the Earth has two distinct types: continental crust and oceanic crust.

These two types have different chemical compositions and physical properties and were formed by different geological processes.

The thickness and density of the crust vary under the oceanic and continental areas.

Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km. The mean thickness of the continental crust is around 30 km. It is much

thicker in the areas of major mountain ranges, extending up to 70 km in the Himalayan region.

Oceanic crust is denser as compared to the continental crust. Continental crust has the mean density of 2.7 g/cm3. It is mainly composed

of silicon and aluminium. Therefore, it is often termed as “sial.” The mean density of material in oceanic crust is 2.9 g/cm3. It is mainly

composed of basaltic rocks. The crust makes up about 1% of Earth’s volume.

The Mantle

Our knowledge of the upper mantle, including the tectonic plates, is derived from analyses of earthquake waves, heat flow, magnetic, gravity studies and laboratory experiments on rocks and minerals.

The portion of the interior beyond the crust is called the mantle. The mantle extends from Moho’s discontinuity to a depth of 2,900 km. It has an average density higher than that of the crust (3.4 g/cm3). The mantle is divided into upper and lower mantle. Asthenosphere – The upper portion of the mantle is called asthenosphere,

extending up to 400 km. The word “astheno” means weak. It is the main source of magma that finds its way to the surface during volcanic eruptions. It lies below the lithosphere.

Lithosphere – The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The lithosphere is subdivided into tectonic plates.

The lower mantle extends beyond the asthenosphere. It is in the solid state. Major constituent elements of the mantle are magnesium and silicon. Hence,

this layer is termed as “sima”. The mantle makes up about 84% of Earth’s volume


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The Core

The earthquake wave velocities have helped in understanding the existence of the core of the earth.

The innermost layer surrounding the earth’s centre is called core, which is about 3500 km in radius.

The core-mantle boundary is located at a depth of 2,900 km. The core consists of two sub-layers. The outer core is in the liquid state while

the inner core is in the solid state. The core is the densest layer of the earth. The density of material at the

mantle-core boundary is around 5 g/cm3, and at the centre of the earth at 6,300 km, the density value is around 13 g/cm3.

The core is made up of very heavy material mostly constituted by nickel and iron.

It is sometimes referred to as the “nife” layer. The core makes up about 15% of Earth’s volume.

Seismic Discontinuities

Seismic discontinuities aid in distinguishing divisions of the Earth into the inner core, outer core, lower mantle, upper mantle, and the crust

Conorad discontinuity – it refers to the zone between upper crust and lower crust.

Mohorivic discontinuity – also called as moho discontinuity is the zone that separates the Earth’s crust from the upper mantle. It can be detected by a sharp increase downward in the speed of earthquake waves there.

Repiti discontinuity – it refers to the zone between upper mantle and lower mantle.

Gutenberg discontinuity – It refers to the zone separating the lower mantle from the core. It is located at a depth of about 2,900 km.

Lehmann discontinuity – it refers to the zone separating solid inner core from the liquid outer core.


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Temperature, Pressure and Density of the Earth’s interior

The temperature increases towards the centre of the earth. However, the rate of increase of temperature is not uniform from the surface towards the earth’s centre. It is faster at some places than at others.

The temperature at the centre is estimated to lie somewhere between 3000°C and 50000C.

Such a high temperature inside the earth may be due to chemical reactions under high-pressure conditions and disintegration of radioactive elements.

The pressure also increases from the surface towards the centre of the earth due to huge weight of the overlying rocks.

Due to increase in pressure and presence of heavier materials towards the earth’s centres, the density of earth’s layers also goes on increasing. The materials of the innermost part of the earth are very dense.


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2. Endogenetic Movements

Though the surface of earth appear to be static the interior of the earth is in a

dynamic state and this dynamism of the earth results in endogenetic movements.

The Movements of the earth

The surface of the earth is dynamic. It has been moving vertically and

horizontally.

Since the origin of the earth, there have been major changes in the distribution

of continents and oceans.

The earth has experienced innumerable earth movements which have brought

about vast changes in its surface.

The lithosphere is broken into a number of plates known as the Lithospheric

plates.

The movement and interaction of these plates cause changes on the surface of

the earth.

The forces which act in the interior of the earth are called as Endogenetic forces

and the forces that work on the surface of the earth are called as Exogenetic

forces.

In general terms, the endogenetic forces are mainly land building forces and the

exogenetic processes are mainly land wearing forces.

The actions of exogenetic forces result in wearing down (degradation) of relief.

But, the endogenetic forces continuously build up parts of the earth’s surface and

hence the exogenetic processes fail to even out the relief variations of the surface

of the earth.

So, variations remain as long as the opposing actions of exogenetic and

endogenetic forces continue.


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Geomorphic Processes

The endogenetic and exogenetic forces that bring about changes in the

configuration of the surface of the earth through physical and chemical actions

on earth materials are known as geomorphic processes.

Diastrophism and volcanism are endogenetic geomorphic processes.

Weathering, mass wasting, erosion and deposition are exogenetic geomorphic

processes.

Endogenetic geomorphic processes

The energy emanating from within the earth is the main force behind

endogenetic geomorphic processes.

This energy is mostly generated by radioactivity, rotational and tidal friction and

primordial heat from the origin of the earth.

This energy due to geothermal gradients and heat flow from within induces

diastrophism and volcanism in the lithosphere.


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Due to variations in geothermal gradients and heat flow from within, the action

of endogenetic forces are not uniform and hence the tectonically controlled

original crustal surface is uneven.

As explained above, diastrophism, volcanism and earthquake are the

endogenetic geomorphic processes

Endogenetic Movements

Endogenetic forces sometimes produce sudden movements and at the other

times produce slow movements.

Sudden movements like earthquakes and volcanoes cause mass destruction over

the surface of the earth.

While diastrophic movements are rather slow. Diastrophism refers to

deformation of the Earth’s crust, and more especially to folding and faulting.

Slow Movements

The movement which bring about changes in the Earth’s crust gradually taking

hundreds or thousands of years and which cover a period much longer than a

human life span are called slow movements. These movements act on the earth’s

crust either vertically or horizontally.

Diastrophism

Diastrophism is included within slow movements. All processes that move, elevate

or build up portions of the earth’s crust come under diastrophism. They include:

1. Orogenic processes

2. Epeirogenic processes

Through the processes of orogeny, epeirogeny, there can be folding, faulting and

fracturing of the crust. All these processes cause pressure, volume and

temperature (PVT) changes which in turn induce metamorphism of rocks.


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Epeirogenic Processes

Epeirogeny is a continental building process.

Due to epeirogeny, there may be mild deformations of the surface of the Earth.

Epeirogenic processes involve uplift or warping of large parts of the earth’s crust.

Epeirogenic or continent forming movements act along the radius of the earth;

therefore, they are also called radial movements.

Their direction may be towards (subsidence) or away (uplift) from the centre.

The results of such movements may be clearly defined in the relief.

Orogenic Processes

Orogeny is a mountain building process.

Orogenic processes involve severe folding and affecting long and narrow belts of

the earth’s crust.

In contrast to epeirogenic movement, orogenic movement is a more complicated

deformation of the Earth’s crust

Orogenic processes are associated with crustal thickening, notably associated

with the convergence of tectonic plates.

Orogenic processes may push deeply buried rocks to the surface.

If the orogeny is due to the colliding of the two continental plates, very high

mountains can result. E.g. Himalayas.

Sudden Movements

Contrary to the slow movements, there are certain movements which bring about

abrupt changes in the crust. The examples of such movements are volcanic

eruptions and earthquakes.


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Earthquake

Earthquakes refer to trembling of the earth surface due to a sudden release of

energy from within the earth’s interior along a fault line or weak zone.

Earthquake can occur at any time of the year, day or night. Its impact is very

sudden.

The place in the crust where the movement starts is called the focus. The place

on the surface above the focus is called the epicentre.

A sudden release of energy creates a vibration of the earth surface

Vibrations travel outwards from the epicentre as earthquake waves.

These earthquake waves create mass destruction on the earth surface.

The greatest damage is usually closest to the epicentre and the strength of the

earthquake decreases away from the centre.

Volcanism

A volcano is a vent in the earth’s crust through which molten material erupts

suddenly toward the earth’s surface.

Volcanism is responsible for formation of many intrusive and extrusive volcanic

forms.

Volcanoes are classified on the basis of nature of eruption and the form

developed at the surface. Major types of volcanoes include – Shield Volcanoes,

Composite Volcanoes, Caldera, Flood Basalt Provinces and Mid-Ocean Ridge

Volcanoes

Volcanic landforms

The solidification of the lava forms volcanic landforms either inside the surface of

the earth or outside its surface. When the lava is not able to reach the surface of

the earth it enters in the fissures of the earth and depending on the shape of the

fissure and its position with respect to the surface of the earth different types of


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intrusive landform develop. Major intrusive forms include – Batholiths, Laccoliths,

Lopolith, Phacolith, Sills and Dykes.

Volcanic Landforms

Batholiths – A large body of magmatic material that cools in the deeper depth of

the crust develops in the form of large domes.

Laccoliths – These are large dome-shaped intrusive bodies with a level base and

connected by a pipe-like conduit from below. These are located at deeper depths.

Lopolith is a saucer shape intrusive form, concave to the sky body.

Phacolith – These are lense shaped intrusive mass formed by the solidification of

lava either on the crest of anticline or trough of the syncline.

Sills or Sheets – These structures are formed by the solidification of lava in a

surface parallel to the earth’s surface. The thinner ones are called sheets whereas

the thick ones are called sills.

Dykes – These are a wall-like structure formed by the solidification of lava

perpendicular to the surface of the earth.


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3. Exogenetic Earth Movements

The natural forces which help in levelling of the earth’s surface by the process of

degradation, transport and aggradation are called exogenetic agents.

Exogenetic Movements

The surface of the earth is ever changing by the endogenetic and exogenetic forces.

The exogenetic movements on the earth’s surface gradually try to level the uneven

surface of the earth. The agents of weathering and erosion are continuously

involved in undoing the changes created by the endogenetic movements of the

earth.

Geomorphic Processes

The endogenetic and exogenetic forces that bring about changes in the

configuration of the surface of the earth through physical and chemical actions

on earth materials are known as geomorphic processes.

Diastrophism and volcanism are endogenetic geomorphic processes.


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Weathering, mass wasting, erosion and deposition are exogenetic geomorphic

processes.

Geomorphic agents

Any Exogenetic element of nature (like water, ice, wind, etc.,) capable of

removing, transporting and depositing earth materials can be called a

geomorphic agent.

Running water, groundwater, glaciers, wind, waves and currents, etc., can be

called geomorphic agents.

Exogenetic Geomorphic Processes

The exogenetic processes derive their energy from atmosphere determined by

the ultimate energy from the sun and also the gradients created by tectonic

factors.

As explained above, Weathering, mass wasting, erosion and deposition are

exogenetic geomorphic processes.

All the exogenetic geomorphic processes are covered under a general term,

denudation. The word ‘denude’ means to strip off or to uncover.

Weathering, mass wasting/movements, erosion and transportation are included

in denudation.

The basic reason that leads to weathering, mass movements and erosion is the

development of stresses in the body of the earth materials.

Force applied per unit area is called stress. Stress is produced in a solid by pushing

or pulling which induces deformations in the body.

Different kinds of stress are produced in the earth materials viz. sheer stress,

gravitational stress, molecular stress etc.

Exogenetic geomorphic processes are greatly influenced by climatic elements

such as temperature and precipitation. Hence, the exogenetic geomorphic

processes vary from region to region.


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Exogenetic geomorphic processes also depend upon type and structure of rocks.

As different types of rocks offer varying resistance to various geomorphic

processes.

Weathering

Weathering is the action of elements of weather and climate over earth

materials.

Weathering is defined as mechanical disintegration and chemical decomposition

of rocks through the actions of various elements of weather and climate.

Weathering processes are responsible for breaking down the rocks into smaller

fragments

Erosion cannot be significant if the rocks are not weathered. That means,

weathering aids mass wasting and erosion.

Weathering is a static process as very little, or no motion of materials takes place

in weathering, it is an in-situ or on-site process.

Weathering processes are conditioned by many complex geological, climatic,

topographic and vegetative factors.


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Types of the weathering process

There are three major groups of weathering processes. However, very rarely does

any one of these processes ever operate completely by itself. In reality, the

weathering process involves combinations of all three types of weathering

processes. These processes are:-

1. Chemical weathering processes

2. Physical or Mechanical weathering processes

3. Biological weathering processes

Chemical weathering processes

Chemical weathering causes rocks to decompose or dissolve and reduce them to

a fine clastic state through chemical reactions by oxygen, water or acids.

The mineral contained in the rocks undergo chemical changes when they get in

contact with atmospheric air and water.

Presence of Water, air (oxygen and carbon dioxide) and high-temperature help

in speeding up the weathering process.

Types of the chemical weathering process

There are different weathering process related to chemical action viz. hydration,

carbonation and oxidation. These weathering processes are interrelated and go

hand in hand and hasten the weathering process.

Solution

When something is dissolved in water or acids, the water or acid with dissolved

contents is called solution.

This process involves removal of solids in solution

The process of weathering through solution depends upon the solubility of a

mineral in water or weak acids.


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Soluble rock-forming minerals like nitrates, sulphates, and potassium etc. are

easily leached out without leaving any residue in the rainy climate.

Carbonation

Carbonation is the reaction of carbonate and bicarbonate with minerals

It is a common process helping the breaking down of feldspars and carbonate

minerals.

It takes place in rocks containing carbonates of calcium, sodium, magnesium,

potassium etc. when they come in touch with rainwater which contains dissolved

carbon dioxide.

Hydration

Hydration is the chemical addition of water.

Many rock minerals swell and contract during wetting and drying and a repetition

of this process results in their disintegration.

Salts in pore spaces undergo rapid and repeated hydration and help in rock

fracturing.

Oxidation

In weathering, oxidation means a combination of a mineral with oxygen to form

oxides or hydroxides.

Oxidation occurs where there is ready access to the atmosphere and oxygenated

waters.

In the process of oxidation, rock breakdown occurs due to the disturbance caused

by the addition of oxygen

The minerals most commonly involved in this process are iron, manganese,

sulphur etc.


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Reduction

When oxidised minerals are placed in an environment where oxygen is absent,

reduction takes place.

Such conditions usually exist below the water table, in areas of stagnant water

and waterlogged ground.

Physical Weathering Processes

The disintegration of rocks by some applied forces is called physical or mechanical

weathering.

These applied forces could be due to the action of gravity, heat and water.

Many of these forces are applied both at the surface and within different earth

materials leading to rock fracture

Most of the physical weathering processes are caused by thermal expansion and

pressure release.

These processes are small and slow but can cause great damage to the rocks

because of continued fatigue the rocks suffer due to the repetition of contraction

and expansion.

Types of the physical weathering process

Unloading

The process of unloading involves removal of overlying rock load because of

continued erosion

Unloading causes a release of vertical pressure on the rock resulting in expansion

of upper layers which further results in disintegration of rock masses

Due to disintegration fractures are developed in the rock mass, roughly parallel

to ground surface

In areas of a curved ground surface, rock fractures tend to produce Large, and

smooth rounded domes called exfoliation domes


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An exfoliation dome

Temperature Changes

With the rise in temperature, every mineral expands, and as the temperature

falls, a corresponding contraction takes place.

Because of diurnal changes in the temperatures, there is a regular internal

movement among the mineral grains

These regular movements make the rocks weak due to continued fatigue and

cause fracture and further disintegration of rock masses

This process is most effective in dry climates and high elevations where diurnal

temperature changes are drastic.

Frost Weathering

Frost weathering occurs due to the growth of ice within pores and cracks of rocks

during repeated cycles of freezing and melting.

This process is most effective at high elevations in mid-latitudes where freezing

and melting is often repeated.

Rapid freezing of water causes its sudden expansion which causes joints, cracks

and small inter granular fractures to become wider and wider till the rock breaks

apart.


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Salt Weathering

Many salts in rocks like calcium, sodium, magnesium, potassium expand due to

thermal action, hydration and crystallisation.

Salt weathering causes splitting of individual grains within rocks, which eventually

fall off.

This process of falling off of individual grains may result in granular disintegration

or foliation.

Salt weathering is common in desert areas due to high-temperature ranges

Biological Weathering Processes

This refers to disintegration and decomposition of rock masses due to growth or

movement of organisms.

Burrowing and wedging by organisms like earthworms, termites, rodents etc.,

help in exposing the new surfaces to moisture and chemical attack causing their

decomposition.

Human activities like ploughing and cultivating cause mixing of air, water in the

minerals, thereby aiding in weathering process.

Decaying plant and animal matter help in the production of humic, carbonic and

other acids which enhance decay and solubility of some rocks.

Plant roots penetrate the cracks in the rocks and exert tremendous pressure on

the earth materials mechanically breaking them apart.

Mass Movements

These movements transfer the mass of rock debris down the slopes under the

direct influence of gravity.

The debris may carry with it air, water or ice.

The process of weathering aids in mass movements. Mass movements are very

active over weathered slopes rather than over unweathered materials.


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No geomorphic agent like running water, glaciers, wind, waves and currents

participate in the process of mass movements.

Mass movements are aided by gravity

Mass movement is also aided by weak unconsolidated materials, thinly bedded

rocks, faults, steep slopes, abundant precipitation and torrential rains and

scarcity of vegetation etc.

Classification of Mass Movements

Heave (heaving up of soils due to frost growth and other causes), flow and slide are

the three forms of movements. The figure, given below shows the relationships

among different types of mass movements, their relative rates of movement and

moisture limits.

Types of Mass Movements

Mass movements can be grouped into two major classes:

1. Slow movements

2. Rapid Movements


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Slow movements

Creep

Creep refers to the movement of materials which is extremely slow and

imperceptible in normal conditions

Creep, generally occur on moderately steep, soil-covered slopes.

Depending upon the type of material involved, several types of creep viz., soil

creep, talus creep, rock creep, rock-glacier creep etc., can be identified.

Creep

Solifluction

Solifluction refers to slow downslope flowing soil mass or fine-grained rock debris

saturated or lubricated with water.

This process is quite common in moist temperate areas

Rapid Movements

Earthflow

Earthflow refers to the movement of water-saturated clayey or silty earth

materials down steep slopes


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These movements are most prevalent in humid climatic regions and occur over

gentle to steep slopes.

Mudflow

In the region of sparse vegetation and heavy rainfall, thick layers of weathered

materials get saturated with water and flow down along definite channels.

It looks like a stream of mud within a valley.

Mudflows frequently occur on the slopes of erupting or recently erupted

volcanoes.

Mudflows can cause great destruction to human habitations

Avalanche

This is also a type of debris flow.


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Debris avalanche can be much faster than the mudflow.

Debris avalanche is similar to snow avalanche.

It is more characteristic of humid regions with or without vegetation cover

It occurs in narrow tracks on steep slopes.

Debris Avalanche

Landslide

Landslide involves relatively rapid and perceptible movements of the rock mass.

The materials involved are relatively dry.

The size and shape of the detached mass in the landslide depends on the nature

of discontinuities in the rock, the degree of weathering and the steepness of the

slope

Depending upon the type of movement, a landslide can take place either

by slump involving back rotation with respect to the slope or by rapid rolling or

sliding of earth debris without backward rotation, referred to as debris slide.

Similarly, sliding down of individual rock masses is referred to as the rock slide.


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Landslide

Erosion

Erosion involves acquisition and transportation of rock debris.

Erosion results in degradation of the surface relief i.e. wearing down of the

landscape.

It is erosion that is largely responsible for continuous changes that the earth’s

surface is undergoing.

When massive rocks break into smaller fragments through weathering and any

other process, erosional geomorphic agents like running water, groundwater,

glaciers, wind and waves remove and transport it to other places

Abrasion by rock debris carried by these geomorphic agents also aids greatly in

erosion.

Thus, weathering aids erosion, but it is not a pre-condition for erosion to take

place.


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Deposition

Deposition is a consequence of erosion.

Gradually, the erosional agents lose their velocity and hence, the materials

carried by them start to settle themselves.

The coarser materials get deposited first and finer ones later.

By deposition, depressions get filled up.

The same erosional agents, viz. running water, glaciers, wind, waves and

groundwater act as aggradational or depositional agents also.


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4. Classification of Mountains

Mountains- An uplifted portion of the earth’s surface is called a hill or a mountain.

Difference between Hills and Mountains

When the summit or top rises to more than 900 m above the base, then it is termed

as Mountain while those with less than this elevation are called Hills.

Classification of Mountains

On the basis of their origin or mode of formation, the mountains are classified as

Structural or Tectonic

Residual or Dissected And

Volcanic

Structural Mountains- These mountain systems are hundreds of kilometres wide

and thousands of kilometres long. Many of them lie near or parallel to continental

coastlines. All great mountain systems of the earth are of this type. Both the fold

and the block mountains are included in this type.

Fold Mountains

Fold mountains are formed due to the folding of the crustal rocks by the

compressive forces which in turn are generated by endogenetic forces.

Folded mountains which have originated before tertiary period are called old fold

mountains. – Caledonian, Hercynian, Vindhyachal and Aravalis. These are also

called relict fold mountain because of denudation

Some new fold mountains are the Alps in Europe, the Rockies of North America,

the Andes of South America, the Himalayas of Asia and Atlas of North Africa.

These young fold mountains are still rising under the influence of the earth’s

tectonic forces. They are known for a variety of rock structures, deep gorges and

the high pyramidal peaks.


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The granitic core of such mountains is surrounded by metamorphic rocks,

merging with sedimentary layers along the margins.

The phenomenon of folding and faulting is most complex in the central areas of

these mountains.

The Urals, the Appalachians, the Tien Shan . and the Nan Shan were formed

during an earlier mountain-building period.

The highlands of Scotland and Norway and the Sayan and Stanovoy mountains in

Russia are of still earlier period.

Fold Mountains

Block Mountains

These mountains are formed when great blocks of the earth’s crust may be raised

or lowered during the late stages of mountain-building

During the uplift of structural mountains, sometimes magma flows upward into

the crust.


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On its cooling and hardening beneath the surface, it contracts and the overlying

rock may crack into large blocks moving up or down.

An intense folding of rocks is generally followed by faulting of strata due to the

horizontal force of tension.

The land between the two parallel faults either rises forming block mountains or

horsts or subsides into a depression termed as a rift valley or

An old fold mountain may also be left as block mountains due to continuous

denudation. These mountains have flat tops, steep fault scarps and the subsided

portions between parallel fault are flat-bottomed.

The Vosges in France, Black Forest mountains in Germany and the Salt Range in

Pakistan are cited as typical examples of block mountains. Sierra Nevada of

California (USA); Wasatch range in the Utah province are also examples of Block

mountains.

River Rhine in Europe flows through a rift valley.

The Great Rift Valley of the world runs for about 6,000 kilometres from East Africa

to Syria through the Red Sea.


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Block Mountain

Volcanic Mountains

As these are formed by the accumulation of volcanic material, they are also

known as mountains of accumulation.

The matter is thrown out and deposited around the crater to form a mountain. If

the lava is thin and basic in its composition, it spreads a long distance forming a

flatter cone of gentler slope and of low elevation. If it is thick and of acid

composition, a small volcanic cone sharply pointing out is the result.

