Lithological Processes, Hazards and Management

1. Theory of Plate Tectonics

Sea-floor spreading: Hypothesis

o Mid-ocean ridges places of generation of new crusto Continents split magma welled up along line of rift cool and form crust

rift constantly replenished by upwellings from mantleo Ocean floors grow outwards from rifts

Supporting evidenceo Rock magnetism

Earth’s polarity reversed at least 9 times in past 4.5b years Time of formation of new basaltic oceanic crust magnetized in

direction of prevailing magnetic field fossil magnetism Parallel and symmetrical about ridge Alternating bands of rocks and reverse polarities Eg. Reykjanes Ridge Date magnetism of bands calculate rate of sea-floor spreading eg.

Iceland 1cm / year on each side of ridgeo Geothermal heat flow

Measured using thermistor probe Much hotter than normal over ridge due to injection of mantle

materialo Centres of seismic activityo Dating of volcanic activity

Eg. Iceland: most recent activity down centre of ridge, progressively older volcanoes to east and west of ridge

o Pattern of sedimentation Age and thickness of sedimentation increases with increasing distance

Subduction: Hypothesis: crust destruction Supporting evidence

o Most intense seismic activityo Pattern of earthquake foci

More recent activity at shallow depth Benioff zone about 45 degree angle: marks line of disturbance caused

by passage of ocean crust as it is subducted, zone of descending focio Geothermal heat flow

Cooler than normal over trenches due to cold crust descending into mantle

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Mantle convection currents: Crust attached to upper mantle and they move together Source of energy for tectonic movements: Earth’s internal heat transferred through

more liquid asthenosphere Hot material in mantle rises to base of lithosphere, move in lateral direction, cools

and descends to be reheated

2. Global Structural Landforms

Divergent plate boundaries: Zones of tension

o 2 convective flows in mantle are divergent viscous drag on overlying plate cause it to be torn apart

o Hot spots Heat flow well above surrounding average dome overlying crust

Rift valleyso Formation

Hot spots crustal stretching and formation of tensional cracks As plates move from area of upwelling, broken slabs displaced

downwards downfaulted valleys May be due to two parallel faults valley floor sink between 2

inward-facing scarpso Features

Fault: parallel, step, grid Block mountains and intervening basins eg. Basin and Range country

of western North America Horsts (upstanding) and graben (downthrown) eg. Rhine Rift Valley Volcanoes due to crustal weaknesses set up by faulting eg. Central

Icelandic Depression coincides with belt of most recent activity Lakes at depressions

o Eg. East African Rift Mid-ocean ridges

o Formation Rift valleys lengthening and deepening, extending into ocean

valley becomes narrow linear sea with outlet to ocean New sea floor continuously generated mid-ocean ridges

o Features 10 000s km long, 100s km wide 0.6km – 3km or more above sea floor Central rift running down middle of mid-ocean ridge eg. Red Sea Transform faults staggered paths

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Volcanic islands eg. Iceland: only major site where mid-ocean ridge appears above sea level

o Eg. Mid-Atlantic Ridge

Constructive Plate Boundaries: Zones of compression

o Oceanic-oceanic collision: denser plate subductedo Continental-oceanic collision: oceanic plate subducted eg. Nazca and South

American plateo Continental-continental collision: fused into a single block with a mountain

range eg. India and Asian plate Ocean trenches

o Long narrow troughs in ocean bed that mark zones of subductiono Fringes a continent due to continental-oceanic collision eg. Peru-Chile Trench

which borders west coast of South Americao Occur on deep ocean floor due to oceanic-oceanic collision, flanked by

volcanic island arc Continental volcanic arcs

o Continental-oceanic collision causing folding and faulting to form mountain chains like the Andes

o Descending oceanic plate reaches depth of about 100km, partial melting of water-rich oceanic crust and some overlying mantle occurs newly formed andesitic / granitic magma less dense than surrounding mantle rocks and will slowly rise emplaced in overlying continental crust cools and crystallises at depth magma may migrate to surface

o Found 100-400 km from trench Island arc volcanoes

o Formation of chains of islands during oceanic-oceanic collisiono Eg. Japanese Islands

Fold mountainso Result of continental-continental collisiono Eg. the Himalayas due to collision between Australian-Indian plate and

Eurasian plateo Sweeps up and deforms sediment accumulated along margins of both

continentso Intense folding and faulting

Transform Plate Boundaries: Zones of shearing limited construction and destruction Occurs along transform fault, marked by zones of intensely shattered rock Linear scars, offset stream channels, elongated ponds Eg. San Andreas fault

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Hot Spots: Flow of geothermal heat through crust from mantle considerably higher Sites of volcanism and lavas produced are rich in alkali metals Cause upwarping of curst forming domes Eg. Hawaiian Chain of Islands: only volcanoes above hot spot is active

3. Extrusive Volcanism

Components of Volcanic Eruptions: Lava: magma that reaches earth’s surface

o Basaltic (most common), andesitic, granitic (in increasing silica content)o Pahoehoe flows

Twisted and ropy structure Wrinkles due to advancement of still molten subsurface lava when

those at the top have started to congeal Nearer to volcanic eruptions

o Aa flows Surface of rough, jagged blocks with sharp edges and spiny

projections Surface of flow cools and forms a crust, interior molten so still

advances Hardened crust broken into angular blocks Further from volcanic eruptions

o Pillow lava Extruded underwater Lava chilled quickly brittle chilled surface cracks, making an

opening for still molten magma inside to ooze out like toothpaste cycle repeats pile of lava pillows

Pyroclastso Gases in highly viscous magma difficult to escape internal pressureo Once released, superheated gases expand thousand fold as they blow

pulverized rock and lava from vento Ash fall, ash flow, bombs, blockso Lahar eg. Mt St Helens: Toutle River: destroyed nearly all homes and bridges

along river Gases

o Pyroclastic flows: hot gases combined with large rock fragments and ash Due to hot, buoyant gases, can travel downslope in nearly frictionless

environment

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Types of Volcanic Eruptions: Magma and viscosity

o Viscosity directly related to silica content due to the strong bonds of networks that must be ruptured for flow to occur

Granitic lava viscous, short thick flows vs. basaltic lavao Higher temperature, lower viscosity

When lava flow cools, it begins to congealo Gas content: dissolved gases increase fluidity of magma and escaping gases

provide enough force to propel molten rock from vent Magma and nature of eruption

o Low pressure, less amount of gas magma can dissolveo Reduced confining pressure as magma moves from deep in the earth to near-

surface environment gases expand to 100s of times their original volume come out of solution and form bubbles

o Fluid basaltic magmas allow expanding gases to migrate upward and escape from vent easily vs. granitic magma of which gases collect as bubbles that increase in size and pressure till they explosively eject the semi-