Sometimes lava is thrown out along with ash and cinders. Such a volcanic cone is

termed as ash and cinder cone.

Mount Mauna Loa in Hawai islands is an example of the former type.

Fuji Yoma of Japan and Mt Popa in Central Myanmar are examples of the latter

one.


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Volcanic Mountain

Residual or Dissected Mountains

They owe their present form due to erosion by different agencies.

That is why they are also known as relict mountains or mountains of

circumdenudation.

They have been worn down from previously existing elevated regions.

Hills like the Nilgiris, the Parasnath, the Girnar and Rajmahal in India are examples

of this type.

But Nilgiris got their present height as a result of subsequent uplift.


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All mountains of the Peninsula with the exception of the Aravallis are relict

mountains


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5. Fold Mountains

Fold mountains are the result of folding of the Earth’s crustal rocks by compressive

forces. They are considered as the “true mountains” and the term Orogenesis or

mountain building is commonly used for Fold mountains. Examples – Rockies

(North America), Andes (South America), Alps (Europe), Atlas(Africa), Himalayas

(Asia) etc.

Fold Mountain

In this article, we will discuss in detail about the various types of fold mountains,

their characteristics, location and touch upon the theories of fold mountain

formation.

Fig.- Compressive Forces leading to the folding of Earth’s crust. (Source –

Certificate Physical and Human Geography by G. C. Leong)

A Brief on Mountains in general

Mountains are natural elevated second order relief features (refer to Table 1) on

the Earth’s surface.

The Penguin Dictionary of Geography defines a mountain as “any natural elevation

on earth’s surface with a summit small in proportion to its base, rising more or less

abruptly from the surrounding level.”


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Based on their mode of formation, mountains can be classified into four main types

1. Fold Mountain

2. Block Mountain

3. Volcanic Mountain

4. Residual Mountain

Table 1: Classification of features on Earth’s surface

First order

relief Oceans and Continents Eg - Asia, Atlantic

Second

order

relief

Features on the oceans and continents due to

Endogenous Processes (caused by forces

from within the Earth)

Eg: Fold mountains,

Volcanic mountains, Rift

valley, Trenches

Third

order

relief

Features on the ocean and continents due to

Exogenous Processes (caused by forces on or

above the Earth's surface like wind erosion).

Eg: River valley,

waterfalls, Gorges,

Canyons etc.

Types of Fold Mountains

On the basis of Nature of Fold

1. Simple folded mountains – folds are arranged in waves like pattern with a well-

developed system of anticline and synclines.

2. Complex folded mountains – folds are complex in nature due to extreme

compressional forces like overfold, recumbent fold and nappe.


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Fig.- Types of Folding (Source – Certificate Physical and Human Geography by G.

C. Leong)

On the basis of the Period of Origin

1. Old fold mountains – Those mountains which originated before the Tertiary

period. These mountains have been so greatly denuded (or eroded) that they have

become residual fold mountains. For example, Aravalis, Appalachians etc.


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Table 2: Major mountain building phases

Era Mountain

Building phase

Age/Years

before present Examples

Pre-

Cambrian Archean

2500-3800

Million years Aravali Range in India

Pre-

Cambrian Proterozoic

570-2500 Million

years

Wopmay Orogen in

northwest Canada

Paleozoic Caledonian 320 Million years Scandinavian Highlands,

Scotland mountains

Paleozoic Hercynian 240 Million years Ural, Pennines,

Appalachians

Cenozoic Tertiary 65 Million years Alps, Rockies, Andes,

Himalayas

2. Young or New fold mountains – These are the fold mountains of the Tertiary

period. They are further subdivided based on their location

(i) Andean type of fold mountains – At the ocean – continental convergent

boundaries (C-O). They are prone to both earthquakes and volcanic activities.

Example – the Rockies, Andes

(ii) Himalayan type of fold mountains – At the continental – continental

convergent boundaries (C-C). There is an absence of active volcanism here.

Example – Great Himalayas


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Characteristics of Fold Mountains

Rock Type – Formed due to the folding of sedimentary rocks by strong

compressive forces. Furthermore, these rocks are of marine origin i.e. formed

due to deposition and consolidation of sediments in water bodies.

Shallow water deposits – The marine fossils found in the sedimentary rocks

belong to such organisms which can survive only in shallow waters.

Size – They are the loftiest, most extensive and elongated mountain chains on

the Earth’s surface. Their length is far greater than their width. For example, the

Himalayas have an east-west length of 2400 km but their maximum width is only

400 km.

Volcanicity – They may or may not have active volcanism but volcanic rocks

of ancient times may be found there. For example, Himalayas don’t have active

volcanism but volcanic rocks are found in Pir Panjal, Dalhauji (Himachal Pradesh)

and Bhimtal (Kumaun).

Earthquake – Generally the region is prone to earthquakes due to the presence

of active plate boundaries.

They are one of the youngest mountains of the world.

Generally found in an arc shape with one side having a concave slope and the

other having convex slope.


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Where are the Fold Mountains Located?

Preliminary analysis of the above world map shows that

They aren’t located randomly on the Earth’s surface.

Young fold mountains are generally located on the margins of the continents.

They are present mostly in the northern and western direction of the continents,

like the Atlas in Africa, the Rockies and the Andes in North and south America.

If we consider the former Tethys sea, then the Himalayas were also once located

along the margins of the continent.

They mark some of the major plate boundaries.

Old fold mountains are present inside the current continents. They represent the

ancient plate boundaries and orogenic movements of those times.

Fold Mountains of India

1. Himalayan mountains – Young fold mountains, formed during the Tertiary period.


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2. Aravali Range – Oldest fold mountain of India, formed during the Archean period

(2500 million years ago). Erosion over time has reduced their size

3. Vindhyan Range – formed during a Proterozoic period (500 million years ago),

erosion over time has left them to

4. Satpura Mountains – meaning “seven folds”, they are the fold mountains of the

Precambrian era and are highly deluded.

5. Eastern Ghat – they were once fold mountains but have been eroded by the east-

flowing peninsular rivers.

Fold Mountain formation

Various theories have been proposed to explain the formation of fold mountains.

A good theory should be able to explain various unique characteristics of fold

mountains and their location.

Thermal Contraction Model of Harold Jeffrey

He proposed mountains as the wrinkles on the Earth surface formed when Earth’s

crust cooled and contracted while differentiating from other parts

He used an analogy of layer of cream separating from a hot milk vessel cooling

slowly.

Limitation

Cannot explain the variation in age of various mountains on the Earth’s surface.

It proposes same types of rocks and random distribution of mountains.


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Figure – Illustration of Harold Jeffery model, Showing layer differentiation and

surface features formation

Horizontal Displacement Theory by F.B.Taylor

According to him mountains formed due to equator side movement and collision

of two ancient continents (Laurasia and Gondwanaland).

Force causing movement – Gravitational and tidal pull of Sun and Moon.

Limitation

Can best explain only Transeurasian mountains (the Alps + the Himalayas) but not

other mountains

Incorrect reason for the movement of continents


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Continental Drift Theory by Alfred Wegener

It was an extension of Taylor’s displacement hypothesis.

He proposed mountains as an accumulation of oceanic sediments (SIMA)

scrapped by floating continents (SIAL) along the leading edge.

This helped explain the marine origin of sediments and their general presence on

the western and northern side of the continents.

Limitation

Wrong concept of SIAL and SIMA.

The mechanism suggested for movement (Tidal pull of Sun and Moon and the

pole-fleeing force) was wrong

Modern Theory of Plate Tectonics

According to Plate Tectonics mountains were formed due to colliding lithospheric

plates along the convergent boundaries.

The colliding plates compress, accumulates and uplifts sediments between the

two plates.


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Arthur Holmes’ concept of Mantle Convection Currents explains the driving force

behind the lithospheric plate movements.

At Oceanic-Continental (O-C) convergent plate boundary, the denser oceanic

crust gets subducted under the relatively lighter continental crust.

Figure – Ocean-Continental Convergence

The subduction causes lateral compressive force which ultimately squeezes and

folds the sediments.

At Continental-Continental (C-C) convergent plate boundary, the amount of

sedimentation is maximum resulting in the formation of the highest mountains

of the world.

C-C convergence is also associated with the strongest compressive forces, hence

the fold mountains here develop complex folds and reverse faults.


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Figure – Continental – Continental Convergence

Geosynclinal Theory of Mountain Building

Geosynclines are the long and relatively narrow depressions on the Earth’s

surface.

They are characterised by the continuous sediment accumulation, which causes

gradual subsidence of the floor of Geosyncline.

Figure – Sedimentation causing subsidence of Geosyncline Floor (Source –

Physical Geography by Savindra Singh)


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Geosyncline as a concept helps in explaining the origin of an enormous amount

of marine sediments that were uplifted into fold mountains.

Tethys sea was one of the major Geosyncline, which was uplifted to form the

Himalayan system of mountains.

Figure – Mountain Formation as per Kober’s Geosynclinal Theory (Source

– Physical Geography by Savindra Singh)


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Fold Mountains and Human Life

Fold mountains impact the climate, vegetation, lifeforms and human activities in

the area. A sudden increase in altitude presents a large variation in the climate of

the region. For example, in the Andes mountain, equatorial rainforest exist just

miles away from its snow-covered peak Cotopaxi.

Similarly, the Himalayan fold mountains are responsible for the unique climate of

the Indian subcontinent. They block the cold Siberian winds, preventing people

from harsh winters. Also, they are responsible for the orographic rainfall from the

south-eastern monsoonal winds.

Fold mountains have significant economic importance as well. These areas house

major tourist spots of the world. They are a pleasant holiday destination in

summers and provide an opportunity for adventure sports.

Their steep slope and melting water from glaciers provide huge potential for Hydro

Electric Power, which is a cleaner energy source than the coal-based thermal

power.


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Apart from this forest produce like timber, fuel-wood etc, agriculture activities

(limited to sun-facing slope) and mining materials are some of the major economic

benefits from these mountains.

However, fold mountains are prone to disasters both natural and man-made.

Almost all of them are prone to Earthquakes and many are also vulnerable to

Volcanic eruptions as well. Soft soil of these mountains makes these areas prone

to landslides as well in the event of heavy rainfall or earthquakes. Human activities

have further destabilized the balance of nature in the area and increased the

vulnerability in the region as seen during the Nepal earthquake, 2015 and

Uttrakhand mountain Tsunami of 2103.


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6. Block Mountains

Block faulted mountains

fault block mountains definition

block faulted mountains or simply Block mountains are the mountains formed as a

result of faulting caused by the tensile and compressive forces caused by the

endogenetic forces coming from inside the earth. The faults are the cracks formed

in the Earth’s crust due to the endogenic forces. The uplifted blocks are known as

horsts, while the lowered blocks are termed as graben.

Topography of block faulted mountains are generally very smooth and they have

steep slopes. The folded zones of these mountains gradually lose their plastic

properties. The surface of these mountains become smooth due to denundations

due to various external forces.

block mountains examples

1. Block mountains are found all over the world in all continents. In Europe Block

mountains are found in the Vosges and the Black Forest mountains around

faulted Rhine Rift Valley. In North America, Block mountains are found in

California in the Sierra Nevada mountains.

2. The block mountains examples include the great African Rift Valley in Africa.

3. The block mountains examples in India includes the Satpura and Vindhya ranges

in the central western part of India. These are the block faulted

mountains formed due to the development of cracks in the crust of the earth.


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fault block mountains diagram

fault block mountains diagram – 01

fault block mountains diagram – 02

Formation of Block mountains

Different theories have been given for the formation of Block mountains. The

important ones are tension theory, compression theory, and erosion theory.


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Tension theory for the formation of block faulted mountains

Tension theory suggests the existence of a weak point on the Earth’s crust which

experiences the tremendous amount of tensile force. This tensile force pushes the

side rocky crust. The side rocky crust is pushed down, while the central block

remains stagnant at its position along the point of tension. The central block thus

becomes higher in elevation than the side blocks leading to the formation of block

faulted mountains.

Compression theory for the formation of Block mountains

Compression theory suggests that the compressional forces set up due to the

movement of earth can compress the middle block of rocks, as the force acts

inward to a point. Due to these compressional forces, the middle block rises

forming the Block mountains. The central block remains at a higher elevation than

the side blocks.

In some cases, the compressional forces may produce a reverse fault and the

Central block may be lowered in relation to the surrounding blocks. In this case, the

surrounding blocks are higher in elevation and form Block mountains.

Erosion theory for the formation of Block mountains

Some geologists like J.F. Spurr, on the basis of their research, have given the

erosional theory for the formation of Block mountains. The erosional theory has

been given after the research in the great basin range mountains of United States

of America. According to this theory, after the origin of these mountains in the

Mesozoic era, these mountains were subjected to intense erosion. Due to the

differential erosion, it led to the formation of the denuded Great Basin Range block

mountains of USA. However, the erosional theory for the formation of these

mountains remains debatable among the scientists.

Types of Block mountains

The Block mountains have been categorized as the tilted Block mountains and the

lifted Block mountains.

Lifted Block mountains: In the lifted Block mountains, there are two steep sides

represented by two boundary fault scraps. Examples of lifted type Block

mountains are the Sierra Nevada and Teton mountains of North America.


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Tilted Block mountains: The tilted block mountains have only one exposed fault

scarp and one gentle slope. The basin range mountains of USA and Rhine valley

of Europe etc. have the features of tilted Block mountains.

The Block mountains have diverse species of flora and fauna throughout the world

due to the diverse range of topographic features, temperature, and rainfall.


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7. Rocks and types of Rock

Rocks are the most common material on the Earth; they ordinarily lie everywhere

on the ground. They constitute most of the landforms, and there is a close relation

between rocks and landforms. The earth’s crust is composed of rocks. A rock is an

aggregate of one or more minerals that have been fused together into a solid lump.

For example, granite, a common rock, is a combination of the minerals

quartz, feldspar and biotite.

Properties of rocks

The rocks may be made up entirely of one mineral or various minerals.

Rocks do not have a definite composition of mineral constituents. Feldspar and

quartz are the most common minerals found in rocks.

Rock may be hard or soft. For example, granite is hard, soapstone is soft.

Rocks may have varied colours. Some rocks are dark and some are light coloured.

For example, Gabbro is black and quartzite can be milky white.


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Types of rocks

Rocks are classified according to characteristics such as mineral and chemical

composition, permeability, the texture of the constituent particles, and particle

size. In the long run, all types of rocks can transform from one type into another,

as described by the rock cycle model. These transformations usually take

thousands or millions of years.

There are many different kinds of rocks which are grouped into three families by

their mode of formation. They are:

1. Igneous Rocks

2. Sedimentary Rocks

3. Metamorphic Rocks

Igneous Rocks

Igneous rocks are called as primary rocks as they are formed out of magma and

lava from the interior of the earth.

The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma cools

and solidifies.

Igneous rocks are classified based on texture. The texture of these rocks depends

upon arrangement and size of grains and other physical properties or condition

of the materials.

Most igneous rocks are extremely hard and resistant. For this reason, they are

quarried for road construction and polished as monuments and gravestones.

Igneous rocks do not contain fossils as their forming material are super hot

magmatic materials.

Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the

examples of igneous rocks.


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Formation of Igneous rocks

During the volcanic eruptions, the molten rock materials – Magma and Lava, find

their way to the surface.

When magma in its upward movement cools and turns into a solid form, it is

called igneous rock.

This process of solidification can happen on the surface of the earth or inside the

earth’s crust.

The rate of cooling of the magma determines the size of its grain structure.

Magma cooled on the surface are fined grained whereas magma cooled slowly

inside the earth surface have coarse grain structure.

Formation of Igneous rocks

Classification of Igneous rocks

The lava that is released during volcanic eruptions on cooling develops into igneous

rocks. The cooling may take place either on reaching the surface or also while the

lava is still in the crustal portion. Depending on the location of the cooling of the

lava, igneous rocks are classified into two types:


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1. Volcanic rocks (Extrusive rocks) – cooling at the surface – extrusive rocks are

formed when molten magma, erupted from volcano, cools on the surface. The

magma on the surface (lava) cools faster on the surface to form igneous rocks

that are fine grained. Pumice and basalt are examples of extrusive igneous rocks.

2. Plutonic rocks (Intrusive rocks) – cooling in the crust – Intrusive igneous rocks are

formed when the magma cools off slowly under the earth’s crust and hardens

into rocks. Intrusive rocks are very hard in nature and are often coarse-grained.

Gabbro and granite are examples of intrusive igneous rocks.

Sedimentary Rocks

The word ‘sedimentary’ is derived from the Latin word sedimentum, which

means settling.

Sedimentary rocks are those that are formed through deposition and lithification

(compaction and cementation) of sediment particles at the Earth’s surface, with

the assistance of running water, wind, ice, or living organisms.

They may be coarse or fine-grained, soft or hard.

Sediments are naturally occurring particles derived from weathering and erosion

of pre-existing rocks.

The particles that form sedimentary rocks may be brought by winds, streams,

glaciers and even animals.

They are non-crystalline and often contain fossils of animals, plants and other

micro-organisms.

Sedimentary rocks are distinguished from other rock types in their characteristic

layer formation and are termed stratified rocks. The strata may vary in thickness.

Layers may be distinguished by differences in colour, particle size, type of

cement, or internal arrangement.

Classification of sedimentary rocks is done according to their age and different

kinds of rocks formed during the same period are grouped together.


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Most of the solid surface of our planet (roughly 70%) is represented by

sedimentary rocks.

Sandstone, Limestone and Shale are some examples of sedimentary rocks.

Weathering, Erosion and Deposition (W.E.D.)

Formation of sedimentary rocks

Sedimentary rocks are formed from sediments accumulated over long periods,

usually under water.


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All types of rocks (igneous, sedimentary and metamorphic) of the earth’s surface

are exposed to denudational agents (water, winds, ice and glacier etc.) and are

broken up into various sizes of fragments.

Such fragments are transported by different exogenous agents and deposited or

settled down in low lying areas or underwater, through the process of

sedimentation.

These deposits finally turn into rocks through compaction and compression by

weight of overlying material. This process is called lithification.

In many sedimentary rocks, the layers of deposits retain their characteristics even

after lithification. That is why we can see a number of layers of different thickness

in sedimentary rocks like shale, sandstone etc.

Formation of sedimentary rocks

Classification of sedimentary rocks

Depending upon the mode of formation, sedimentary rocks are classified into three

major groups:


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1. Mechanically formed — they are known as Clastic sedimentary rocks because of

their formation by accumulations of clasts: little fragments of broken up rock

material which have been piled up and “lithified” by compaction and

cementation. Conglomerate, sandstone, shale, limestone, loess etc. are some of

the examples of mechanically formed sedimentary rocks.

2. Organically formed— these rocks are formed by an accumulation of living

organisms such as corals or shellfish. These rocks have significant amounts of

organic material. The most famous rocks formed in this way are of the calcareous

type such as Chalk. However, carbonaceous rocks are also organically formed but

from vegetative matter. Geyserite, chalk, limestone, coal etc. are some examples

of organically formed sedimentary rocks.

3. Chemically formed — many of these forms when standing water evaporates,

leaving dissolved minerals behind. These sedimentary rocks form when mineral

constituents in solution become supersaturated and inorganically precipitate.

Chert, limestone, halite, potash etc. are some examples of chemically formed

sedimentary rocks.

Metamorphic Rocks

The word metamorphic means ‘change of form’ whereby ‘meta’ means change

and ‘morph’ means ‘form.’

Metamorphic rocks are formed due to the transformation of a pre-existing rock.

They are predominantly sedimentary or igneous rocks that have undergone

physical and chemical changes under the action of extreme heat and pressure.

Formation of metamorphic rocks can take place under different physical

conditions i.e. in different temperatures (up to 200 °C) and pressures (up to 1500

bars)

Their natural characteristics undergo a massive transformation in such extreme

conditions of temperature and pressure.

Metamorphic rocks make up a large part of the Earth’s crust and form 12% of the

Earth’s land surface


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Gneissoid, granite, syenite, slate, schist, marble, quartzite etc. are some

examples of metamorphic rocks.

Formation of Metamorphic rocks

Metamorphic rocks can simply be formed when the sedimentary or igneous rocks

moves deep inside the earth and come under the influence of high temperature

and pressure of the overlying material.

Formation of metamorphic rocks can take place through tectonic processes

including continental collisions, which may cause changes in pressure and

temperature.

The intrusion of magma on the earth’s surface is also an important cause of

metamorphic transformation.

Formation of metamorphic rocksMetamorphism is a process by which already

consolidated rocks undergo re-crystallisation and re-organisation of materials

within original rocks.


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Dynamic Metamorphism – Dynamic metamorphism refers to mechanical

disruption and reorganization of the original minerals within rocks due to breaking

and crushing without any significant chemical changes.

Thermal Metamorphism – Due to thermal stress the material of rocks undergo

crystal transformation which alters the chemical properties of the rock. There are

two types of thermal metamorphism.

1. Contact Metamorphism – In contact metamorphism the rocks come in contact

with hot intruding magma and lava and the rock materials recrystallise under high

temperatures. Quite often new materials form out of magma or lava are added

to the rocks.

2. Regional Metamorphism – In regional metamorphism, recrystallisation of rocks

takes place as a result of deformations caused by tectonic shearing along with

high pressure and temperature.

Foliation or Lineation – In the process of metamorphism in some rocks grains or

minerals get arranged in layers or lines. This arrangement of minerals or grains in

metamorphic rocks is referred to as lineation or foliation.

Banding – Sometimes minerals or materials of different groups are arranged into

alternating thin to thick layers appearing in light and dark shades. Such

arrangement in metamorphic rocks is commonly referred to as banding. Rocks that

display banding are called banded rocks.

Classification of Metamorphic rocks

Types of metamorphic rocks depend upon original rocks that were subjected to

metamorphism. Metamorphic rocks are classified into two major groups —

1. Foliated rocks – foliated rocks are formed where pressure squeezes or elongates

the crystals due to differential stress, they have a clear preferential alignment.

Rocks that were subjected to uniform pressure from all sides, or those that lack

minerals with distinctive growth habits, will not be foliated. For example, the

slate is a foliated metamorphic rock, originating from shale.


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2. Non-foliated rocks – they are formed where the crystals have no preferential

alignment. Some rocks, such as limestone are made of minerals that simply don’t

elongate, no matter how much stress you apply

Rock cycle

Rock Cycle

Like most Earth materials, rocks are created and destroyed in natural cycles. Rocks

do not remain in their original form for long but may undergo a transformation.

The rock cycle is a model that describes the formation, breakdown, and

reformation of a rock as a result of sedimentary, igneous, and metamorphic

processes. Rock cycle is a continuous process transforming old rocks into new ones.


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Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic)

are formed from these primary rocks. Igneous rocks can be changed into

metamorphic rocks. Sedimentary rocks are formed from the fragments derived out

of igneous and metamorphic rocks. Sedimentary rocks themselves can turn into

fragments and the fragments can be a source for the formation of sedimentary

rocks.

In the subduction zones in the earth’s crust. The crustal rocks can get inside the

mantel where they melt and become a source of the igneous rock, thereby

completing the rock cycle.

Related terms

Mineral – a mineral is an organic or inorganic substance that occurs naturally on

the earth surface. They have an orderly atomic structure and a definite chemical

composition and physical properties. A mineral consists of two or more elements.

However, sometimes single element minerals like copper, sulphur, gold, silver,

graphite etc. can be found.

Magma and Lava – The mantle contains a weaker zone called asthenosphere. It is

from this that the molten rock materials find their way to the surface. The material

in the upper mantle portion is called magma. Once it starts moving towards the

crust or it reaches the surface, it is referred to as lava

Petrology – Petrology refers to the science of rocks. Under petrology, rocks are

studied with respect to all their aspects viz., mineral composition, structure,

texture, occurrence, origin, alteration and relationship with other rocks.

Weathering – Weathering is an action of elements of weather and climate over

earth materials. Weathering includes mechanical disintegration and chemical

decomposition of rocks through the actions of various aspects of weather and

climate.

Erosion – Erosion involves acquisition and transportation of rock debris. When

massive rocks are broken into smaller pieces through the process of weathering or

any other process, erosional geomorphic agents like groundwater, running water,

wind, waves and glaciers remove and transport it to other places depending upon

the dynamics of each of these agents.


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Diagenesis – Diagenesis is a process which includes the formation of sedimentary

rocks by compaction and cementation of grains, or by crystallization from water or

solutions and recrystallization.

Cementation – Cementation is the process by which clastic sediment is lithified by

precipitation of mineral cement, such as calcite cement, among the grains of the

sediment.

Compaction – compaction is a process through which the porosity of a given

sedimentary material is reduced as a result of its mineral grains being squeezed

together by the weight of overlying sediment.


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8. Continental Drift Theory

To explain the present distribution of oceans and continents, various theories have

been proposed. Continental Drift Theory is considered a pivotal theory which

provided various conclusive proof to establish the movement of the earth crust but

the theory failed to establish the mechanism behind the process.

People for a very long time, till the early 20th century, thought that the continents

were fixed land masses. But it was in 1912, Alfred Wegener, a geologist came up

with the theory of continental drift. He further expanded the idea in his book “The

Origin of Continents and Oceans” which was published in 1915. Though Abraham

Ortelius, a Dutch cartographer was the first one to work on ideas of symmetric

coastlines on the sides of Atlantic ocean.

Continental Drift Theory

Continental drift means the movement of the continents across the ocean bed. This

drifting happens very, very slowly, over hundreds of million years!

According to Alfred Wegener, all the continents formed a single continental mass

known as Pangea (Pan=all + Gea=earth).

Pangea was surrounded by a mega-ocean, Panthalassa (Pan=all +

Thalassa=ocean).

Wegener further argued that about 225 million years ago, Pangea began to split.

It first broke into two large continental masses- Laurasia (the northern

component) and Gondwanaland (the southern component).

The intertwining part between Laurasia and Gondwanaland was known as Tethys

Sea, a shallow and meandering waterbody.

Subsequently, Laurasia and Gondwanaland continued to break into smaller

continents that we see today.


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Figure showing different stages of Continental Drift

Stages of Continental Drift

1. First stage – During the Carboniferous period in which a supercontinent Pangea

was surrounded by mega-ocean, Panthalassa.

2. In the second stage – Around 200 million years ago, Flight of continents took

place, continents began to drift gradually and broke into pieces, Laurasia

(Angaraland) and Gondwanaland. ( India was a part of Gondwanaland.)

3. In the third stage – During the Mesozoic era, the space between Laurasia and

Gondwanaland got filled with Tethys Sea and it gradually got widened.

4. Fourth stage – around 100 million years ago-Westward drift of North America

and South America led to the opening of Atlantic Ocean.

5. Fifth stage is the Orogenetic Stage-in which mountain building activity took

place. While Himalayas and Alps were formed with the folding of sediments of

Tethys Sea, and westward drift of North and South America led to folded edges

and formation of Rockies and Andes.


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Forces responsible for Continental Drift

Due to the cumulative effect of gravitational forces, ‘pole-fleeing force’ and force

of buoyancy, the continental drift was equator wards. The ‘pole-fleeing force’ is

due to increase in centrifugal force from poles towards the equator as the earth

is not perfectly round but there is a bulge at the equator.

Due to tidal currents resulting from rotation of the earth, the continental drift

was westwards. Tidal currents are due to the attraction of moon.

However, later, these two forces were found to be insufficient reasons for drifting

of the continents which is counted as the criticism of Wegener’s theory.

Evidence in support of the Continental Drift Theory

To justify his theory, Alfred Wegener came up with some evidence which are listed

below.

1. Jig-Saw Fit evidence or the Matching of continents– the shorelines of South

America and Africa when are facing each other shows a remarkable fit. Similarly,

Africa, Madagascar and east coast of India fit into each other when matched.

Figure showing jig-saw fitting of continents of Africa with South America


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2. Rocks of Same Age across the Oceans– The radiometric dating methods have

correlated rock formation across different continents. It suggests that the belt of

ancient rocks formed 2,000 million years ago from Brazil coast matches the

mountain belt found in Western Africa. the Caledonian and Appalachian

mountains also show similarity. It also suggests that the early marine deposits

along the coastline of Africa and South America belong to the Jurassic age

indicating that the ocean did not exist prior to that time.

Figure showing matching mountain ranges across different continents

3. Tillite evidence- Tillite refers to the sedimentary rock made out of deposits of

glaciers. In revealing evidence, it has been found that the Gondwana system of

sediments from India has its counterparts in six various landmasses of the

Southern Hemisphere- Africa, Falkland Island, Madagascar, Antarctica and

Australia and India. It demonstrates that these landmasses had remarkably same

antiquities.


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Figure showing continuity of glaciers across different landmasses

4. Placer deposit- In the Ghana coast (West Africa), rich placer deposits of gold are

found. But there is a complete absence of source rock in the area. It is amazing

to know that the gold-bearing veins are in Brazil. It suggests that the gold deposits

of Ghana are derived from the Brazil plateau when the two continents lay side by

side.

5. Fossil evidence- The observations that Lemurs occur in Africa, Madagascar and

India suggest that a contiguous landmass “Lemuria” existed connecting the

three. Another amazing fact is that skeleton of Mesosaurus (a small reptile,

adapted to shallow brackish water) are found only in South Africa and Iraver

formations of Brazil.


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Figure showing fossils of similar species in different continents

Criticisms of the Continental Drift Theory

There are flaws in jig-saw fitting, though it seems appropriate if 500-fathom

Isobath is considered as an outline of the continents.

Wegener talks about the role of forces like buoyancy, tidal currents and gravity.

But these were not strong enough to drift continents.

He advocates directional movement either westward or equatorward but

movements have taken place in all directions.

Alfred Wegener failed to explain the Pre-carboniferous history. He did not explain

that why the drift began only in Mesozoic-era and not before.

The theory did not take oceans into consideration.

The theory did not explain the formation of oceanic ridges and Island arcs.

Earth’s crust is believed to be too rigid to permit large-scale motions. Wegener’s

ideas have not offered a suitable mechanism justifying the displacement of larger

masses for long journeys.


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Present status of the Continental Drift Theory

The Continental Drift Theory was not accepted by most of the scientific community

and was hotly debated off and on for decades even after his death in 1930. In the

1920s, the concept of conventional currents in the upper mantle was developed.

But Alfred Wegener was not able to incorporate the concept of conventional

currents as the most justifying reason for the movement of continents due to his

untimely death.

Although, the Continental Drift theory have become obsolete the main idea of the

theory of drift of continent was the driving force behind all other modern theories

including the theory of plate tectonics and seafloor spreading.


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9. See Floor Spreading – Paleomagnetism

The theory of Sea Floor Spreading states that new oceanic crust is being formed

continuously at mid-oceanic ridges, while the older rocks move away from the

ridge. That is, it explains why the age, thickness, and density of the oceanic crust

increases with distance from the mid-oceanic ridge.

Sea Floor Spreading

In this article series, we are learning about the theories which explain the present

distribution of oceans and continents, the concepts which explain the distribution

of earthquakes and volcanoes, folds and faults. In the previous article, we have

learned about the Continental Drift Theory given by Alfred Wegener. Although

Wegener is right in stating that the continents drift over time, but his assumption

that the continents were blocks of rock that slid across the ocean floor has been

found to be incorrect. What we know today is that the continents and ocean floor

are part of the same layer of the earth.

In this article, we will learn about Sea Floor Spreading which explains continental

drift by the help of the theory of plate tectonics. But, before coming directly into

the concept of Sea Floor Spreading, we must understand some basic concepts that

explain Sea Floor Spreading.

These concepts are Ocean Floor Mapping, Distribution of earthquakes and

volcanoes, Convectional Current theory and Paleomagnetism.

Ocean Floor Mapping

The ocean floor is found to be having mountain ranges, plains, canyons, submarine

ridges, deep trenches and other relief features. It has also been found that along

the mid-oceanic ridges, volcanic eruptions are most active. Further, the dating of

rocks suggests that the oceanic crust rocks are much younger than the continental

rocks. Also, the rocks which are equidistant from the crest of oceanic ridges on both

sides have been found with utmost similarities in terms of their age, constituents,

chemical composition and magnetic properties. And the sediments on the ocean

floor near the ridge has been found to be thinner.


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Figure showing typical relief features of the ocean floor

Convectional Current Theory

Convectional current theory was proposed by Arthur Holmes.

First of all, it is to be noted that the heat which is generated from the radioactive

decay of substances deep inside the Earth (the mantle) creates magma which

consists of molten rocks, volatiles, dissolved gases among other material.

The Convectional current theory states that this magma, heat and gases seek a

path to escape which leads to the formation of convection currents in the mantle.

According to the theory of Seafloor spreading, convectional cells are the force

behind drifting of continents.

Also, please note that the ocean plates get subducted under the continental

plates (since ocean plates are denser than continental plates), when these two

types of plates converge.

The collision of plates is followed by earthquakes and volcanoes.


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The Distribution of Earthquakes and Volcanoes

Another observation is that, if we map the distribution of earthquakes and

volcanoes, it is surprising to know that volcanic activities, earthquakes and other

associated activities at plate margins are a result of convection currents in the

mantle.

Do observe in the map shown below, that the volcanic activity in the oceans is

almost parallel to the coastlines.

Since magma is less dense than the crust, it rises up to the oceanic crust

with convection currents, leading to the formation of a volcano.


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Figure showing ridges as the hot-spots of the earthquake and volcanic activity

Palaeomagnetism

Palaeomagnetism, in simple words, refers to the study of the earth’s magnetic

properties. The magnetic studies have revealed that the ocean floor consists of

parallel bands of oceanic crust which have alternating magnetic polarity. These

magnetic bands are symmetric and are mirrored around the mid-oceanic ridge.

Alternating magnetic polarity or the reversal of magnetic polarity refers to change

in the Earth’s magnetic field, that is, the north magnetic pole becomes south

magnetic pole and vice versa.

These alternating magnetic bands had happened because the new rocks which are

formed near the ridge, while the older rocks, which formed millions of years ago

when the magnetic field was reversed, have been pushed farther away.

Hence, this further explains the seafloor spreading.


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Figure showing parallel bands of oceanic crust with alternating magnetic polarity

around the mid oceanic ridge

The concept of Sea Floor Spreading

The concept of seafloor spreading was put forward by H.Harry Hess, an American

geologist.

He suggested that new sea-floor forms at the oceanic ridges and spread outwards

from the line of origin. Further, he claimed that continents would be pushed aside

by the same forces that cause the ocean to grow. That is, constant eruptions at the

crest of oceanic ridges cause the oceanic crust to rupture and new lava to

wedge out of it, pushing the oceanic crust on either side. The ocean floor thus

spreads.


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Figure showing sea-floor spreading

The intense heat generated by radioactive substances in the mantle beneath the

lithosphere seeks a path to escape and forms convection currents. The rising

convectional currents continuously bang thick but strong oceanic SIMA crust and

the hammering effect leads to the development of cracks, joints, fractures and

disintegration of oceanic ridges. Pyrospheric material oozes out and pushes the

older oceanic SIMA away from the rift zone. Seafloor spreading occurs at diverging

plate boundaries.

Upwelling of the magmatic material leads to the formation of mid-oceanic ridges

and substitution of older material by newer one. Successive eruption results into

seafloor spreading.


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The rate of movement is slow, 2.5 cms per year and resembles two giant conveyor

belts carrying sea floor from the zones of accretion(divergence area- mid-oceanic

ridge) to zones of consumption ( convergence area- trenches).

The theory of seafloor spreading solved many of the unsolved problems

1. It solved the problem of younger age crust found at the mid-oceanic ridges and

older rocks being found as we go away from middle part of the ridges.

2. It also explained why the sediments at the central parts of the oceanic ridges are

relatively thin.

3. The sea-floor spreading also proved the drifting of continents as propounded by

Alfred Wegener and helped in the development of the theory of plate-tectonics.

In the next article, we will learn about a related concept, the theory of plate-

tectonics.


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10. Plate Tectonics

In this geomorphology study notes, we have till now covered concepts like

continental drifting, seafloor spreading and convectional current theory which

gives the explanation for many geomorphological features present on the earth.

But still, there remain some unanswered questions like- how do the fold mountains

form, what are the causes behind the occurrence of earthquakes, what are the

reasons behind volcanic activity on land and so on. For answering these and other

similar questions, we will learn about a very important concept in this article, that

is, the theory of Plate Tectonics.

Formulation of the theory of plate tectonics

With the emergence of the concept of seafloor spreading, and wealth of new

evidence at the beginning of the 1950s and 1960s, the interest in the problem of

distributions of oceans and continents was revived. Also, the following six

developments were instrumental in the formulation of the theory of plate

tectonics:

Development of mid-oceanic ridges and sea floor spreading

Palaeomagnetism

The findings of the age of ocean floors

Discoveries of island arcs and submarine trenches

The precise documentation of volcanoes and earthquakes, identification of

susceptible seismic zones, and spots vulnerable to volcanic activity

Identification of hotspots, their strengths, size and retrospective ejections.

What is a ‘tectonic plate’, ‘plate tectonics’ and ‘tectonic activity’?

The Earth consists of four concentric layers: inner core, outer

core, mantle and crust. The crust and uppermost of solid mantle are known as

lithosphere. Whereas asthenosphere is highly viscous, mechanically weak

and semi-molten region of the upper mantle of the Earth. And, lithosphere floats

over asthenosphere.


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Figure showing the structure of the earth

A tectonic plate is a massive, irregularly shaped slab of solid rock, generally

composed of both continental and oceanic lithosphere. Plates move horizontally

over the asthenosphere as rigid units.

On the basis of size, a tectonic plate may be a major plate or a minor plate. For

example, Pacific plate is a major plate whereas Nazca plate is a minor plate.

On the basis of nature, a plate may be referred to as continental plate or oceanic

plate depending on which of the two occupy a large portion of the plate. For

example, Pacific plate is mostly an oceanic plate whereas Eurasian plate may be

called as a continental plate.

While a tectonic plate is a rigid lithospheric slab, plate tectonics is a collective term

for evolution, nature and motion, deformation, the interaction of plate margins and

resultant landforms.

The earth’s crust is continuously experiencing movements in horizontal as well as

vertical direction resulting in breaking and bending of crustal rocks and this process

of deformation is known as the tectonic activity.


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Theory of Plate Tectonics

The theory of plate tectonics proposes that the earth’s lithosphere is divided into

seven major and several minor plates. The movement of the plates results in the

building up of stresses within the plates and the continental rocks above, which

leads to folding, faulting and volcanic activity. The major plates are surrounded by

fold mountains, ridges, trenches and faults.

These plates have been moving very slowly across the globe throughout the history

of the earth. Moreover, it may be noted that all the plates without exception, have

moved in the geological past, and shall continue to move in the future as well.

Alfred Wegener in his theory of continental drift had thought that continents move,

but, this is incorrect. He further believed that all continents were initially existent

as a super-continent, Pangea. However, later discoveries have revealed that

continental masses, resting on plates have been moving, and Pangea was a result

of the convergence of different continental masses that were part of one or the

other plates.

Figure showing seven major plates and some minor plates

The seven major plates are:


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1. North American plate (with the western Atlantic floor separated from the South

American plate along the Caribbean islands)

2. South American plate (with western Atlantic floor separated from the North

American plate along the Caribbean islands)

3. Pacific plate

4. Antarctica and the surrounding oceanic plate

5. Eurasia and the adjacent oceanic platee)

6. Africa with the eastern Atlantic floor plate

7. India-Australia-New Zealand plate


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While Pacific plate is the largest of them all, South American plate is the smallest.

Minor plates:

1. Carribbean Plate

2. Cocos Plate

3. Caroline Plate

4. Juan de Fuca Plate

5. Juan Fernandez micro Plate

6. Iranian Plate

7. South sandwich Plate

8. Myanmar Plate

9. Anatolian Plate

10. Nazca Plate

11. Nubian Plate

12. Philippines Plate

13. Okhotsk Plate

14. Scotian Plate

15. Eastern micro Plate

16. Somalian Plate

17. Arabian Plate

18. Solomon Plate

19. Fiji Plate


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20. Bismarck Plate

The force behind the Movement of Plates

Previously, we have studied that intense heat is generated from the radioactive

decay of substances deep inside the Earth (the mantle) which creates magma

consisting of molten rocks, volatiles and dissolved gases. These produce

convectional currents when the magma, heat and gases seek a path to escape in

the mantle. The force behind the movement of the plates are these convectional

currents generated by the upwelling of hot magma which causes the overlying

lithospheric slabs to uplift and stretch.

Figure showing how convection currents play role in movement of plates

Rates of Plate Movement

The rate of plate movement is determined by the bands of normal and reverse

magnetic fields that parallel the mid-oceanic ridge. The rates of plate movement

have a considerable variation. For example, while the Arctic Ridge has the slowest

rate (less than 2.5 cm/yr), the East Pacific Rise in the South Pacific has the fastest


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rate (more than 15 cm/yr). An interesting fact is that the movement of Indian plate

from south to equator was one of the fastest plate movements in history.

Importance of the theory of Plate Tectonics

For geologists, it is a fundamental principle for study. It is the unifying theory of

geology, which further explains large-scale geological phenomena, such as

earthquakes, volcanoes, and the existence of ocean basins and continents.

Plate tectonics theory explains why there are lots of volcanoes in Iceland and

Japan, but far fewer in Russia and Africa. This is because Iceland was created by

a mid-oceanic ridge. Similarly, Japan is located on a fault line. The constant

pressure around the fault line causes many earthquakes and volcanic eruptions.

For geographers, the theory of Plate tectonics aids in the interpretation of

landforms. It ultimately explains why and where deformation of Earth’s surface

occurs.

Further, the concept of plate tectonics explains mineralogy. New minerals pour

up from the core along with the magmatic ejections. The plate boundaries are the

pathways through which rocks from the mantle come out as deposits on

lithosphere. These rocks are the source of many minerals. The famous Pacific

Ring of fire known for its violent volcanic activity is also a ring of mineral deposits.


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11. Divergent Boundary

The Earth’s crust is made up of 6 major plates; they are the Pacific plate, American

plate, Eurasian plate, African plate, Indo-Australian Plate and Antarctic plate along

with some minor plates. These plates float on the molten magma of plastic

asthenosphere. These plates form plate boundaries which are of three types

Convergent plate boundaries: in which the lithospheric plates move towards

each other.

Divergent plate boundaries: in which the lithospheric plates move away from

each other.

Transform plate boundaries: in which the lithospheric plates slide past one

another.

Divergent Plate Boundaries

Divergent plate boundaries can be defined as the locations where the lithospheric

plates move away from each other. Divergent plate boundaries occur above the

rising convection currents in the asthenosphere. These rising currents push up the

bottom of the lithospheric plate, and the magma flows laterally below it. This

lateral flow of the current drags the lithospheric plates in the direction of flow of

current. At the point of the crest of upliftment the lithospheric plate is stretched,

which breaks it and pulls the above plates apart, creating a divergent plate

boundary.

divergent plate boundaries


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Facts associated with divergent plate boundaries

According to the continental drift theory, all the different continents were part of

one giant supercontinent which was known as Pangea. Due to the plate tectonics

and rising Convection currents in the asthenosphere, the supercontinent Pangea

broke apart and the different continents emerged and they began their slow

migration towards their present locations. The divergent plate boundaries are the

places where the lithospheric crust is being extended, thinned and broken due to

extension stress. At the divergent plate boundaries, new lithospheric plates are

created while the old lithospheric plates get destroyed somewhere else at the

convergent plate boundaries.

The divergent plate boundaries and the continents initially produce rifts, which

ultimately becomes rift valleys. Most of the active divergent plate boundaries occur

between the oceanic tectonic plates. The divergent plate boundaries are also

associated with volcanic Islands which occur when the lithospheric plates move

apart from each other and produce gaps from which the molten lava comes out. At

the divergent boundaries, new crust is created as the Magma is pushed up from

the mental to the surface. The divergent plate boundaries are also associated with

earthquakes which strike along the rift.

Divergent plate boundaries – Oceanic

When the divergent plate boundaries occur below the oceanic lithosphere, the

rising convection currents from the magma lifts the lithospheric plate and produces

a mid-oceanic ridge. The lithospheric plate is stretched due to the extensional

forces and produce a deep fissure. When this fissure opens, the pressure on the

heated magma is reduced. New magma flows through this fissure, which comes out

and solidifies. This process is repeated in a cyclic manner.

Divergent plate boundaries examples include the mid-Atlantic Ridge. The area

around the ridge is higher compared to the surrounding sea floor, due to the uplift

of the oceanic plate by the convection currents below it. In these areas, the

divergent plate boundaries are associated with submarine mountain ranges such

as the mid-Atlantic ridge, volcanic activities in the form of fissure eruptions,

seafloor spreading and shallow earthquakes.


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Divergent plate boundaries location

Seafloor spreading

Divergent plate boundaries are associated with seafloor spreading whose evidence

has been found with younger oceanic crust near the ridges. The oceanic crust

becomes progressively older away from the spreading centre. There is a gradual

formation of new Oceanic crust close to the ridges and gradual spreading of these

oceanic plates over time. The seafloor spreading over the past 200 million years

has enlarged the size of Atlantic Ocean which has grown from a tiny inlet of water

between Europe, Africa, and the Americas into the vast ocean of present times.

Divergent boundaries – Continental

The divergent boundaries below the thick Continental plate are not vigorous

enough to form a clean single break through the continental plate. When the two

Continental plates are pulled apart, faults develop on both sides of the rift. The

central block of the plate slides downwards and earthquakes occur due to this

movement. In the early part of the rift forming process, streams and rivers flow

through this sinking rift valley which can form a long linear lake. If the rift grows

deeper, its level can fall below the sea level and the ocean water can enter inside


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it. This can lead to the formation of a narrow sea inside the rift. If this rifting process

continues, new ocean basin could be formed in that place.

This type of divergent plate boundaries includes the East African Rift Valley, which

is in the early stage of development. The continental plate of that area has not been

completely rifted as the East African Rift Valley is presently above the sea level. The

example of the completely developed rift valley is the Red Sea, where the plates

have been fully separated and the central Rift Valley has dropped below the sea

level.


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12. Earthquakes

An Earthquake is a tectonic movement caused by endogenetic (originating within the earth) thermal conditions inside earth’s interior which is transmitted through the surface layer of the earth.

What is an Earthquake?

It is shaking or trembling of the earth surface causing energy to release suddenly. An earthquake can range from a faint tremor to a wild motion capable of shaking building apart. Minor tremors caused by gentle waves of vibration within the earth’s crust occur every few minutes. Major earthquakes, usually caused by movement along faults, can be very disastrous particularly in a densely populated area.

Earthquakes themselves may cause only restricted damage in the regions of occurrence, but their aftershocks can be very catastrophic.An aftershock is an earthquake of the smaller magnitude that occurs after the main shock. They are also known to cause Tsunami waves.

Nearly 54% of land area in India is prone to earthquakes. Earthquakes are by far the most unpredictable and highly destructive of all the natural disasters. It not only damages and destroys the settlements, infrastructure etc. but also result in loss of lives of men and animals.

Focus and epicentre of an earthquake

Focus – The point within the earth’s crust where an earthquake originates is called the focus. It is also referred as seismic focus or hypocenter. It generally lies within the depth of 60 kilometres in the earth crust.

Epicentre – The point vertically above the focus on the earth’s surface is known as ‘epicenter’. Earthquake travel in the form of the longitudinal wave from the focus to epicentre. The intensity is the highest at the epicentre. That is why the maximum destruction occurs at and around the epicentre. The intensity of vibrations decreases as one moves away from the epicentre.


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Focus and Epicentre of an Earthquake

Causes of earthquakes

Plate Movements

Folding, faulting and displacement of rock strata, upwarping and downwarping of crust are some of the main causes of earthquakes. Some examples of this type of earthquakes are the San Francisco earthquakes of California in 1906, the Assam earthquakes of 1951, the Bihar earthquakes of 1935.


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Volcanic eruptions

The violent volcanic eruptions put even the solid rocks under great stress. It causes vibrations in the earth’s crust. But, these earthquakes are limited to the areas of volcanic activity such as the circum-pacific ring of fire and the Mid-Atlantic Ridge. An important example of this type of earthquake includes the earthquake preceding the eruption of Mauna Loa volcano of Hawaii Island in 1868.

Forces within the earth

Gaseous expansion and contraction within the earth can cause stress in strata which build up over time and is released suddenly in the form of earthquakes.

Exogenic forces

An earthquake may also be caused due to landslide and collapse of cave or mines etc. which can cause a sudden release in energy.

Man-made causes

These may range from dam building, mining, dredging, road building, drilling etc.

Types of earthquakes

Tectonic earthquakes – The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.

Volcanic Earthquakes – A special class of tectonic earthquake is sometimes recognised as a volcanic earthquake. They are generated due to violent volcanic eruptions. However, these are confined to areas of active volcanoes.

Collapse Earthquakes – In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.

Explosion Earthquakes – Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.


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Reservoir-induced Earthquakes – The earthquakes that occur in the areas of large reservoirs are referred to as reservoir-induced earthquakes.

On the basis of focus, earthquakes can be classified as a shallow focus ( focus located at depth of up to 50 km), intermediate focus ( 50 – 250 km depth) and deep focus earthquakes (foci depth of up to 700 km).

Mechanism of tectonic earthquake

Tectonic earthquakes are the most common earthquakes occurring on earth surface.

They occur due to movement of tectonic plates past each other which builds up stress along the fault line.

A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. The friction between the plates locks them together due to which they

cannot glide past each other. However, at some point of time, their tendency to move apart overcomes

the friction and they slide past one another abruptly. This causes a sudden release of energy along the fault, and the energy waves

radiate in all directions.

Earthquake waves

Earthquake waves are energy waves, generated by the sudden release of energy from within the earth surface during an earthquake. The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.

Earthquake waves are basically of two types — body waves and surface waves.

Body Waves

Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves.

Travelling through the interior of the earth, body waves arrive before the surface waves emitted by an earthquake.


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These waves are of a higher frequency than surface waves.

Surface Waves

The body waves interact with the surface rocks and generate a new set of waves called surface waves.

These waves move along the surface. The surface waves are the last to report on a seismograph. Surface waves are considered to be the most destructive waves. They cause displacement of rocks, and hence, the collapse of structures

occurs.

Types of Body waves

There are two types of body waves. They are called P and S-waves.

Earthquake Waves

P waves

These are also called ‘primary waves’. P-waves move faster and are the first to arrive at the surface. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials. P-waves vibrate “parallel” to the direction of the wave. This exerts pressure on the material in the direction of the propagation.


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Hence, it creates density differences in the material leading to ‘stretching’ and ‘squeezing’ of the material.

S waves

These are called secondary waves. S-waves arrive at the surface with some time lag. They can travel only through a solid medium. S-waves vibrate perpendicular to the direction of the wave in the vertical

plane. Hence, they create ‘troughs’ and ‘crests’ in the material through which they

pass.

Shadow Zone of earthquakes

When an earthquake occurs, earthquake waves radiate out spherically from the earthquake’s focus. Earthquake waves get recorded in seismographs located at far off locations.

A shadow zone is an area of the Earth’s surface where seismographs do not detect any earthquake waves.

For each earthquake, there exists an altogether different shadow zone. As P waves are refracted by the liquid outer core, the shadow zone of P-

waves appears as a band around the earth between 103° and 142° away from the epicentre.

S waves cannot pass through the liquid outer core and are not detected beyond 103°. Thus, the entire zone beyond 103° is referred as Shadow zone of S-waves

The shadow zone of S-wave is much larger than that of the P-waves. It is also a little over 40 percent of the earth surface.


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Measurement of earthquakes

The earthquake events are scaled either according to the magnitude or intensity of the shock.


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Richter scale – The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10.

Mercalli scale – The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.

Earthquake-prone zones in India

Over 55% of the land area in India is vulnerable to earthquakes. Some of the most vulnerable states are Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, and the Darjiling and subdivision of West Bengal and all the seven states of the northeast.

Apart from these regions, the central-western parts of India, particularly Gujarat and Maharashtra have also experienced some severe earthquakes.

Bureau of Indian Standards, based on the past seismic history, grouped the country into four seismic zones, viz. Zone II, III, IV and V. Of these, Zone V is the most seismically active region, while zone II is the least. The current division of India into earthquake-prone zones does not use Zone I.

The Modified Mercalli (MM) intensity, which measures the impact of the earthquakes on the surface of the earth, broadly associated with various zones, is as follows:

Seismic Zone Description Intensity on MM scale

Zone II Low intensity zone VI (or less)

Zone III Moderate intensity zone VII

Zone IV Severe intensity zone VIII

Zone V Very severe intensity zone IX (and above)


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Areas covered under different Seismic Zones

Zone – V covers 10.79% area of the country. It comprises entire northeastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttaranchal, Rann of Kutch in Gujarat, part of North Bihar and Andaman & Nicobar Islands.

Zone – IV covers 17.49% area of the country. It comprises remaining parts of Jammu and Kashmir and Himachal Pradesh, National Capital Territory (NCT) of Delhi, Sikkim, Northern Parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan.

Zone – III covers 30.79% area of the country. It comprises Kerala, Goa, Lakshadweep islands, remaining parts of Uttar Pradesh, Gujarat and West Bengal, Parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Orissa, Andhra Pradesh, Tamil Nadu and Karnataka.

Zone – II covers 40.93% area of the country. It comprises major parts of peninsular region. Karnataka Plateau falls in this zone.


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Seismic Zone Map of India

Distribution of earthquakes in the world

The occurrence of an earthquake is a phenomenon of almost every part of the world. But, there are two well-defined belts where they occur more frequently. These belts are the Circum-Pacific belt and the Mid-world mountain belt.

The Circum Pacific Belt comprises the western coast of North and South America; the Aleutian Islands and island groups along the eastern coasts of Asia such as Japan and Philippines. As it encircles the Pacific Ocean from end to end, it is named as such. The earthquakes in this belt are associated with the ring of mountains and volcanoes. It is estimated that about 68 % of earthquakes of the world occur in this belt alone.

The Mid-world Mountain Belt extends from the Alps with their extension into the Mediterranean the Caucasus and the Himalayan region and continues into


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Indonesia. About 21 % of total earthquakes of the world originate in this belt. Remaining 11 % occur in the other parts of the world.

Consequences or effects of the earthquakes

Damage to property: when an earthquake occurs, all buildings from cottage to palaces and stronger skyscrapers are greatly damaged or totally destroyed. Earthquakes also cause a great deal of infrastructural damage. Underground pipelines and railway lines are damaged or broken. Dams on river collapse, resultant floods cause havoc.

Loss of lives: Duration of tremors of an earthquake is normally of only a few seconds, but thousands of people may die in this short period. More than 25,000 people died in Gujarat earthquake of 2001. Earthquakes also cause the death of wildlife and result in a destruction of their habitat.

Floods: Flood may result as an indirect consequence of an earthquake due to dam or levee failure.

Changes in river courses: Sometimes river channels are blocked or their courses are changed due to the impact of the earthquake.

Tsunamis: Tsunamis are extremely high sea wave caused by an earthquake. It wreaks havoc on settlement of coastal areas. It sinks large ships. The effect of a tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high. Tsunamis are waves generated by the tremors and not an earthquake in itself.

Soil liquefaction: Soil liquefaction occurs when, because of the shaking, water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits.

Cracks and fissures: Sometimes cracks and fissures develop in roads railway tracks, and fields, making them useless. The well known San Andreas Fault formed during the earthquake of San Francisco (California).

Landslides and Avalanches: landslides and avalanches may be triggered due to an earthquake.


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Fires: Earthquakes can cause fires by damaging electrical power or gas lines. it may also become difficult to stop the spread of a fire once it has started.

Prediction of earthquake

Earthquake can occur at any time of the year, day or night. Its impact is very sudden. There are no warning signs of earthquakes. Extensive and sincere research has been conducted in the forecast or prediction of an earthquake.

For the first time in India, a system to detect earthquakes and disseminate warnings was installed in Uttarakhand, in 2015. It issues warnings 1-40 seconds before earthquakes of magnitude 5 occur. All sensors under this system that warn of earthquakes are based on the detection of P and S waves generated during an earthquake. The P wave, which is harmless and travels faster than the S wave, is detected by the sensors for advance warning.

IIT Roorkee is conducting research to develop first of its kind sensors to be deployed in all seismic prone major cities in North India.


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13. Tsunami

Tsunami and Its Causes

“Tsunami” meaning “harbour wave” in literal translation comes from the Japanese

characters for harbour (“tsu”) and wave (“name”). A tsunami also called seismic sea

waves, is one of the most powerful and destructive natural forces. It is a series of

extremely long waves caused by a large and sudden displacement of the ocean due

to earthquake, volcanic eruptions etc. When they reach the coast, they can cause

dangerous coastal flooding and powerful currents that can last for several hours or

days.

Global distribution of Tsunami

Globally, 70% of the confirmed tsunami sources have been in the Pacific Ocean, 9%

in the Caribbean Sea, 15% in the Mediterranean Sea and the Atlantic Ocean and 6%

Indian Ocean. Most of these Tsunamis were generated by earthquakes.

Tsunamis are frequently observed along the Pacific ring of fire, particularly along

the coast of Alaska, Philippines, Japan and other islands of South Asia and

Southeast Asia including Malaysia, Indonesia, Myanmar, Sri Lanka and India etc.


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Characteristics of Tsunami

Tsunamis are among Earth’s most infrequent hazards and most of them are small

and nondestructive

Tsunamis generally consist of a series of waves, with periods ranging from

minutes to hours

Tsunamis radiate in all directions from the point of origin and they can cover

entire ocean basins.

There is no season for tsunamis. We cannot predict where, when or how

destructive the next tsunami will be.

Not all tsunamis act the same. And, an individual tsunami may impact coasts

differently. A small tsunami in one place may be very large a few miles away.

Most tsunamis are caused by large earthquakes. Though, not all earthquakes

cause tsunamis.


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Tsunamis are waves generated by the tremors and not by an earthquake itself.

The effect of Tsunami would occur only if the epicentre of the tremor is below

oceanic waters and the magnitude is sufficiently high.

A tsunami can strike any ocean coast at any time. They pose a major threat to

coastal communities.

The speed of the wave in the ocean depends upon the depth of water. It is more

in the shallow water than in the ocean deep. As a result of this, the impact of a

tsunami is more near the coast and less over the ocean

Over deep water, the tsunami has very long wavelengths (often hundreds of

kilometres long) when a tsunami enters shallow water, its wave-length gets

reduced and the period remains unchanged, which increases the wave height.

Tsunamis have a small amplitude (wave height) offshore. This can range from few

centimetres to over 30 m height. However, most tsunamis have less than 3 m

wave height.

How is a tsunami different from a wind-generated wave?

Most ocean waves are generated by wind. Tsunamis are not the same as wind

waves. First of all, they have different sources. Also while wind waves only affect

the ocean surface, tsunamis move through the entire water column, from the

ocean surface to the ocean floor. Waves can also be described based on their


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wavelength (horizontal distance between wave crests), period (time between wave

crests), and speed. These characteristics highlight the differences between wind

waves and tsunamis.

Tsunami Wind Wave

Source

Earthquakes, landslides, volcanic

activity, certain types of weather,

near earth objects

Winds that blow across

the surface of the

ocean

Location of

energy

Entire water column, from the

ocean surface to the ocean floor Ocean surface

Wavelength 100-500 Km 20-30 meters

Wave Period 5 minutes – 2 hours 5-20 seconds

Wave Height 10-30 meters from centimeters to

few meters

Wave Speed 800-900 Kmph (in deep water), 30-

50 Kmph (near shore) 10-100 Kmph

Causes of Tsunami

A tsunami is caused by a large and sudden displacement of the ocean. Large

earthquakes below or near the ocean floor are the most common cause. But

landslides, volcanic activity, near earth objects (e.g., asteroids, comets), certain

meteorological conditions and nuclear tests can also cause tsunamis.


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Earthquake – Tsunami can be generated when the sea floor ruptures abruptly due

to tectonic earthquakes, causing vertical displacement of the overlying water. Most

of the earthquakes which generate tsunamis occur on thrust faults. These

earthquakes occur mainly in the areas where tectonic plates move toward each

other in subduction zones.

As per data, ten to fifteen percent of the most damaging tsunamis are generated

by strike-slip earthquakes involving a horizontal movement of the earth.

Example – 2004 Indian Ocean Tsunami was an earthquake-induced Tsunami,

caused by an earthquake (Mw 9.2) in the Indian Ocean.

Landslides – “landslide” is a general term that involves the ground movement of

different types, including rock slide, block slide, debris flows, avalanches, and

glacial calving (referring to the breaking off of large pieces of ice from a glacier).


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Tsunamis can be generated when a landslide enters the water and displaces it. Such

generation of Tsunami depends on the amount of rock material that displaces the

water, the speed with which it is moving, and the depth it moves to.

Landslide-generated tsunamis may be larger than seismic tsunamis near their

source, but they usually lose energy quickly and rarely affect distant coastlines. A

landslide big enough to cause a transoceanic tsunami has not occurred in the

recorded history.

Example – 1998 Papua New Guinea Tsunami was generated by a landslide cause by

an earthquake.

Volcanoes – volcanoes generated Tsunamis are very infrequent, both above and

below water. However, different types of volcanic activity can displace enough

water to generate tsunamis e.g. submarine explosions, caldera formations etc.

Like other non-seismic tsunamis, such as those generated by landslides, volcanic

tsunamis usually lose energy quickly and rarely affect distant coastlines.

Example – 1883 Indonesia Tsunami was caused by the explosion of

Krakatau volcano.

Near Earth Objects – It is very rare for a near earth object like an asteroid or comet

to reach the earth and its potential to generate Tsunami is still uncertain, as there

are no records of a Tsunami caused by near earth objects, in recent human history.

However, scientists are of the opinion there are two ways near earth objects could

generate a tsunami.

Large objects (more than 1,000 meters in diameter) that make it through Earth’s

atmosphere without burning up could hit the ocean, displacing water and

generating an “impact” tsunami.

If this happens above the ocean, the explosion could release energy into the ocean

and generate an “airburst” tsunami.

Meteotsunamis – Some meteorological conditions, for example, air pressure

disturbances often associated with fast moving weather systems, can displace

bodies of water enough to generate Tsunamis. These “meteotsunamis” are similar

to tsunamis generated by earthquakes, but usually with lower energies.


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The development of these Meteotsunamis depends largely on the direction,

intensity of air pressure and ocean depth. Meteotsunamis are regional, and it is

found in some part of the world frequently due to regional factors such as

topography of earth’s surface both below and above the ocean.

Example – 2013 New Jersey, USA Tsunami caused by a high-speed windstorm

associated with thunderstorms.

Nuclear Weapon or tests – it is an example of man-made disaster. Massive

explosions created by a nuclear weapon of nuclear tests have the potential to cause

Tsunami. There have been dangers of using this as a tectonic weapon.

There has been considerable speculation on the possibility of using nuclear

weapons to cause tsunamis near an enemy coastline. In fact, In World War II,

the New Zealand Military Forces, in a failed attempt, tried to create small tsunamis

with explosives.

Effects of Tsunami

After the tsunamis reach the coast, an enormous energy stored in them is released

which causes colossal loss of lives as well as the infrastructure of the place. As the

port cities are economic hubs and densely populated the damage caused by the

tsunami is devastating.

The Tusanami of 2004 in the Indian Ocean is one of the devastating natural

disasters in the modern time. It took a toll of nearly 230000 people leaving in the

coasts of Indian Ocean.

Unfortunately escaping a tsunami is nearly impossible. Hundreds and thousands

of people are killed by tsunamis, most commonly by drowning, electrocution,

explosions from gas and collapsing of buildings etc.

Flooding and contamination of drinking water can cause disease such as Malaria

to spread in the tsunami-hit areas.

Tsunamis not only destroy human life, but also have a devastating effect on

animal and plant life and other natural resources. A tsunami changes the

landscape. It uproots trees and plants and destroys animal habitats.


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Contamination of soil and water is the second key environmental impact of a

tsunami.

There may be nuclear pollution due to radiation resulting from damaged nuclear

plants, as it happened in Fukushima, Japan in March 2011.

Tsunamis are extremely dangerous to coastal life and coastal property. They

produce unusually strong currents, rapidly flooding the land and causing great

damage to coastal property and life.

The flow and force of the water and the debris it carries can destroy boats,

vehicles, and buildings and other structures as the tsunami moves across the

land.

Victims of tsunami events often suffer psychological problems such as PTSD (Post

Traumatic Stress Disorder) which can last for days, years or an entire lifetime.

Massive economic costs hit communities and nations when a tsunami happens.,

severely affecting the economy of the nation.

Poor nations are more prone to large-scale destruction as the infrastructure are

not well developed, and warning systems are not robust or unavailable. Also,

their ability to cope with such massive disaster remains inadequate.

The water can be just as threatening (if not more so) as it returns to the sea,

taking debris and people with it. Flooding and dangerous currents can last for

days.

Tsunami Early Warning System

Tsunami is the most unpredictable natural disaster in the world and to prevent the

Tsunami is next to impossible. Hence, the only way to effectively mitigate the

impact of a tsunami is through an early warning system.

Tsunamis are detected in advance using a tsunami warning system (TWS) and early

warnings are issued to safeguard the life of people. It is made up of two equally

important components: a network of sensors to detect tsunamis and a

communications infrastructure to issue timely alarms to permit evacuation of the

coastal areas.


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There are many regional and international early warning systems installed all across

the globe. National governments warn citizens through a variety of means,

including SMS messages, radio and television broadcasts, and sirens from

dedicated platforms, mosque loudspeakers and police vehicles with loudspeakers.

India had volunteered to join the International Tsunami Warning System after the

December 2004 tsunami disaster. The Indian Tsunami Early Warning Centre

(ITEWC) embedded with specific systems called Deep Ocean Assessment and

Reporting of Tsunamis (DART), established in 2007 at Indian National Centre for

Ocean Information Sciences, (INCOIS – ESSO) Hyderabad, autonomous body under

Ministry of Earth Sciences, is up and running to provide tsunami advisories for the

events occurring in the global oceans.

It has been recognized as one of the best systems in the world. The ITEWC includes

a real-time seismic monitoring network of seventeen broadband seismic stations

to detect tsunamigenic earthquakes and to provide timely warnings to the

vulnerable community. It also receives earthquake data from all other global

networks to detect earthquakes (of M>6.5).

Since its inception in October 2007, so far ITEWC has monitored 339 earthquakes

of M > 6.5. ITEWC also acts as one of the Regional Tsunami advisory Service

Provider (RTSP) along with Australia & Indonesia for the Indian Ocean region.

Way forward and recommendation

2004 Indian Ocean Tsunami was a brutal reminder of disaster preparedness in

India. While the current early warning system in India is state of the art, it is still

inadequate in terms of preparedness for Tsunami. Following suggestions can be

observed for enhancing India’s preparedness for future Tsunami events:-

Adopting integrated multi-hazard approach with emphasis on cyclone and

tsunami risk mitigation in coastal areas

Strict implementation of the coastal zone regulations

Plantation of mangroves and coastal forests along the coastline


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Identification of vulnerable structures and appropriate retrofitting for tsunami

resistance of all such buildings as well as appropriate planning, designing,

construction of new facilities

Capacity building programmes and public awareness campaigns should be held

at Tsunami prone areas

Streamlining the relief distribution system and evacuation plans in Tsunami

prone areas

Component of planning for reconstruction and rehabilitation should be added to

disaster management plans at all levels

Emphasis on mental health and to socio-psychological issues during post-disaster

period should be accorded in every plan

Conclusion

Tsunami is one of the most hazardous and unpredictable natural force. Tsunamis

have no seasons and they can occur at anytime and anywhere. We certainly cannot

prevent Tsunami. But what we can do is take necessary steps to minimize the

damage caused by it. Tsunami is a global and transnational event. Hence, it is

important that all countries across the world should join hands to evolve new

scientific ways to predict Tsunamis and to design mitigation strategies to cope with

this disastrous force.


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14. Volcanism

The volcano is a narrow opening in the earth’s crust through which the molten rock

material, magma (lava), volcanic ashes are emitted outward through an eruption.

Such types of openings (vents) are found in those parts of the crust where the rock

strata are relatively weaker than the surrounding areas.

Volcanism

Volcanism refers to an exogenous activity which includes the formation of magma,

its upward movement, ejection of magma (lava) on the earth’s surface, and its

cooling and solidification.

volcano diagram 1 – Structure of Volcano

Magma refers to the molten rock material present inside the earth in the

asthenosphere. When this molten material comes out to the Earth’s surface

through an opening of a volcano, it is known as the Lava. The process through which

this molten material or magma comes out from asthenosphere to the Earth’s

surface is known as volcanism.


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Types of volcanic eruptions

Ejection of Lava to the surface occurs through either Fissure eruption or through

Central eruption.

Fissure eruption

In Fissure volcanic eruptions, the Lava comes out to the surface through the cracks

of the rock strata and hence the fissure eruptions are not much explosive. The

fissure eruptions are smooth and the Lava spreads to a larger area, so they form

landscapes such as plateaus etc.

volcano diagram 2 – Lava flow

Central eruption

In the central volcanic eruptions, the lava comes out to the surface through narrow

pipes and thus causes an explosion, during the ejection of magma onto the surface.

The explosive nature of eruptions leads to the formation of mountains which are

known as volcanic mountains. The different volcanic Islands throughout the world

are actually volcanic mountains formed through Central eruptions.


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Causes of volcanic eruptions

In the interior of the earth, the radioactive substances undergo chemical

reactions which generate a large amount of heat. Apart from this, some amount

of residual heat which was captured at the centre of the earth during its

formation is also present. This leads to a creation of large temperature difference

between the inner and Outer layers of earth.

This huge temperature difference leads to the formation of convection currents

in the outer Core and the Mantle. Due to this, the molten magma along with the

gaseous materials comes out to the earth’s surface at the first available

opportunity. This mainly occurs in the weak zones of earth surface such as

divergent plate boundaries, and convergent plate boundaries etc.

Sometimes, the earthquakes may expose the fault zones in the rock strata

through which the Magma can escape to the earth’s surface leading to volcanic

eruptions.

Types of Lava in volcanism

Acidic or Andesitic or composite Lava

The acidic or composite Lava is highly viscous and has a high melting point. It has

a high percentage of silica content, low density and light colour.

The acidic lava flows slowly and they rarely travel far before solidification. This

leads to the formation of the cone-like structure having steep sides.

Due to the rapid solidification of this acidic Lava, the openings obstruct the flow

of new Lava, which results in loud explosions and pyroclasts (volcanic bombs).


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volcano diagram 3 – Cone-like Structure

Basic or Shield or Basaltic Lava

The basaltic or basic lava is highly fluid, and their temperature is about 1,000°C.

Basaltic lava is poor in silica, but are rich in Iron and manganese.

They have a dark colour and high fluidity. Due to their high fluidity, the basaltic

Lava is not very explosive, and they spread over great distances as thin sheets of

Lava.

The volcano formed by Basic Lava is gently sloping and they form a flattened

shield or dome with a wide diameter.

Active, dormant and Extinct volcanoes

The volcanoes erupting fairly frequently are known as active volcanoes. Kilauea

volcano of Hawaii, Grímsvötn volcano of Iceland and Etna volcano in Italy etc are

examples of active volcanoes which have been volcanoes erupting in the recent

past.


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Those volcanoes in which the eruption has not taken place regularly in the recent

past are known as dormant volcanoes. The volcanoes erupting after undergoing

long intervals of repose are the dormant volcanoes. The Fujiyama volcano of

Japan, Krakatoa volcano of Indonesia and the Narcondam island volcano of

Andaman and Nicobar islands are the examples of dormant volcanoes.

Extinct volcanoes are those where the volcanic eruption had taken place in

historic times but they are not active today. Mount Chimborazo in Ecuador,

Mount Kenya in Eastern Africa, and Popa in Myanmar etc. are the examples of

extinct volcanoes.

Negative effects of volcanic eruptions

Volcanic eruptions are a highly damaging natural disaster and are highly

destructive in nature. volcanic eruptions have been responsible for the

destruction of whole cities and towns by the advancing lava.

Violent earthquakes are associated with volcanic eruptions which have often

caused damage to life and property. The mudflows of volcanic ashes which get

saturated by rainfall can bury the nearby areas.

The earthquakes are associated with volcanism and in coastal areas, they can

cause tsunamis which have often caused the large destruction of life and

property.

Different gases released from volcanic eruptions such as carbon dioxide,

hydrogen fluoride, Sulphur Dioxide etc are hazardous to human life and

environment. The volcanic gases such as Sulphur Dioxide etc have also been

responsible for causing acid rain.

Large volcanic eruptions inject a large number of Sulphur aerosols in the

stratosphere which can lead to the lowering of surface temperature and increase

in the depletion of Ozone layer. The release of SO2 from volcanic eruptions has

been responsible for lowering of earth’s temperature leading to crop failures and

famines.


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Positive impacts of the volcanic eruption

Volcanic eruptions are responsible for the formation of new landforms such as

islands, plateaus, Volcanic Islands, and mountains etc. The volcanic lava, ash and

dust are very fertile for the cultivation of different plants. The weathering of

Volcanic rocks leads to the formation of fertile soil.

volcanic eruptions are also the source of mineral resources. They bring useful and

important minerals resources to the surface of Earth. For example, the diamond

mines of the kimberlite rocks of South Africa, are actually the part of an ancient

volcano.

The areas surrounded by the active volcanoes give rise to the formation of springs

and geysers. These springs and geysers can even be used for the generation of

geothermal electricity. The Yellowstone National Park of USA generates

electricity from the geothermal electricity. The Puga Valley of Ladakh in India is

also a promising spot for geothermal electricity.

The landforms formed by the volcanic eruptions are also great tourist spots and

have a great natural beauty. For example, the Yellowstone National Park of USA

is a great tourist spot.

Apart from these, Volcanic rocks are also used as raw materials for various

building and Engineering purposes etc.

Hot springs and Geysers

When the underground water which percolates down through the porous rocks

is subjected to the heat of the underlying Rock Strata which is in the contact of

hot magma it gives rise to geysers and hot springs.

When the water comes in contact with the intense heat of these rocks, it gets

heated and rises in the form of capillaries and narrow roots through the porous

rocks. When this heated water comes to the surface it undergoes expansion and

gets converted into steam leading to the formation of geysers and Springs.


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Diagram- Geyser

Geysers: when the heated water at high pressure comes out of the surface and

bursts into steam, it is known as Geysers. In most of the cases, a carter like

structure is formed at its mouth.

Springs: When the hot water comes out to the surface in a smooth manner it is

known as a spring.

Most of the world’s geysers are found in the areas of Iceland, New Zealand and

the Yellowstone National Park of USA. The hot springs and geysers of Japan and

Hawaii are great tourist attractions.

Geysers are found in very few regions, while the hot water springs are found all

over the world.


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volcano diagram – Volcanic_hot_spring

Distribution of the volcanoes around the world

Till now, around 480 major active volcanoes have been found out of which

around 400 are found in the areas around the Pacific ocean. While the others are

in the Alpine Himalayan belt, Atlantic Ocean, Indian ocean etc. The Himalayas do

not have an active volcano.

The converging plate margins and the mid-oceanic ridges are the areas of high

volcanic activity and earthquakes. The volcanic zones and earthquake zones are

more prominent around the converging plate boundaries.

Pacific Ring of Fire

The circum Pacific region or Pacific Ring of Fire has the largest concentration of

active volcanoes. It has almost two-thirds of active volcanoes.


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The Aleutian islands of Kamchatka, Japan, the areas of Philippines, Indonesia,

Islands of Solomon, Tonga and North Island, New Zealand, the Andes to Central

America and up to Alaska are the part of Pacific rim of fire.

volcano diagram – volcanoes in pacific rim

volcanoes along the Atlantic coast

The Atlantic coast has a comparatively fewer number of active volcanoes. But it

has many dormant volcanoes such as Saint Helena, Cape Verde islands etc. The

volcanoes of Iceland and Azores along the Atlantic coast are active volcanoes.

Volcanoes in the Mediterranean region

The Alpine folds, such as Vesuvius, stromboli (also known as the Lighthouse of

Mediterranean) and the Aegean Islands are the areas of the Mediterranean

region where active volcanoes are found.


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Volcanoes in the great rift region

Mount Kilimanjaro and Mount Kenya of the East African Rift Valley have some

extinct volcanoes. Mount Cameroon is the only volcano active in West Africa.

Volcanoes in other parts of the world

Other regions such as West Indian Islands have experienced some volcanic activity

in the recent past. Mount Pelee of the Lesser Antilles is a volcanic Island where the

last eruption took place in 1929.

Volcanoes in India

The Barren Island of Andaman and Nicobar Islands which is in the northeast of

Port Blair is a volcanic island. The Barren Island volcano was the last active

recently in 2017 and in 1991 and 1995.

Narcondam which is in the north-east of Barren Island is another volcanic Island

in India. Narcondam volcano has not been active in the recent past. Other parts

of India do not have an active volcano.

Distribution of earthquakes

The distribution of earthquakes in the world coincides closely with that of

volcanoes. The Circum Pacific area, along with the Pacific Ring of Fire, is the

region of greatest systemic activity with most frequent occurrences of

earthquakes. These areas also have the most number of volcanic activity and

Volcanic Islands. Around 70% of world’s total earthquakes take place in the

Circum Pacific belt.

Around 20% of earthquakes take place in the Mediterranean Himalayan belt

which includes the Asia Minor, the Himalayas and the parts of northwestern

China. The earth’s crust is relatively stable in other parts of the world and they

are less prone to the threat of earthquakes. However, no place in the world can

be completely immune to the earthquakes.


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Some important volcanic eruptions

Mount Vesuvius in Italy

Mount Vesuvius is a composite volcano, which is around 4000 feet above the bay

of Naples. The volcano erupted in AD 79 and buried the city of Pompeii located

in the Southwest of this volcano. The volcanic ashes and the torrential rainfall

afterwards buried the city and killed its inhabitants.

Since the city was buried with the volcanic ashes, it was good infertility for the

cultivation of crops. This tempted farmers to begin a settled life on the slopes of

Mount Vesuvius. This volcano again erupted in December 1631 and destroyed

around 15 towns and killed their inhabitants.

Mount Krakatoa in the Sunda Strait

In August 1883, Mount Krakatoa located in the centre of the strait, between Java

and Sumatra, exploded violently; this was one of the greatest volcanic explosion

known to the mankind.

The Krakatau island was not inhabited, so nobody was killed due to the lava flows

and ashes. However, it set up the Tsunami waves of over hundred feet high which

submerged the coastal areas of Indonesia and drowned around 36000 people.


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Mount Tambora, Indonesia in 1815

volcano diagram -Mount_Tambora_Volcano

It is one of the deadliest volcanic eruption in the recent human history which was

responsible for the death of around 120,000 people. The volcano erupted on 10

April 1815 and was the most powerful volcanic eruption in the last 500 years. It

sent volcanic ashes and gases like SO2 in the sky. It also led to the creation a series

of Tsunami waves.

Due to the emission of large amount of SO2, the world experienced large

temperature drop which was responsible for the crop failures in various parts of

the world. Thousands of people died due to starvation in China, while the price

of Grains quadrupled in Switzerland after 2 years of the volcanic eruption.


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15. Volcanic Landforms – Extrusive,

Intrusive

The landforms formed due to the solidification of lava either inside or outside the

earth surface are known as volcanic landforms.

Extrusive and Intrusive Landforms

The landforms formed by the action of volcanoes and volcanic eruptions are

known as volcanic landforms. The geological processes control the characteristics

of various volcanic landforms. On the basis of cooling of magma, the volcanic

landforms are divided into extrusive igneous rocks landforms and intrusive

igneous landforms.

Plutonic rocks are formed when the magma cools within the earth’s crust.

The extrusive igneous rocks are formed when the cooling of Lava occurs above

the Earth’s surface.

Extrusive igneous rocks landforms

When the Lava and other volcanic materials are thrown out to the Earth’s surface

during volcanic eruptions, the extrusive igneous landforms are formed. It includes

volcanic Lava, pyroclastic debris, ash, volcanic bombs, and gases such as Sulphur

dioxide, nitrogen compounds and other gases.

The conical vent and fissure vent

The narrow cylindrical vent through which the lava flows out to the earth’s crust

during a volcanic activity is known as a conical vent. Conical vents are more

common in the composite (or strato volcanic) volcanic features.

The fissure is a narrow linear vent through which the lava comes out to the

earth’s crust during a volcanic eruption. The fissure vents are more commonly

found in the areas of basaltic volcanism.The fissure vents are often few meters

wide, which can be several kilometres long.


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Shield volcanoes

Shield volcanoes are characterized by gentle upper slopes and a little steeper

lower slopes. They are composed of relatively fluid lava flows which have been

built over a central vent. Mostly, the low viscosity basaltic lava which is high in

fluidity form Shield volcanoes. It leads to the formation of the extrusive igneous

rocks.

Shield volcanoes are mostly non-explosive, but they can become explosive if

water gets inside the vent.

Shield volcanoes are the largest volcanoes in the world. They extend to greater

heights and distances. Examples of Shield volcanic landforms include Mauna Loa

volcanoes of Hawaii.

Shield volcano

Cinder cone volcanoes


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A Cinder cone has the features of a steep conical hill with loose pyroclastic

fragments which include volcanic clinkers, cinder, volcanic ash (scoria) around

the vent.

Cinder cone volcanoes are made entirely of the loose grainy cinders, and they

lack lava. Cinder cone usually has very steep sides along with a small crater on its

top. They are small volcanoes.

Cinder_cone_diagram

Composite volcanoes

Composite volcanoes (strato-volcanoes) are mainly cone shaped with moderate

steep Sites. The andesitic lava, along with the pyroclastic materials and ashes

which find their way to the ground gets accumulated in the vicinity of vent

openings. This leads to the formation of layers, which makes the volcanic mounts

appear as composite volcanoes.


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Composite volcanoes are also known as stratovolcanoes. Most common and

highest volcanoes have the features of composite cones. For example, Stromboli,

the Lighthouse of Mediterranean, mount Fuji etc.

Composite volcanoes are associated with the eruption of a cooler and more

viscous lava than the basaltic lavas. The composite volcanoes often cause

explosive volcanic eruptions.

Composite volcano

Flood basalt provinces (Lava plateaus)

When a very thin and fluid lava comes out to the Earth’s surface, and flow after

intervals for long periods of time, spreading to a large area; it produces a layered,

undulating- wave-like flat surfaces.

These types of extrusive igneous rocks and landforms are known as flood basalt

landforms or Lava plateaus. The Deccan traps of India, Snake basin of USA,

Canadian Shield etc. are the examples of Flood basalt provinces.


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Mid-oceanic ridge volcanoes

The mid-oceanic ridges occur in the underwater oceans. There is a system of

70000 km long mid-oceanic ridges that stretch along all the major ocean basins.

The central portion of the mid-oceanic ridges is associated with frequent volcanic

eruptions.

The lava which comes out through these eruptions are Basaltic and have less silica

content, so they are less viscous. Due to less viscosity, they flow through longer

distances and cool slowly. This outpour of lava through volcanic eruptions is

responsible for the phenomenon of seafloor spreading.

mid-oceanic ridge

Caldera lake

When the Lava ceases to flow after the volcanic eruption, the creator of

volcanoes turns into a lake, which is known as Caldera lake. The rainwater and

snowmelt often get accumulated in these enclosed depressions leading to the

formation of lakes.


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Lonar in Maharashtra, Krakatoa in Indonesia, and Lake Caldera in southern

Oregon etc. are the examples of Caldera lakes.

Caldera lake

Intrusive volcanic landforms

The intrusive igneous rocks or plutonic rocks are formed when the Magma cools

within the earth’s crust and does not erupt to the surface. Various forms of

intrusive igneous rocks are formed due to the intrusive activity of volcanoes.


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Intrusive landforms

Batholiths

Batholiths are the intrusive igneous rocks masses formed due to the cooling and

the solidification of Magma inside the earth. These intrusive igneous rocks appear

on the surface after the erosional process erode the materials lying above these

rocks.

The batholiths form the core of large mountains, and they get exposed to the

surface after the erosional activities. Batholiths are granitic intrusive igneous

rocks.

Laccoliths

Laccoliths are the large dome-shaped intrusive igneous rocks which are

connected by a pipe-like conduit with the magma.

These intrusive igneous rocks resemble like a composite volcano structure, but

they are found below the earth’s surface. Example Karnataka plateau.


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Lopolith

Lopolith is formed when the Magma moves upwards, and a portion of this magma

moves in a horizontal direction where it finds a weak plane. When it develops into

saucer shape, it is known as Lopolith.

Phacolith

When a wavy mass of intrusive igneous rocks are formed at the base of synclines

or on the top of anticline having a definite conduit with the magma chambers

below, they are called the laccoliths.

Sills

Sills are the intrusive igneous rocks which are formed by the solidified and near

horizontal lava layers inside the earth. The thinner deposits of these rocks are called

sheets, while the thicker horizontal deposits are known as sills.

Dykes

When the Magma moves upwards through the cracks and fissures, and solidifies

almost perpendicularly to the earth’s surface, developing a wall like structure, they

are known as dykes. Dykes are the most common intrusive igneous rocks in

Western Maharashtra and other parts of Deccan traps.


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16. Volcanism Types Based on Out Flow of

Lava

On the basis of outflow of lava, volcanism can be classified into four types-

Exhalative, Effusive, Explosive and subaqueous Volcanism.

Volcanism Types Based on Outflow of Lava

Exhaltive volcanism – vapour or fumes

Exhalative volcanism is characterized by the discharge of materials in the gaseous

form. It includes gases like steam, Sulphur Dioxide, carbon dioxide, carbon

monoxide, hydrogen, nitrogen, hydrogen sulphide, hydrochloric acid etc.

These gases can escape to the earth’s atmosphere from the vents which can be

in the form of hot Spring, geyser, solfataras, and fumaroles etc. The landforms

associated with exhaustive volcanism are sinter mounds, mud volcanoes and the

cones of the of the precipitated minerals etc.

Fumaroles

Effusive Volcanism – Lava outpouring


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When the outflowing lava has low viscosity and high fluidity, various gases

coming out of the volcano can escape easily. The erupting magma forms lava

flows, and these eruptions are known as effusive volcanic eruptions.

Erupting Lava is poor in silica content such as basalt, and they flow through very

large distances. The Deccan traps are formed by the effusive outpouring of Lava.

Many parts of the Deccan trap has developed into the finely grained basalt

plateau.

Lava outpouring

Explosive volcanism – violent ejection of the solid material

If the Magma has a large amount of gases which get trapped inside it, the

pressure increases and builds until the magma erupts explosively out of the vent.


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The explosive eruptions can produce pyroclastic flows sweeping down the

valleys, which can destroy everything which comes under their path.

Explosive volcanism leads to the fragmentation and ejection of the solid materials

through the volcanoes. The various materials coming out of the vents of an

explosive volcanism are

Tephra, which includes all the fragmented rejects coming out of the

volcanoes.

Volcanic ashes, which are the finest and sand-sized tephra.

The gravel-sized particles which are in the molten or in the solid state are

known as Lapilli.

The Boulder-sized solid ejects are known as blocks.

When the lumps of Lava are thrown out of the volcano, they are known as

volcanic bombs.

The layers of the volcanic dust and ashes are known as tuff.

The lighter and smaller particles like the lapilli and ash can travel for large

distances and can remain suspended in the atmosphere for longer periods of

time. Volcanic bombs and blocks, which are heavier in weight fall near to the

vents of the volcanoes.


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Explosive eruption

Subaqueous volcanism

Subaqueous volcanism takes place in the underwater oceans and seas, and other

water bodies. In the underwater ocean floors, the outflowing lava comes in the

contact of water and consolidates to produce structures, which are like the heap

of pillows. These types of structures belonging to the Precambrian age have been

found in the Karnataka region.

When the outflowing lava is highly viscous and erupts at lesser depths, they

develop a structure of glassy margins on the pillows leading to the formation of

hyaloclastite. Hyaloclastites are most commonly found in Iceland.


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pillow lava

Negative effects of Volcanism

Volcanic eruptions release gases like Sulphur dioxide, carbon dioxide, carbon

monoxide etc in the atmosphere. These gases are hazardous for the people,

agriculture, and environment.

The sulphur dioxide released from volcanic eruptions can cause acid rain which is

harmful to humans as well as biodiversity and environment. The release of

Sulphur aerosols in the stratosphere can lower the surface temperature leading

to the depletion of Ozone layer.


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Volcanic_injection

Classification of volcanism based on the mode of eruptions

Hawaiian eruption: in the Hawaiian eruption, the basalt lava comes out in the

form of effusive outpouring from the craters or fissures. In a single flow, the lava

spreads over wide areas, or it flows down the valley is like lava rivers. The great

basalt plateaus of Colombia is its example.

Strombolian eruption: in the Strombolian eruption, more viscous lava has

erupted like a fountain at the regular intervals of about 15 minutes. The

Stromboli volcano is located on the Lipari island near Italy.

Vulcanian Eruption: this volcanic eruption is associated with a short, violent and

a relatively smaller eruption of viscous lava. During volcanic eruptions, volcanic

bombs, blocks and ashes etc are ejected in the surrounding areas. The volcano


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becomes dormant for several decades and even for centuries after each eruption

cycle.

Pelean eruption: it occurs as a result of the flow of very viscous, gas-rich and

acidic lava erupting violently over the crater rim. The lava and gases do not move

upwards toward the sky but spread downslopes as a nuee ardente.

Icelandic type eruption: it is characterized by the outflow of molten basaltic lava

flowing from the long and parallel fissures and often leading to the formation of

Lava plateaus.


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Counterclockwise: a Plinian eruption column, Hawaiian pahoehoe flows, and a

lava arc from a Strombolian eruption.


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17. Pseudo Volcanic features

Those topographic features that resemble the volcanic landforms but are non-

volcanic in origin are known as pseudo volcanic features. The pseudo volcanic

features include meteorite crater, mud volcanoes, and salt plugs.

Pseudo Volcanic features

Craters

A crater is a depression usually circular formed by either the extrusion of volcanic

material or by the impact of any meteorite.

A rootless cone is a pseudo creator which resembles a real volcanic crater, but it

is not of volcanic origin. It does not have an actual volcanic vent from which the

lava has erupted. The rootless cone is characterized by the absence of the magma

conduit which connects the crater to the Magma chamber below the Earth’s

surface.

The meteorite craters are formed by the impact of falling meteorites from the

space on earth. They resemble like a crater lake; for example, the Lonar lake in

Buldhana is an example of meteorite crater lake which has been formed as a

result of the impact of a giant meteorite. Other examples of meteorite craters

include Siberian crater, Shiva crater on the Mumbai Offshore basin etc.

Apart from these, the craters developed due to the anthropogenic activities are

also referred as pseudo volcanic features. It includes the craters formed by the

explosion of bombs, mine blasts, etc. which resemble the features of a crater.


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Lonar_Meteorite_Crater

Salt plugs or salt dome

The salt plug is a dome-like structure formed by the stratified rocks which contain

the central core of salt, which has been formed by the upward movement of the

salt deposits. Under high pressure, the salt deposits deform plastically and

deform and pierce the overlying sediments like an intrusive landform.

Salt extrusions can take the form of salt hills having many features of plug domes

or lava cones having peaks and sinkholes, which are visually similar to the volcanic

craters formed due to subsidence.


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Salt_Dome

Mud volcanoes

A mud volcano is a landform formed by the eruption of mud, water, and gases

etc. Mud volcanoes can be formed due to several geological processes. The mud

volcanoes are not the true volcanoes as they do not produce Lava and are not

necessarily driven by the activities of magma. Thus they can be classified under

the pseudo volcanic features.

The Mud volcanoes have usually been found in the subduction zones around the

world. They mostly release methane gas along with smaller quantities of Nitrogen

and carbon dioxide etc.

Some mud volcanoes are non-volcanic in origin. The volatile hydrocarbons which

have been given off from the petroleum-bearing beds can cause mud eruptions,

which can be called as mud volcanoes. These kinds of mud volcanoes have been

found at Baku along with the Caspian sea, in Burma, and in southern Balochistan

etc.


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Mud_Volcano


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18. Hotspot Volcanism

A volcano is a rupture in the crust of the Earth from where gases, ashes and molten

rock material – magma – escape to the ground. A volcano is called an active volcano

if the materials mentioned are being released or have been released out in the

recent past. Pacific Ring of Fire has one of the maximum numbers of active

volcanoes in the world. Most of the volcanoes are found underwater.

Hotspot Volcanism

Volcanoes are found where tectonic plates are diverging or converging. The vast

majority of earthquakes and volcanic eruptions occur near plate boundaries or

margins, but there are some exceptions. For example, the Hawaiian Islands, which

are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more

than 3,200 km from the nearest plate boundary.

Hotspot Volcanism refers to this intra-plate volcanism, which describes a volcanic

activity that occurs within tectonic plates. The position of these hotspots on the

Earth’s surface is independent of tectonic plate boundaries.

Hotspot volcanism is unique because of its occurrence. It does not occur at the

boundaries of Earth’s tectonic plates, where all major volcanic activity takes place.

Instead, it occurs within the plates at abnormally hot centres known as mantle

plumes.


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Hotspot Volcano

What is a Hotspot?

“Hotspot” refers to an area in the Earth’s mantle from where hot plumes rise

upward, forming volcanoes on the overlying crust.

A hotspot is fed by a region deep within the Earth’s mantle from where these

mantle plumes rise through the process of convection.

The heat from mantle plumes facilitates the melting of rock at the base of

the lithosphere, where the brittle, upper portion of the mantle meets the

Earth’s crust.

High heat and lower pressure at the base of the lithosphere (tectonic plate)

facilitates melting of the rock.

This molten material (rock), called magma, rises through cracks in the crust and

erupts to form volcanoes.

Hot-spots are relatively fixed in comparison to the plates

As the tectonic plate moves over the stationary hot spot, the volcanoes are rafted

away and new ones form in their place, resulting in chains of volcanoes, such as

the Hawaiian Islands.


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Mantle Plumes

Hotspot volcanism occurs at abnormally hot centres known as the mantle plume

A mantle plume is an upwelling of abnormally hot rock within the Earth’s mantle,

first proposed by Tuzo Wilson in 1963.

In 1971, geophysicist Jason Morgan further developed the hypothesis of mantle

plumes. In this hypothesis, convection in the mantle transports heat from the

core to the Earth’s surface in thermal diapirs.

These mantle plumes are almost like lava lamps, with a rising bulbous head fed

by a long, narrow tail that originates in the mantle.

As the plume head reaches the lithosphere, it spreads into a mushroom shape

that reaches roughly 500 to 1000 kilometres in diameter. These features are

called diapirs.

When the head of a plume encounters the base of the lithosphere, it undergoes

widespread decompression. As a result, melting takes place, and large volumes

of basalt magma are formed which, finds its way to the earth surface when an

explosion takes place.

Layers of Earth


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Mantle Plume

Distribution of Hotspots

Because of differing definitions of what a hot spot, there is also diverging opinions

about the numbers of hotspots in the world. Forty to fifty hotspots are thought to

exist around the world, although this number varies greatly. Major hot spots in the

world include the Iceland hotspot, under the island of Iceland in the North Atlantic,

the Reunion hotspot, under the island of Reunion in the Indian Ocean etc.


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Distribution of Hotspot across the globe

Wilson’s hotspot theory

“Hotspot” theory was given by J. Tuzo Wilson, the Canadian geophysicist in 1963.

Wilson in his study found that in certain locations around the world, such as

Hawaii, volcanism has been active for very long periods of time.

Based on this he gave the idea of “Hotspots” referring to small, long-lasting, and

exceptionally hot regions which existed below the plates and provided a localized

source of high heat energy (mantle plumes) to sustain volcanism.

This led to a new hypothesis by Wilson that the distinctive linear shape of the

Emperor Seamount chain in the Hawaiian Islands resulted from the movement of

the Pacific Plate over a deep, stationary hotspot in the mantle, located below the

current position of the Hawaiian Island.

Heat emerging from this hotspot produced a perennial source of magma by partly

melting the overriding Pacific Plate.

The magma, being lighter than the surrounding solid rock rises through the

interior of the earth to erupt onto the seafloor, forming an active seamount.


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Over time, a large number of eruptions cause the seamount to grow until it finally

emerges above sea level forming an island volcano.

He suggested that continuing plate movement eventually carries the island

beyond the hotspot. Hence the source of magma is cutoff and volcanism ceases.

As one island volcano cease to exist, another develops over the hotspot, and the

cycle is repeated.

This process growth and death of volcano, over many millions of years, has left a

long trail of volcanic islands and seamounts across the Pacific Ocean floor.

According to Wilson’s hotspot theory, the farther the volcanoes of the Hawaiian

chain travel beyond the hotspot, the older and more eroded they get.

Cycle of Volcanism

Interestingly, a volcano located above a hot spot does not erupt forever.


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Attached to the tectonic plate below, the volcano moves and is eventually cut off

from the hot spot.

Without any source of heat, the volcano becomes extinct and cools. The plate

beneath the volcano (and above the hot spot) also cool.

This cooling causes the rock of the volcano and the tectonic plate to become

denser

The volcano and the plate gradually subside as they move away from the hot

spot.

As the volcano subsides below sea-level, the top is eroded flat by waves.

In time, new and active volcanoes are developed over the hot spot, and this cycle

of volcanism goes on.

Even giant volcanoes, like Mauna Loa on Hawaii, will eventually disappear into

the ocean.

Hotspot Features

Seamounts – Volcanic activity at hot spots can create submarine mountains

known as seamounts. Depending on the amount of volcanic activity, seamounts

can rise hundreds or thousands of meters from the seafloor.

Chain of Islands – Hotspot seamounts that reach the surface of the water can

create entire chains of islands, such as the U.S. state of Hawaii.

Hot spots can also develop beneath continents, for example, The Yellowstone

hotspot, U.S.A

Geysers – Hotspots don’t always create volcanoes that spew rivers of lava.

Sometimes, the water and steam have erupted like a volcano from within the

earth surface due to heating up of the groundwater by the magma. These

eruptions are called geysers. A famous geyser is Old Faithful in Yellowstone

National Park.


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Hotspot tracks – While a plume that feeds hot spot volcanoes remains stationary

relative to the mantle, the plate above it usually moves. The result is that a chain

of progressively older volcanoes is created on the overlying plate. Pacific ocean

has some of the best examples of such “hot spot tracks”.

Reunion Hotspot

The Reunion hotspot is a volcanic hotspot

Presently, it’s lying under the Island of Reunion in the Indian Ocean.

The hotspot is believed to have been active for over 66 million years.

About 66-68 million years ago present-day India was above the hot spot

Deccan traps, a vast bed of basalt lava that covers part of central India is thought

to have been formed by a huge eruption of this hotspot 66 million years ago.

The Laccadive Islands, the Maldives, and the Chagos

Archipelago are atolls, resting on tracks created by Reunion Hotspot.

As the plate moved in the northeastern direction more volcanic centres were

formed: the Mauritius Islands from 18-28 million years ago, the Mascarene

Plateau 40 million years ago, the Chagos Ridge 48 million years ago and the

Maldives from 55-60 million years ago.

The youngest volcanoes, Piton de la Fournaise and Piton des Neiges, were formed

in the last 5 million years. Piton de la Fournaise is one of the most active

volcanoes on the Earth surface.


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19. Fluvial Depositional Landforms

In geography, fluvial processes are associated with rivers and streams and the

deposits and landforms created by them. Landforms are small to medium tracts or

parcels of the earth’s surface.

There are two types of landforms created by the fluvial process – Fluvial Erosional

Landforms and Fluvial Depositional Landforms. Fluvial Depositional landforms are

made by river sediments brought down by extensive erosion in the upper course of

the rivers.

Fluvial Depositional Landforms

Rocks and cliffs are continually weathered and eroded in the youth stage or upper

course of the river.

The river moving downstream on a level plain brings down a heavy load of

sediments from the upper course.

The decrease in stream velocity in the lower course of the river reduces the

transporting power of the streams which leads to deposition of this sediment

load.

Coarser materials are dropped first and finer silt is carried down towards the

mouth of the river

This depositional process leads to the formation of various depositional

landforms through fluvial action such as Delta, Levees and Flood Plain etc.

Alluvial Fans

An alluvial fan is a cone-shaped depositional landform built up by streams, heavy

with sediment load.

Alluvial fans are formed when streams flowing from mountains break into foot

slope plains of low gradient.


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Normally very coarse load is carried by streams flowing over mountain slopes.

This load gets dumped as it becomes too heavy to be carried over gentler

gradients by the streams

Furthermore, this load spreads as a broad low to a high cone-shaped deposit

called an alluvial fan that appears as a series of continuous fans.

Alluvial fans in humid areas show normally low cones with a gentle slope from

head to toe and they appear as high cones with a steep slope in arid and semi-

arid climates.


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An alluvial fan at the mouth of Copper Canyon, Death Valley, California

Floodplains

Floodplain is a major landform of river deposition.

Deposition develops a floodplain just as erosion makes valleys.

Rivers in the lower course carry large quantities of sediments

Large sized materials are deposited first when stream channel breaks into a

gentle slope.

Sand, silt and clay and other fine sized sediments are carried over gentler

channels by relatively slow-moving waters

During annual or sporadic floods, these materials are spread over the low lying

adjacent areas. A layer of sediments is thus deposited during each flood,

gradually building up a floodplain

In plains, channels shift laterally and change their courses occasionally leaving

cut-off courses which get filled up gradually by relatively coarse deposits.

The flood deposits of spilt waters carry relatively finer materials like silt and clay.

Active Floodplain – A riverbed made of river deposits is the active floodplain.

Inactive Floodplain – The floodplain above the bank is an inactive floodplain.

Inactive floodplain above the banks basically contains two types of deposits —

flood deposits and channel deposits.

Delta plains – The floodplains in a delta are called delta plains.

Natural Levees

This is an important landform associated with floodplains.

They are found along the banks of large rivers.


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They are low, linear and parallel ridges of coarse deposits along the banks of

rivers on both sides due to deposition action of the stream, appearing as natural

embankments.

At the time of flooding, the water is spilt over the bank. As the speed of flow of

the water comes down, large sized sediments with high specific gravity are

dumped along the bank as ridges.

They are high nearer the banks and slope gently away from the river.

Generally, the levee deposits are coarser

When rivers shift laterally, a series of natural levees can form.

Artificial embankments are formed on the levees to minimize the risk of the

floods.

But sudden bursts in the banks due to the pressure of water can cause disastrous

floods.

An example of such flood can be seen in Hwang Ho river which is also called

“China’s sorrow”.

Floodplain, Natural Levee, Point Bars


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Point Bars

Point Bar is also associated with floodplain

Point bars are also known as meander bars.

A point bar is a depositional feature

It is formed by alluvium that accumulates in a linear fashion on the inside bends

of streams and rivers below the slip-off slope.

They are found on the convex side of meanders of large rivers.

They are almost uniform in profile and in width and contain mixed sizes of

sediments.

Long and narrow depressions can be found in between the point bars where

there is more than one ridge

Rivers build a series of them depending upon the water flow and supply of

sediment.

As the point bars are built by the rivers on the convex side, erosion takes place

on the concave side of the bank.

Meanders

In large flood and delta plains, rivers rarely flow in straight courses. Loop-like

channel patterns called meanders develop over flood and delta plains

Normally, in meanders of large rivers, there is active deposition along the convex

bank and undercutting along the concave bank.

If there is no deposition and no erosion or undercutting, the tendency to

meander is reduced.

The concave bank is known as a cut-off bank which shows up as a steep scarp

and the convex bank presents a long, gentle profile and is known as the slip-off

bank


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Meander growth and cut-off loops and slip-off and undercut banks

Factors responsible for meandering of the rivers

1. The propensity of water flowing over very gentle gradients to work laterally on

the banks

2. Unconsolidated nature of alluvial deposits making up the banks with many

irregularities which can be used by water exerting pressure laterally

3. Coriolis force acting on the fluid water deflecting it like it deflects the wind


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A

satellite scene depicting meandering Burhi Gandak river, showing a number of

oxbow lakes and cut-offs

Oxbow Lakes

In the lower course of a river, meanders become very much more pronounced

As meanders grow into deep loops, the same may get cut-off due to erosion at

the inflexion points and are left as independent water bodies, known as ox-bow

lakes.

Through subsequent floods that may silt up the lake, oxbow lakes are converted

into swamps in due course of time. It becomes marshy and eventually dries up


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Formation of Oxbow Lakes

Braided Channels

A braided channel consists of a network of river channels divided into multiple

threads and separated by small and often temporary islands called ‘eyots’.

Braided channels are commonly found where water velocity is low and the river

is heavy with sediment load

Deposition and lateral erosion of banks are essential for the formation of the

braided pattern.

There is the formation of central bars due to selective deposition of coarser

material which diverts the flow towards the banks causing extensive lateral

erosion

As the valley widens due to continuous lateral erosion, the water column is

reduced and more and more materials get deposited as islands and lateral bars

developing a number of separate channels of water flow.


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Braided Channels

Deltas

Deltas are fan-shaped alluvial areas, resembling an alluvial fan

This alluvial tract is, in fact, a seaward extension of the floodplain

The load carried by the rivers is dumped and spread into the mouth of the river

at sea. Further, this load spreads and piles up as a low cone

Unlike in alluvial fans, the deposits making up deltas are very well sorted with

clear stratification. The coarsest sediments are deposited first and the finer

sediments are carried out further, into the sea.

Deltas extend sideways and seaward at an amazing rate

As the delta grows, the river distributaries continue to increase in length and

Delta continues to build up into the sea.


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Some deltas are extremely large. For example, the Ganges delta is as big as the

whole west of Malaysia

Satellite image of Ganges Delta

Types of Deltas

There are great variations in size, shape, growth and importance of Deltas. A great

number of factors influence the eventual formation of deltas such as depth of the

river, sedimentation, sea-bed, character of tides, waves and currents etc. owing to

these factors several types of deltas can be found.

Bird’s foot delta – It’s a kind of delta featuring long, stretching distributary

channels, which branch outwards resembling the foot of a bird. Deltas that are less

subjected to wave or tidal action culminate to a bird’s foot delta. Example – the

Mississippi River has a bird’s foot delta extending into the Gulf of Mexico


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Mississippi River, Birdfoot delta

Arcuate delta – Arcuate is the most common type of delta. This is a fan-shaped

delta. It’s a curved or bowed delta with the convex margin facing the sea. Arcuate

deltas have a smooth coastline due to the action of the waves and the way they are

formed. Examples – The Nile, Ganges and Mekong river deltas


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Nile river, Arcuate delta

Cuspate delta – A few rivers have tooth-like projections at their mouth, known as

the cuspate delta. Cuspate deltas are formed where the river flows into a stable

water body (sea or ocean). The sediments brought down by the rivers collide with

the waves. As a result, Sediments are spread evenly on either side of its channel.

Example – Ebro river delta in Spain


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Ebro river, Cuspate delta

Estuarine delta – some rivers have their deltas partly submerged in the coastal

waters to form an estuarine delta. This may be due to a drowned valley because of

a rise in sea level. Example – Amazon river delta

Conditions favourable for the formation of delta

Active vertical and lateral erosion in the upper course of the river to provide

extensive sediments to be eventually deposited as deltas

The coast should be sheltered preferable tideless

The sea adjoining the delta should be shallow or else the load will disappear in

the deep waters

There should be no large lakes in the river to ‘filter off’ the sediments

There should be no strong current running at right angles to the river mouth,

washing away the sediments


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20. Fluvial Erosional Landforms

In geography, fluvial processes are associated with movement and energy

associated with rivers and streams, and landforms created by them. Landforms are

small to medium tracts or parcels of the earth’s surface.

There are two types of landforms created by the fluvial process

1. Fluvial Erosional Landforms

2. Fluvial Depositional Landforms

The removal and transport of sediment due to fluvial processes result in erosional

landforms. In this article, we will discuss erosional landforms of Fluvial Process.

Fluvial Erosional Landforms

Unlike other geomorphic agents like wind and ice etc., which are confined to

certain areas, the effect of running water is felt all over the globe wherever water

is present. Thus running water forms the most potent geomorphic agent for

denuding the Earth’s surface through erosion.

Different aspects of Fluvial Erosive Action

In rivers, erosion and transportation go on simultaneously. There are different ways

in which fluvial erosion takes place, such as:-

Corrasion or abrasion – Corrasion is a process of mechanical erosion of the earth’s

surface caused by mechanical grinding of the river’s traction load (coarser material)

against the bed and banks of the river. There are two distinct ways in which

Corrasion can take place:

1. Lateral Corrasion – Lateral Corrasion takes place sideways and widens the V-

shaped valley

2. Vertical Corrasion – it is the downward action. It deepens the river channel

Hydraulic action – It is a mechanical process, in which the moving water current

flows against the banks and bed of a river, thereby removing rock particles. Some


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of the water splashes against the river banks and surges into cracks and crevices.

This helps to disintegrate the rocks. The river water picks up the loose fragments

from its banks and bed and transports them away.

Attrition – this is a form of fluvial erosion in which the bed load is eroded by itself

due to wear and tear of the transported material when they roll and collide into

one another. The coarser boulders are broken down into smaller stones and

pebbles.

Corrosion or solution – this is the chemical or solvent action of water on soluble or

partly-soluble rocks with which the river comes into contact. For example, calcium

carbonate in limestone is easily dissolved and removed in solution.

While the first three processes of fluvial erosion come under mechanical erosion,

the last or the fourth process i.e. corrosion comes under chemical erosion by fluvial

action.


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Forms of Erosion

River erosion takes place in three ways:

1. Headward erosion – it is a process by which a river increases its upstream length.

This is achieved by a river cutting back at its source

2. Lateral erosion – it is a process through which river channel is extended in its

width due to sideways erosion at the outside banks of the rivers

3. Vertical erosion – Vertical erosion takes place at the base of the river. The

channel of the river gets deepened through vertical erosion

The fluvial cycle of erosion

Three distinct stages of youth, maturity and old age can be identified during the

lifetime of a stream. At different stages of the erosional cycle, the valley acquires

different profiles. The characteristics related to each stage of landscape

development in running water regimes are summarised as below:

Youth

Streams are few during this stage with poor integration and flow over original

slopes


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The valley developed is thus deep, narrow and distinctly V-shaped with no

floodplains or with very narrow floodplains.

Downcutting predominates over lateral corrasion

Streams divides are broad and flat with marshes, swamp and lakes.

Some of the outstanding features which are developed in this stage are gorges,

canyons waterfalls, rapids and river capture etc.

Mature

During this stage, streams are plenty with good integration.

Lateral corrasion tends to replace vertical corrasion

The valleys are still V-shaped but wide and deep due to an active erosion of the

banks;

Trunk streams are broad enough to have wider floodplains within which streams

may flow in meanders confined within the valley.

Swamps and marshes of youth stage, as well as flat and broad inter-stream areas,

disappear. The stream divides turn sharp.

Waterfalls and rapids disappear.

Meander and slip off slopes are the characteristic features of this stage

Old

The river moving downstream across a broad level plain is heavy with sediments.

Vertical corrasion almost ceases in this stage though lateral corrasion still goes

on to erode its banks further

Smaller tributaries during old age are few with gentle gradients.

Streams meander freely over vast floodplains. Divides are broad and flat with

lakes, swamps and marshes.


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Depositional features predominate in this stage

Most of the landscape is at or slightly above sea level

Characteristic features of this stage are floodplains, oxbow lakes, natural levees

and Delta etc.

Fluvial Erosional Landforms

Most of the erosional landforms associated with running water are made by

youthful rivers vigorously flowing over steep gradients. With time, stream channels

over steep gradients turn gentler due to continued erosion, and as a consequence,

lose their velocity, facilitating active deposition. There are two components of

running water. One is the sheet that refers to overland flow on the land surface.

Another is streams and rivers that refer to linear flow as in valleys.

River Valleys

The extended depression on the ground through which a stream flows

throughout its course is called a river valley.

At different stages of the erosional cycle, the valley acquires different profiles

Valleys start as small and narrow rills

The rills will gradually develop into long and wide gullies

The gullies will further deepen, widen and lengthen to give rise to valleys.

Depending upon dimensions, shape, types and structure of rocks in which they

are formed, many types of valleys like the V-shaped valley, gorge, canyon, etc.

can be recognised.


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V-shaped river valley

1) V-shaped Valley

The river is very swift as it descends the steep slope, and the predominant action

of the river is vertical corrasion

The valley developed is thus deep, narrow and distinctly V-shaped


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Formation of the V-shaped valley

2) Gorge

A gorge is a deep and narrow valley with very steep to straight sides

A gorge is almost equal in width at its top as well as its bottom.

Gorges are formed in hard rocks.

Example – Indus Gorge in Kashmir


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A gorge

3) Canyon

A canyon is a variant of the gorge.

Unlike Gorge, a canyon is wider at its top than at its bottom.

A canyon is characterised by steep step-like side slopes

Canyons commonly form in horizontal bedded sedimentary rocks

Example – Grand Canyon carved by Colorado River, USA


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Grand Canyon, Colorado River, USA

Waterfalls and Rapids

When rivers plunge down in a sudden fall of some height, they are called

waterfalls

Their great force usually wears out a plunge pool beneath

Waterfalls are formed because of several factors like the relative resistance of

rocks lying across the river, the relative difference in topographic reliefs e.g. in

Plateau etc.

A rapid is similarly formed due to an abrupt change in gradient of a river due to

variation in resistance of hard and soft rocks traversed by a river

Waterfalls are also transitory like any other landform and will recede gradually

and bring the floor of the valley above waterfalls to the level below.


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Potholes and Plunge Pools

Potholes are more or less circular depressions formed over the rocky beds of hill-

streams, because of stream erosion aided by the abrasion of rock fragments.

Once a small and shallow depression forms, pebbles and boulders get collected

in those depressions and get rotated by flowing water and consequently the

depressions grow in dimensions.

Eventually, such depressions are joined leading to deepening of the stream

valley.

At the foot of waterfalls also, large potholes, quite deep and wide, form because

of the sheer impact of water and rotation of boulders. These deep and large holes

at the base of waterfalls are referred to as plunge pools.

These pools also help in the deepening of valleys


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Incised or Entrenched Meanders

Incised or entrenched meanders are found cut in hard rocks. They are very deep

and wide.

In streams that flow rapidly over steep gradients, normally erosion is

concentrated on the bottom of the stream channel.

Entrenched meander normally occurs where there is a rapid cutting of the river

bed such that the river does not get to erode the lateral sides.

Meander loops are developed over original gentle surfaces in the initial stages of

development of streams and the same loops get entrenched into the rocks

normally due to erosion or gradual uplift of the land over which they started.


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They are widened and deepened over a long period of time and can be found as

deep gorges and canyons in the areas where hard rocks are found.

They give an indication of the status of original land surfaces over which streams

have developed.

Incised meanders are said to be an impact of river rejuvenation.

River Terraces

River terraces refer to surfaces relating to old valley floor or floodplain levels.

They may be bedrock surfaces without any alluvial cover or alluvial terraces

consisting of stream deposits.

River terraces are basically products of erosion as they result due to vertical

erosion by the stream into its own depositional floodplain.

There can be a number of such terraces. They are found at different heights

indicating former river bed levels.

The river terraces may occur at the same elevation on either side of the rivers in

which case they are called paired terraces

Paired and unpaired river terraces


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Peneplain

A peneplain (an almost plain) is a low-relief plain which is formed as a result of

stream erosion

The peneplain is meant to imply the representation of a near-final (or

penultimate) stage of fluvial erosion during times of extended tectonic stability.

A peneplain

Drainage Patterns

The drainage pattern of a stream refers to the typical shape of a river course as

it completes its erosional cycle

They are governed by the topography of the land, resistance and strength of base

rocks and the gradient of the land

There are various types of drainage patterns which are described briefly as

below:-


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Various types of drainage patterns

Dendritic drainage pattern

It is the most common form of drainage system.

The drainage pattern resembling the branches of a tree is known as “dendritic”

In a dendritic system, there are many contributing streams, which are then joined

together into the tributaries of the main river

The examples of Dendritic Pattern include the rivers of northern plain such Indus.

Trellis drainage pattern

In the trellis drainage pattern, the primary tributaries of rivers flow parallel to

each other and they are joined by secondary tributaries at the right angle.

The geometry of a trellis drainage system is similar to that of a common

garden trellis used to grow vines.

Trellis drainage is characteristic of folded mountains,

Examples of trellis pattern include the drainage system of the Appalachian

Mountains in North America and Seine and its tributaries in Paris basin (France)

etc.


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Parallel drainage pattern

A parallel drainage system is a pattern of rivers caused by steep slopes with

some relief.

The parallel drainage pattern is observed in a uniformly sloping region where the

tributaries seem to be running parallel to each other.

A parallel pattern sometimes indicates the presence of a major fault that cuts

across an area of steeply folded bedrock.

Examples of this system include the rivers of Lesser Himalaya

Rectangular drainage pattern

Rectangular drainage develops on rocks that are of approximately uniform

resistance to erosion, but which have two directions of joining at approximately

right angles.

In the rectangular drainage pattern, the mainstream curve at right angles and the

tributaries join the mainstream at right angles.

Example – Colorado river the USA

Angular drainage pattern

Angular drainage pattern is commonly observed in foothill regions.

Angular drainage patterns form where bedrock joints and faults intersect at more

acute angles than rectangular drainage patterns. Angles are both more and less

than 90 degrees

the mainstream is joined by the tributaries at acute angles.

Radial drainage pattern

When the rivers originate from a hill and flow in all directions, the drainage

pattern is known as ‘radial’.


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Volcanoes usually display excellent radial drainage. Other geological features on

which radial drainage commonly develops are domes and laccoliths.

The rivers originating from the Amarkantak range present a good example of it.

Centripetal drainage pattern

When the rivers discharge their waters from all directions in a lake or depression,

the pattern is known as ‘centripetal’.

Examples – streams of Ladakh, Tibet and Loktak Lake in Manipur (India)

Annular drainage pattern

In an annular drainage pattern streams follow a roughly circular or concentric

path along a belt of weak rock, resembling in plan a ring-like pattern.

Example of such system include Black Hill streams of South Dakota, USA


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21. Glacial Landforms

Glacier

A glacier is a large mass of ice that is persistently moving under its own weight

over the land or as linear flows down the slopes of mountains in broad trough-

like valleys

Glaciers are formed in the areas where the accumulation of snow exceeds

its ablation (melting and sublimation) over many years, often centuries.

Glaciers move under the influence of the force of gravity.

The movement of glaciers is slow, unlike water flow. Glaciers flow like very slow

rivers.

Their movement could be a few centimetres to a few metres a day or even less

or more.

Types of Glaciers

Glaciers are categorized by their morphology, thermal characteristics, and

behaviour. Glaciers are mainly of four types – continental glaciers, ice caps,

piedmont glaciers and valley glaciers.


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1. Continental Glaciers – Continental glaciers are continuous masses of ice that are

much larger than alpine glaciers. By definition, they have areas larger than

50,000 km2, some examples of Continental Glaciers are Antartica, Iceland,

Greenland etc.

2. Ice caps – Ice caps are the covers of snow and ice on the mountain ranges from

which the valley or mountain glaciers originate. Though they can also be found

at the lower altitudes. Ice caps have an area of less than 50,000 km2.

3. Piedmont Glaciers – The piedmont glaciers form a continuous ice sheet at the

base of mountains. The Malaspina Glacier in Alaska is one of the most famous

examples of this type of glacier

4. Valley Glaciers – A glacier that fills a valley is called a valley glacier. The valley

glaciers are commonly known as Alpine Glacier and are found in the valleys

created by lofty mountains such as Himalaya in India.

Mechanism of erosion and deposition

Erosion by glaciers is tremendous because of friction caused by sheer weight of

the ice.

The rate of erosion is determined by several factors such as the velocity of flow,

gradient of the slope, the weight of the glacier, the temperature of the ice and

the geological structure of the valley

A glacier erodes its valley through two processes ‘plucking’ and ‘abrasion.’

Plucking – By “Plucking”, the glacier freezes the joints and beds of the underlying

rocks tears out individual blocks and drags them away

Abrasion – By “abrasion”, the glacier scratches, scraps, polishes and scours the

valley floor with the debris frozen into it. These fragments are powerful agents

of denudation

As glaciers move over bedrock, large blocks and fragments of rocks are plucked

from the land by glaciers. This mass of rocks and debris creates huge erosion

potential and erodes the bed and sides of the valley through which glaciers flow.


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The movement of glaciers continuously erodes the bedrock and levels of the

plain. Eventually, the slope is so much reduced that no further movement is

possible and so glacier stops and deposits the debris in the vast outwash plain.

Glacial Erosion

Glacial Landforms

Glaciation generally gives rise to erosional features in the highlands and

depositional features on the lowlands, though these processes are not mutually

exclusive because a glacier plays a combined role of erosion, transportation and

deposition throughout its course


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Erosional Landforms

Cirque

Cirques are horseshoe shaped, deep, long and wide troughs or basins with very

steep to vertically dropping high walls at its head as well as sides.

Cirques are often found along the head of Glacial Valley

The accumulated ice cuts these cirques while moving down the mountain tops.

After the glacier melts, water fills these cirques, and they are known as cirque

lake.

Horns

Horns form through head-ward erosion of the cirque walls.

If three or more radiating glaciers cut headward until their cirques meet, high,

sharp pointed and steep-sided peaks called horns form.


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Horn

Aretes

Arete is a narrow ridge of rock which separates two valleys.

Aretes are typically formed when two glacial cirques erode head-wards towards

one another

The divides between Cirque side walls or head walls get narrow because of

progressive erosion and turn into serrated or saw-toothed ridges referred to as

aretes with very sharp crest and a zig-zag outline.


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Arete

Glacial Valleys

Glaciated valleys are trough-like and U-shaped with wide, flat floors and relatively

smooth, and steep sides.

When the glacier disappears, and water fills the deep narrow sections of the

valley, a ribbon lake is formed.

Glacial Valley


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Fjords/Fiords

A fjord or fiord is a long, narrow and steep-sided inlet created by a glacier

They are formed where the lower end of a very deep glacial trough is filled with

sea water

Fjords are common in Norway, Chile, and New Zealand etc.

Fjord/Fiord

Hanging Valleys

A hanging valley is a tributary valley that is higher than the main valley. Hanging

valleys are common along glaciated fjords and U-shaped valleys.

The main valley is eroded much more rapidly than the tributary valleys as it

contains a much larger glacier

After the ice has melted tributary valley, therefore, hangs above the main valley

The faces of divides or spurs of such hanging valleys opening into main glacial

valleys are quite often truncated to give them an appearance like triangular

facets.


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Often, waterfalls form at or near the outlet of the upper valley

Thus, the hanging valley may form a natural head of water for generating

hydroelectric power

Hanging Valley

Depositional Landforms

Outwash plains

An outwash plain is a plain at the foot of the glacial mountain

They are made up of fluvioglacial sediments, washed out from the terminal

moraines by the streams and channels of the stagnant ice mass.

As it flows, the glacier grinds the underlying rock surface and carries the debris

along.

When the glacier reaches its lowest point and melts, it leaves behind a stratified

deposition material, consisting of rock debris, clay, sand, gravel etc. with larger


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boulders being deposited near the terminal moraine, and smaller particles

travelling further before being deposited.

The stratified surface thus formed is called as an outwash plain and a downward

extension of the deposited finer particles and clay material is called valley train.

Outwash Plain

Moraines

The unassorted coarse and fine debris dropped by the melting glaciers is called

glacial till.

The long ridges of deposits of these glacial till is called as Moraines

Depending on its position, moraines are classified into be ground, lateral, medial

and terminal moraine.

Terminal Moraines – Terminal moraines are long ridges of debris deposited at

the end (toe) of the glaciers.


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Lateral Moraines – Lateral moraines form along the sides parallel to the glacial

valleys. These moraines partly or fully owe their origin to glaciofluvial waters

pushing up materials to the sides of glaciers.

There can be many lateral moraines on either side in a glacial valley. The lateral

moraines may join a terminal moraine forming a horse-shoe shaped ridge

Ground Moraines – Many valley glaciers retreating rapidly leave an irregular

sheet of till over their valley floors. Such deposits varying greatly in thickness and

in surface topography are called ground moraines.

Medial Moraines – The moraine in the centre of the glacial valley flanked by

lateral moraines is called medial moraine. They are imperfectly formed as

compared to lateral moraines.

Sometimes medial moraines are indistinguishable from ground moraines.

Types of Moraine

Eskers

An esker is a long, winding sinuous ridge of stratified sand and gravel


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Eskers are frequently several kilometres long and, because of their peculiar

uniform shape, are somewhat like railway embankments

When glaciers melt in summer, the water flows on the surface of the ice or seeps

down along the margins or even moves through holes in the ice.

These waters accumulate beneath the glacier and flow like streams in a channel

beneath the ice.

Such streams flow over the ground with ice forming its banks.

The stream underneath carries coarse materials such as boulders, blocks which

gets deposited in the bed and when the glacier melts the deposits forms a sinuous

ridge called eskers.

Eskers


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Drumlins

Drumlins are smooth oval shaped ridge-like features composed mainly of glacial

till with some masses of gravel and sand.

The drumlins form due to the dumping of rock debris beneath heavily loaded ice

through fissures in the glacier.

The long axes of drumlins are parallel to the direction of ice movement.

They may measure up to 1000m in length and 30-35 m or so in height.

One end of the drumlins facing the glacier called the stoss end is blunter and

steeper than the other end called the tail.


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Drumlins


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22. Karst Landforms

Karst topography is named after the typical topography developed in limestone

rocks of Karst region in the Balkans adjacent to the Adriatic Sea. Karst topography

includes typical landforms in any limestone or dolomitic region, produced by the

action of groundwater through the processes of solution and deposition.

Karst Topography

Limestone is an organically formed sedimentary rock. In its pure state, limestone is

made up of calcite or calcium carbonate but where magnesium is also present it is

termed as dolomite. Limestone is soluble in rainwater.

Conditions for the formation of Karst Topography

A region with a large stretch of water-soluble rocks such as limestone at the

surface or sub-surface level

Limestones should not be porous

These rocks should be dense, thinly bedded and well jointed

A perennial source of water and a low water table to allow the formation of

conspicuous features.

Moderate to abundant rainfall to cause the solvent action of water i.e. solution

of rocks

Mechanism of erosion in Karst region

In Karst regions, rocks are permeable, thinly bedded and highly jointed and

cracked.

Thus there is the general absence of surface drainage as the surface water has

gone underground


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After vertically going down to some depth, the water under the ground flows

horizontally through the bedding planes, joints or through the materials

themselves.

Rocks are eroded due to this downward and horizontal movement of water.

It is through the chemical process of solution and precipitation deposition by

surface water and groundwater, varieties of landforms are developed in rocks

like limestones or dolomites rich in calcium carbonate.

Karst Landforms

Erosional Landforms

Sinkhole

Small to medium-sized round to sub-rounded shallow depressions called swallow

holes form on the surface of limestones through solution where rainwater sinks

into the limestone at a point of weakness

They are also known as sinkholes

Sinkholes are a common feature in limestone/karst areas.


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A sinkhole is an opening more or less circular at the top and funnel-shaped

towards the bottom

There is a great variation in sizes of Sinkholes with areas from a few sq. m to a

hectare and with depth from a less than half a metre to thirty metres or more.

These holes grow in size through continuous solvent action

They are also referred to as solution sinks

Doline

They are also referred to as “Collapse sinks”.

They are less common than sinkholes

They might start as solution forms first, and if the bottom of a sinkhole forms the

roof of a void or cave underground, it might collapse leaving a large hole opening

into a cave or a void below


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Uvalas

They are long, narrow to wide trenches, also referred to as “Valley sinks”.

Several sinkholes and dolines may merge together as a result of subsidence to

form a large depression called an Uvala.

Lapies/ Karren

These are grooved, fluted and ridge-like features in an open limestone field.

These ridges or lapies form due to differential solution activity along parallel to

sub-parallel joints.

Eventually, the lapie field may transform into smooth limestone pavements.

Limestone Pavements

A limestone pavement is a natural karst landform consisting of a flat, incised

surface of exposed limestone that resembles an artificial pavement.

These are formed by the solvent action of underground water in the limestones,

causing progressive widening and enlargement of joints and cracks in the

trenches.


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The enlarged joints are called grikes and the isolated, rectangular blocks are

termed as clints.

Caves

Cave formation is prominent in areas where there are alternating beds of rocks

(sandstone, shale, quartzite) with limestone or dolomite in between or in areas

where limestones are dense, massive and occurring as thick beds.

Water percolates down either through the materials or through cracks and joints

and moves horizontally along bedding planes.

Gradually, the limestone dissolves along these bedding planes resulting in the

creation of long and narrow gaps called caves.


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Polije

A polije is a very large, flat-floored depression in the karst region.

They are often formed by merging of several uvalas or partly due to faulting

They are commonly found in subtropical and tropical latitudes

Some of these may also appear in the temperate region. They may also be found

in boreal regions, though very rarely.

During the rainy season, parts of the floor which are at or near the water table

may become temporary lakes

Drier areas are fertile. Usually covered with thick sediments, they are used

extensively for agricultural purposes

Ponor

A ponor is a natural surface opening in the karst regions


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They are found directly underneath the sinkholes

A ponor is kind of a portal where a surface stream or lake flows either partially or

completely underground into a karst groundwater system.

Landforms due to depositions

Depositional landforms in karst region are developed due to the deposition of

calcium carbonate. The calcium carbonate dissolved during the erosional process

starts to precipitate when the water evaporates or when the solution is super-

saturated.

Stalactites, Stalagmites and Pillars are the most spectacular underground features,

found in the limestone caves.


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Stalactites

Stalactites are the sharp, slender, downward-growing icicles of different

diameters that hang from the cave roofs.

Stalactites have a variety of forms

Their bases are normally broad which taper towards the free ends

The water carries calcium in solution and when this lime-charged water

evaporates, it leaves behind the solidified crystalline calcium carbonate.

Stalagmites

Stalagmites form due to dripping water from the surface or through the thin pipe,

of the stalactite, immediately below it

Moisture dripping from the roof trickles down the stalactite and drops to the

floor where stalagmites are formed due to deposition of calcium


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Stalagmites may take the shape of a column, a disc, with either a smooth,

rounded bulging end or a miniature crater-like depression.

Pillars

Over a long period, the stalactite is eventually merged with the stalagmite

Thus, the pillars or columns of different diameters are formed.


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23. Marine Landforms

The Coastal Landforms are formed by the constant action of the waves, tides, and

currents. The coastline under the influence of these denudational agents changes

the coastal landforms and gives shapes to various types of marine landform

features.

Agents of Erosion

Waves

Waves accomplish most of the changes along the coasts.

Constant impact of breaking waves drastically affects the coasts.

When waves break, the water is thrown with great force onto the shore, and

simultaneously, there is a great churning of sediments on the sea bottom.

Storm waves and tsunami waves can cause far-reaching changes in a short period

of time than normal breaking waves.

On calm days, when winds are slight, waves do little damage to the shoreline and

may instead help to build up beaches and other depositional features.

Tides and Currents

Tides and Currents, on contact with the shores, make very little direct attack on

the coastline

Tides affect marine erosion mainly by extending a line of erosion into a zone of

erosion. This zone correspond to the area between the low water level and the

high water level

Currents help to move eroded debris and deposit it as silt, sand and gravel along

the coasts


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The mechanism of marine erosion

The rate of marine erosion depends on the nature of rocks, the amount of rock

exposed to the sea, the effects of tides and currents, and human interference.

Marine agents of erosion operate in the following ways to transform the coastal

landscape

Corrasion – Corrasion is a process of mechanical erosion. Waves armed with rock

debris break on cliff faces and slowly erode it. On-coming currents and tides

complete the work by sweeping the eroded material into the sea.

Attrition – Attrition occurs when waves cause loose pieces of rock debris such as

boulders, pebbles, shingle and fine sand, to collide with each other. Under attrition,

these materials are broken down into finer, smaller and rounder particles which

are largely responsible for the fine sand that forms the beaches.

Hydraulic action – in their forward surge, waves splashing against the coast may

enter joints and crevices in the rocks. The air trapped inside is immediately

compressed. When the waves retreat, the compressed air expands with explosive

violence. Such repeated action causes enlargements of the cracks and rock

fragments are prised apart.

Solvent action – this refers to chemical erosion of rocks. This process is limited to

limestone coasts. On limestone coasts, the solvent action of seawater on calcium

carbonate sets up chemical changes in the rocks and disintegration takes place.

Types of Coasts

Other than the action of waves, the coastal landforms depend upon the

configuration of land and sea floor and whether the coast is advancing (emerging)

seaward or retreating (submerging) landward.

There are different types of coastlines based on a great variety of coastal features.

However, it is important to discuss, two types of coasts (assuming sea level to be

constant) to explain the concept of evolution of coastal landforms:

1. Submerged coasts (high, rocky coasts)

2. Emerged coasts (low, smooth and gently sloping sedimentary coasts)


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Submerged coasts

Submerged coasts are found either because of the sinking of the land or due to

the rise of the sea

The coasts are rocky and river appears to have drowned in the sea creating

estuaries.

Erosional landforms dominate coastal landform and depositional landforms are

absent

Along high rocky coasts, waves break with great force against the land shaping

the hill-sides into cliffs, which further develops a wave-cut platform, caves etc.

As the erosion along the coast takes place a good supply material becomes

available to longshore currents and waves to deposit them as beaches, bars, spits

etc.

Emerged Coasts

Emerged coasts are found due to either uplift of the land or fall in the sea level

They are less common

Along, the low sedimentary coasts the rivers appear to extend their length by

building coastal plains and deltas.

The coastline appears smooth with occasional incursions of water in the form of

lagoons and tidal creeks.

The land slopes gently into the water.

Marshes and swamps may abound along the coasts.

Depositional features dominate.

When waves break over a gently sloping sedimentary coast, the bottom

sediments get churned and move readily building bars, barrier bars, spits and

lagoons.


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The maintenance of these depositional features depends upon the steady supply

of materials.

Large rivers which bring lots of sediments build deltas along low sedimentary

coasts.

Marine Landforms

Erosional landforms

Sea cliffs

The most widespread landforms of erosional coasts are sea cliffs.

Generally, any very steep rock face adjoining the coast forms a cliff

Almost all sea cliffs are steep and may range from a few m to 30 m or even more.

Their steep nature is the result of wave-induced erosion near sea level and the

subsequent collapse of rocks at a higher elevation.


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At the base of the cliff, the sea cuts a notch, which gradually undermines the cliff

so that it collapses

The best-known cliffs are the Chalk cliffs of the English channel and the ‘White

Cliffs’ of Dover

Wave-cut platforms

When the sea waves strike against a cliff, the cliff gets eroded gradually and

retreats.

With constant pounding by waves, as the cliffs recede, an eroded base is left

behind, called a wave-cut platform.

The waves level out these platforms to create a flat surface

Such surfaces may measure from a few metres to hundreds of metres wide and

extend to the base of the adjacent cliff.


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Sea caves

Prolonged attack of waves against the base of the cliff and the rock debris that

gets smashed against the cliff along with lashing waves create holes in regions of

weakness and

These holes get further widened and deepened to form sea caves.

Example Flamborough head, England


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Sea arches

When two caves approach one another from either side of a headland and unite,

they form a bridge like structure, known as arch

These archways may have an arcuate or rectangular shape, with the opening

extending below water level.

The height of an arch can be up to tens of metres above sea level.

It is common for sea arches to form when the waves attack a rock- form from two

opposite sides, the differential erosion

Example – the Neddle Eye near Wick, Scotland


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Sea Stack

Continued erosion, under the attack of the wave, can result in the total collapse

of an arch

The seaward portion of headland will remain as an isolated pillar of rock known

as stack

Like all other features, sea stacks is also temporary and eventually, the stack will

also disappear


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Stump

The stack is gradually eroded, leaving behind only the stump

Stumps are only just visible above the sea level

Blow holes

The occasional splashing of the waves against the roof of a cave may enlarge the

joints when compressed air is trapped inside

A natural shaft is thus formed which may eventually pierce through the surface

Waves breaking into the cave may force blasts of water from the top

Such shaft is termed as Blow-hole or ‘Gloup’.

Example – Holborn Head, Scotland

Geos

The enlargement of blow-holes and the continued action of waves weaken the

cave roof.

When the roof collapses a long, narrow inlet or creek develops.

Such long and deep clefts are called Geos

Example – the Wife Geo, Scotland

Depositional landforms

Beaches

Beaches are characteristic of shorelines that are dominated by deposition but

may occur as patches along even the rugged shores.

Sands and gravels loosened from the land are moved by waves to be deposited

along the shore as beaches


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Most of the sediment making up the beaches comes from land carried by the

streams and rivers or from wave erosion.

Most of the beaches are made up of sand sized materials. Beaches called shingle

beaches contain excessively small pebbles and even cobbles.

Beaches are temporary features.

Dunes

Just behind the beach, the coastal sands lifted and winnowed from over the

beach surfaces will be deposited as sand dunes.

On shore, winds play a major part in the formation of these dunes

Sand dunes forming long ridges parallel to the coastline are very common along

low sedimentary coasts.

Sand dunes are common in the coasts of Belgium, Denmark and the Netherlands


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Bars

When a ridge of sand and shingle formed in the sea in the off-shore zone (from

the position of low tide waterline to seaward), it is called a bar

The off-shore bars and barriers commonly form across the mouth of a river or at

the entrance of a bay.

Bars are submerged features and when bars show up above water, they are called

barrier bars.

Generally, bars are approximately parallel to the coast

Tombolo

Tombolo joins two landmasses by a connecting bar

The tombolo is a deposition landform in which an island is attached to the

mainland by a narrow piece of lands such as a spit or bar.

A tombolo is a sandy isthmus.

An example of Tombolo can be found in Chesil beach in England which links the

Isle of Portland with mainland


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Barriers

An off-shore bar which is exposed due to further addition of sand is termed a

barrier bar.

The off-shore bars and barriers commonly form across the mouth of a river or at

the entrance of a bay.

They usually occur in chains

They are subject to change during storms and other action, but absorb energy

and protect the coastlines and create areas of protected waters

where wetlands may flourish.

Spits

Barrier bar which gets keyed up to the headland of a bay is called a spit.

Spits are projected depositional landforms with one end attached to the land and

the other end projecting into the sea

Spits may also develop attached to headlands/hills.

The mode of formation of spit is similar to a bar or barrier.

A shorter spit with one end curved towards the land is called a hook.

When barrier bars and spits form at the mouth of a bay and block it, a lagoon

forms.

The lagoons would gradually get filled up by sediments from the land giving rise

to a coastal plain.


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24. Arid Landforms

Deserts are regions with scanty rainfall, i.e. rainfall less than 25cm annually but still,

rainwater plays a crucial role in the geomorphology of the arid landforms. However,

the wind is the dominating agent of gradation in the arid regions of the world.

Arid Landforms

Arid landforms are the characterised by badlands, Mushroom rocks, dunes,

yardang, etc. These features are typical in an arid region. In this article, we will

study various types of arid landforms, their mechanism of formation etc.

Deserts

Deserts are regions with very less precipitation concentrated to a very short

duration. Around 20% of the geographical areas in the world are deserts.

There is a certain definite pattern to the location of the world’ deserts

Almost all the deserts are confined within the 15 to 30-degrees Latitude on both

sides of the equator.

Deserts are generally located in the west coasts of the continent as the

Tradewinds are off-shore

They are bathed by cold currents which produce a desiccating effect so that

moisture is not easily condensed into precipitation

Type of deserts

The works of wind and water in the erosion of elevated uplands, transporting the

eroded material and depositing them elsewhere has given rise to five distinct kinds

of desert landscape

Rocky desert

Rocky deserts are also known as “Hamada”.

They consist of large stretches of bare rocks, swept clear of sand and dust by the

wind


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The exposed rocks are thoroughly smoothed and polished

The region is bare and sterile

Example – the best known rocky deserts are those of the Sahara deserts

Stony desert

It is also known as “Reg”

Pebbles and gravels form an extensive sheet of the landscape of these areas.

Stony deserts are more widespread than sandy deserts, contrary to the general

idea of deserts associated with sandy landforms.

Example – Sturt Stony desert, Australia


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Sandy deserts

The commonly accepted idea of a desert is the sandy landscape with dunes.

Vast stretches of dunes are deposited by winds, in these types of desert.

The wind direction can be observed from the patterns of the ripples on dunes.

Example – Thar desert in India is a sandy desert


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Badlands

Badlands are arid regions where the hills are badly eroded under the action of

water due to occasional rainstorms or flow of river streams

They are represented by gullies and ravines

Example – famous ravines of Chambal in India, ravines in South Dakota, USA etc.


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Mountain deserts

Mountain deserts are found are found on highlands such as plateaux and

mountain ranges

Erosion has dissected the desert highlands into harsh, serrated outlines of chaotic

peaks and craggy ranges

They have steep-sided slopes and sharp and irregular edges carved by the action

of frost

Example – Tibesti Mountains in the Sahara desert


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The mechanism of Arid Erosion

Arid landforms are the result of many combined factors, one reacting upon the

other. Low precipitation and high rate of evaporation are the major causes of

aridity. The desert rocks devoid of vegetation, exposed to mechanical and chemical

weathering processes due to drastic diurnal temperature changes, decay faster and

wind and the torrential rains help in removing the weathered materials easily.

Weathering

This is the most potent factor of denudation in arid regions.

Weathering is defined as the breakdown of rocks by agents of weathering acting

in situ.

Mechanical weathering and Chemical weathering dominates in the arid

landforms

Without abundant water in the arid environment, the chemical breakdown of

rocks proceeds extremely slowly. However, the mechanical breakdown of rock

proceeds relatively quickly in the arid climate.

Drastic diurnal temperature changes in deserts cause stress in the rocks due to

continuous expansion and contraction. This stress helps in speeding up the

weathering process through exfoliation of the outer rock surface


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Wind action

As an erosional agent, the wind is more effective in arid regions than humid

regions as in arid regions, there is little moisture and vegetation to bind the loose

material

The wind action creates a number of interesting erosional and depositional

features in the deserts.

Wind erosion is carried out in desert areas in mainly three ways – Deflation,

Abrasion and Attrition

Deflation – Deflation includes lifting and blowing away of dust and smaller

particles from the surface of rocks.

Abrasion – the sand-blasting of a rock surface by winds when they hurl sand

particles against them is called abrasion. The impact of such blasting results in

rock surface being scratched, polished and worn away.

Attrition – when wind-borne particles roll against one another in a collision they

wear each other away. This process is called attrition


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Water action

Many features of deserts owe their formation to mass wasting and running water

as sheet floods.

Though rain is scarce in deserts, it comes down torrentially in a short period of

time.

Stream channels in desert areas are broad, smooth and indefinite and flow for a

brief time after rains.

The desert rocks devoid of vegetation, exposed to mechanical and chemical

weathering processes due to drastic diurnal temperature changes, decay faster

These weathered materials are easily carried away by torrential rainfall


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Landforms of wind erosion

Rock pedestal or mushroom rocks

Many rock-outcrops in the deserts easily susceptible to wind deflation and

abrasion are worn out quickly

This leads to wearing away of the softer layer leaving some remnants of resistant

rocks

Grooves and hollows are cut in the rock surface, carving them into fantastic and

grotesque looking pillars called “Pedestals”

Such rock pillar is further eroded near bases

This process of undercutting produces mushroom with a slender stalk and a

broad and rounded pear shaped cap above.


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Zeugen

These are tabular masses which have a layer of soft rocks lying beneath a surface

layer of more resistant rocks

The sculpting effects of wind abrasion wear them into a weird looking ridge and

furrow landscape

Their formation is initiated by opening up of joints of surface rocks by mechanical

weathering

Deep furrows are developed by wind abrasion eating into the underlying softer

layers

The hard rocks then stand above the furrows as ridges or Zeugen

Such tabular blocks of Zeugen may stand 10 to 100 feet above the sunken

furrows.


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Continuous abrasion by wind gradually lowers the Zeugen and widens the

furrows

Yardangs

The word yardang originated in the interior deserts of central Asia where they

are best developed

Yardangs are a steep-sided irregular ridge of sand lying in the direction of the

prevailing wind

They look quite similar to the ‘ridge and furrow’ landscape of Zeugen

They are formed by the dual action of wind abrasion by dust and sand, and

deflation which is the removal of loose material


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Wind abrasion excavates the bands of softer rocks into long, narrow corridors,

separating the steep-sided, over-hanging ridges of hard rocks, called yardangs.

They are commonly found in the Atacama desert, Chile

Mesas (Table) and buttes

Mesa is a Spanish word meaning ‘table’.

It is a flat, table-like land masses with a very resistant horizontal top layer and

very steep sides

The hard stratum on the surface resist denudation by both wind & water, and

thus protects the underlying layer of rocks from being eroded away

Continuous denudation through the ages may reduce Mesas in an area so that

they become isolated flat-topped hills called Buttes.


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Inselberg

Inselberg is a German word meaning ‘Island-mountains.’

An inselberg is an isolated residual hill rising abruptly from a gently sloping or

virtually level surrounding plain.

They are characterized by their very steep slopes & rather rounded tops

They are often composed of granite or gneiss

They are probably the relics of an original plateau which has been almost entirely

eroded away

Inselbergs are typical features of many deserts and semi-arid landscapes in old

age


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Ventifacts

Ventifacts are pebbles faceted by sand blasting

They are shaped and polished by wind abrasion

Mechanically weathered rock fragments are moved by wind in open settings to

blast against the rock formations carving facets

If wind direction changes another facet is developed

Such rocks have characteristic flat facets with sharp edges

Among the ventifacts, those with the three wind faceted surfaces are known as

Dreikanter.


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Landforms of wind deposition

When the velocity of wind decreases, its carrying capacity also decreases. As a

result, the grains of sands starts to settle down, and it leads to the formation of

depositional landforms in a desert.

Depending upon the size of the particles, velocity and direction of the wind,

different depositional landforms can be found in arid and desert areas:

Dunes

Dunes are hills of sand formed by the accumulation of sand & shaped by the

movement of winds

Dry hot deserts are good places for sand dune formation

They may classifies as active and inactive dunes – active or live dunes are

constantly on move and inactive or fixed dunes are rooted with vegetation


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Types of dunes

Because of their great contrast in shape, size and alignment, they have been given

classified into several types of dunes viz head dune, tail dune, parabolic dune,

pyramidal dune, transverse dune, longitudinal dune etc. However, there are two

most common types of dunes are barchans and seif which are described as below:-

1) Barchan

They are moon or Crescent shaped live dunes

They may occur individually or in groups

They have their points or wings directed away from wind direction i.e., downwind

They are initiated probably by a chance accumulation of sand at an obstacle, such

as patch of grass or a heap of rocks


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They occur transversely to the wind

Thus, their horns thin out & become lower in the direction of the wind.

The windward side is convex & gently sloping whiles the leeward side is concave

& steep.

The crest of sand dunes moves forward as more sand is accumulated by the

prevailing wind.

The sand is driven up the windward side & on reaching the crest slips down the

leeward side so that the dune advances

The migration of Barchans may be a threat to desert life as they may encroach

on an oasis burying palm trees & houses.

They are most prevalent in the deserts of Turkestan and in the Sahara


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2) Seif

Seif is an Arabic word meaning ‘sword’

They are long, narrow ridges of sand, often over a hundred miles long lying

parallel to the direction of the prevailing winds

Seif is similar to barchan with a small difference; it has only one wing.

Prevailing winds increases the length of the dunes into tapering linear ridges

while occasional crosswinds tend to increase their heights & width

Extensive seif dunes can be found in Sahara desert, West Australian desert, Thar

desert etc.

Loess

The fine dust blown beyond the desert limits is deposited on neighbouring lands

as loess.

It is a yellow, friable (easily crumbled) material is usually very fertile.


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Loess is in fact, fine loam, rich in lime, very coherent and extremely porous

Water sinks in readily so that the surface is always dry,

Streams may cut deep valleys through the thick mantle of soft loess to develop

badland topography.

The most extensive deposits are found in north-west China in the loess plateau

of the Hwang-Ho basin.

Landforms due to water action

Desert areas have scanty rainfall. However, there are deserts without rainfall

also.

However, occasional and sudden rainfalls in torrential downpours may produce

devastating effects due to flash floods etc.

Loose materials such as gravel, sand and fine dust are swept down the hillsides.

They cut deep gullies and ravines forming badland topography


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There are so much fine materials in the flash floods that the flow becomes liquid

mud

Bajada

The Bajada is a depositional feature made up alluvial material laid down by the

intermittent streams

Bajada is formed by the coalescence of alluvial fans

These fan-shaped deposits form from the deposition of sediment by a stream

from upland region onto flat land at the base of a mountain

Bajadas are common in arid areas where a large quantity of sediment is deposited

by flash floods

Bajadas frequently contain playa lakes

Playa

In arid areas drainage from upland regions into the lower depression, in times of

sufficient water, create shallow water body or a temporary lake

Such types of shallow lakes are called as playas where water is retained only for

short duration due to evaporation

Quite often the playas contain good deposition of salts.

The playa plain covered up by salts is called alkali flats.

Pediments and Pediplains

A pediment is an erosional plain formed at the base of the surrounding mountain

scarps

They are gently inclined rocky floors close to the mountains at their foot with or

without a thin cover of debris.

They form through the erosion of mountain front through a combination of

lateral erosion by streams and sheet flooding.


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Through parallel retreat of slopes, the pediments extend backwards at the

expense of mountain front

Gradually, the mountain gets reduced leaving an inselberg which is a remnant of

the mountain.

That’s how the high relief in desert areas is reduced to low featureless plains

called pediplains.


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25. Lake – Classification of Lakes

Lakes are among the most varied features of the earth’s surface. A lake is a large body of natural water accumulated in a depression. Lake basins are formed due to endogenous geological processes like tectonism and volcanism and exogenous activities like landslides, glaciation, solution, river and wind action.

Lake and Its Classification

They vary tremendously in size, shape, depth and mode of formation. The tiny ones

are no bigger than ponds or pools, but the large ones are so extensive that they

merit the name of seas, e.g. the Caspian Sea. The Caspian Sea is the largest lake

regarding the area. The deepest lake in the Lake Baikal in Siberia.

Lakes occupy about 1.8 % of the earth’s surface. About 280 000 cu.km of water

exists on earth in the form of lakes. This is 0.19% of the total volume of water in

the hydrosphere.

Advantages of lakes

The major role played by lakes and reservoirs is the regulation of stream flow.


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Lakes provide water for drinking, factory, irrigation and generating hydel-power.

Lakes are a good refuge for an enormous variety of flora and fauna.

The lives of the people, in a region, are greatly influenced by the presence of a

lake in that area.

In some places, lakes are good sources for water supply for drinking.

Lakes help in the growth of the fishing industry.

The salt lakes yield common salt. For example, Sambar lake

Lakes are helpful in controlling the weather and moderating local climate- Lakes

cool the air in summer and warm it during winter. They also enhance the

humidity.

Lakes have an aesthetic appeal and are helpful in recreation; tourists are

attracted due to lakes which have boating, swimming and a good landscape

around.

Lakes are used for navigation. For example the Great Lakes in North America

Lakes also help in flood control as rivers passing through the lakes in their course

seldom cause disastrous floods. The Wular lake and the Dal lake do not allow the

Jhelum river to be flooded and due to lack of such lakes, the Brahmaputra is

subjected to very great floods every year.

Lakes only a temporary feature?

Lakes are thought to be only a temporary feature of the earth’s crust. Eventually,

they will be eliminated by the dual process of draining and sitting up. In regions of

unreliable rainfall, lakes dry up completely during the dry season. In the hot

deserts, lakes disappear altogether by the combined processes of evaporation,

percolation and outflow. Though the process of lake elimination may not be

completed within our span of life, it takes place relatively quickly regarding

geological time.


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Classification of lakes

When there are a large number and variety of lakes, people tend to classify them.

There are several types, kinds and categories of lakes in the world. Classification

helps us to understand and visualize the relationships and helps us to

communicate. The most common classification of lakes is based on the size or

dimension of lakes, whether it is small, big or very large.

Lakes are mainly classified on the basis of:

a) Nature of Inflow-outflow

b) Origin

c) Trophic levels

a) Classification based on inflow-outflow

Temporary and Permanent Lakes

1. Temporary Lakes -These lakes may exist temporarily by filling up small

depressions of undulating grounds after a heavy shower. In such lakes rate of

evaporation is much greater than the rate of recharge through precipitation.

They are usually saline. They are subject to extreme fluctuations in water level.

Example – Badhkal Lake, Faridabad

2. Permanent lakes – Permanent lakes carry more water than could ever be

evaporated. These are very deep. They have some perennial source of inflow of

water such as a glacier. They are usually freshwater lakes. Example – Dal Lake

Freshwater and Salt lakes

1. Freshwater lakes – Most of the lakes in the world are freshwater lakes. They are

usually found in low lying areas and are fed from streams, rivers and runoff from

the surrounding area. e.g. Great Lakes of North America, Lake Baikal in Russia,

Lake Wular and Loktak Lake in India.

2. Salt Lakes – Salt Lake is an inland body of water situated in an arid or semiarid

region, having no outlet to the sea, and containing a high concentration of


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dissolved salt. These lakes exist in regions of low precipitation and intense

evaporation. Because of intense evaporation, the concentration of salts increases

in the water body, turning them saline. Playas or salt lakes are a common feature

of deserts. Example – Great Salt Lake of Utah, USA, Dead Sea etc.

Great Lakes of North America

b) Classification based on origin or mode of formation

The following are the various ways in which lakes can be formed. Each of them is

placed in a different category, though in a few cases the lake could have been

formed by more than one single factor

1) Lakes formed by earth movement

Tectonic Lakes

These lakes are formed by filling up with water in the tectonic depressions

created due to warping, sagging, bending and fracturing of the earth’s crust.


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Such depressions give rise to lakes of immense sizes and depths.

Example – Lake Titicaca, Chile, the Caspian Sea etc.

Rift Valley Lakes

These lakes include some of the oldest, deepest and largest lakes around the

globe.

Due to faulting, a rift valley is formed by the sinking of the land between two

parallel faults, deep, narrow and elongated in character.

Water is collected in these troughs

Often their floors are below sea level.

The best example of this is the East African Rift Valley which includes such lakes

as Lake Tanganyika and the Dead Sea etc.

2) Lakes formed by Volcanism

Crater and Caldera Lakes

A natural hollow called a crater is formed by blowing off of the top of the cone

during a volcanic explosion.

Crater may be widened and enlarged by further subsidence into a caldera.

These depressions are normally dry.

In dormant or extinct volcanoes, due to rainfall straight into these depressions

which have no superficial outlet, a crater or caldera lake is formed.

Examples – Lonar crater lake in Maharashtra, India, Crater Lake in Oregon, USA

and Lake Toba in Sumatra etc.

Lava-blocked Lakes

In volcanic regions, it is common to find a stream of lava that flows across a valley


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This stream of lava may occasionally become solidified and block the valley thus

forming a lake basin

This basin may get filled up due damming up of the river due to solidified lava

Example – The Sea of Galilee which is an inland lake was created due to blocking

of the Jordan valley by lava flow

One more type of lake formed due to subsidence of a volcanic land surface is

included under this type. Under this type of lake, the crust of a hollow lava flow

may collapse. The subsidence leaves behind a wide and shallow depression in

which the lake may form. E.g. Myvatn Lake of Iceland

3) Lakes formed by Glaciation

Cirque or tarn lakes

Cirque, a common landform in glaciated mountains, is often found at the heads

of glacial valleys.

A glacier on its way down the valley leaves behind circular hollows.

These circular hollows, in the heads of the valley up in the mountain, are called

cirques.

Cirques are very deep, long and wide troughs or basins.

The head and sides of these cirques have very steep to vertically dropping high

concave walls

Often, a lake of water can be seen within the cirques after the disappearance of

the glacier. Such lakes are referred to as the Cirque or tarn lakes

They are also called as Ribbon lakes

Example – Red tarn in the English Lake District and Chandra Taal (Himachal

Pradesh) in India


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Cirque or Tarn Lakes

Kettle Lakes

These are depressions in the outwash plain left by melting of a large mass of

stagnant ice

They are irregular in shape, and also these lakes are not very large or deep

Example – Kettle-lakes of Orkney in Scotland

Rock-Hollow Lakes

These lakes are formed by ice scouring when valley glaciers or ice sheets scoop

out hollows or depressions on the surface

Such lakes are abundant in Finland

Two more types of a glacial lake are formed due to “damming up of valleys by

morainic debris deposited by valley glaciers” and “deposition of glacial drifts in

glaciated lowlands”.


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4) Lakes formed by Erosion

Karst Lakes

Karst lakes are formed in depressions, carved out by solvent action of rainwater

on water-soluble rocks such as limestone, gypsum and dolomite.

The collapse of limestone roofs of underground caves may result in the exposure

of long, narrow lakes that were once underground.

The shallow bed of these lakes is usually an insoluble layer of sediment so that

water is impounded, resulting in the formation of lakes.

Many karst lakes only exist periodically but return regularly after heavy rainfall.

Example – the Lac de Chaillexon in the Jura mountains

Otjikoto Karst Lake in Namibia


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Wind deflated lakes

These lakes are formed in arid regions and deserts

The depressions are created in deserts due to deflating action of winds

Groundwater may seep out in these depressions forming lakes

Excessive evaporation causes these to become salt lakes and Playas

Example – Great Basin of Utah, USA

5) Lakes formed by deposition

Ox-bow lakes

In large flood and delta plains, rivers rarely flow in straight courses. Loop-like

channel patterns called meanders develop over flood and delta plains

During a flood, a river may shorten its course by cutting across its meandering

loops, leaving behind a horse-shoe shaped channel as an ox-bow lake

Example – Ox-bow lakes are a common phenomenon in the floodplains of Lower

Mississippi, USA and Rio Grande (Mexico), Kanwar Lake Bird Sanctuary in Bihar,

India is one of Asia’s largest oxbow lakes.


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Meandering or river and Formation of Oxbow Lakes

Barrier Lakes

These lakes are formed by landslides, avalanches and such other processes

These processes cause damming up of the river by blocking the valleys

These lakes are short lived as the large piles of loose fragments soon give way

under the pressure of water. The sudden release of water from these lakes like

this can also cause floods

Example – Lake Gormire in Yorkshire, blocked by a landslide

6) Man-made lakes

Artificial lakes

Besides natural lakes, man has now created artificial lakes

Artificial lakes are created by erecting a concrete dam across a river valley

These dams help in creating a reservoir by impounding river water


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Guru Gobind Sagar Lake which supports the Bhakra Nangal Hydel Project is an

example of an artificial lake in India

c) Classification based on trophic level

Eutrophic Lake

Eutrophic lakes have very high levels of biological productivity.

The excessive level of nutrients, especially phosphorus and nitrogen gives rise to

an abundance of aquatic plants in these water bodies.

Usually, the water body will be dominated either by aquatic plants or algae.

Eutrophication might occur naturally or due to human impact on the

environment.

Some of Highly Eutrophicated Lake in India include Udaisagar Lake (Rajasthan)

and Dal Lake (Kashmir)

A Eutrophic Lake


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Mesotrophic Lake

Lakes with an intermediate level of productivity are called mesotrophic lakes.

The nutrients level of these lakes in medium or moderate.

They usually have clear water with submerged aquatic plants

Oligotrophic Lake

An oligotrophic lake is a lake with low primary productivity, as a result of low

nutrient content.

Algal production in these lakes is relatively low.

Often, they have very clear waters, with high drinking water quality

Paleolakes

A paleolake is a lake that existed in the past when hydrological conditions were

different.

Often, Paleolakes are identified based on relict lacustrine landforms such as

coastal landforms that form recognizable relict shorelines, referred to as paleo-

shorelines.

Paleolakes can also be recognized by characteristic sedimentary deposits that

accumulated in them and any fossils that these sediments might contain.

Evidence of prehistoric hydrological changes during the time of their existence

can be found from the sedimentary deposits of paleo-shorelines and paleo-lakes.

Types of Paleolakes

Former Lake – A former lake is a lake which is no longer in existence. Former lakes

include prehistoric lakes and permanently dried up lakes resulting

from evaporation or human intervention. A good example of a former lake is

Owens Lake in California, USA.


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Shrunken Lake – A shrunken lake is a lake which has drastically decreased in size

over geological time. A good example of a shrunken lake is Agassiz Lake, once

covering much of central North America.


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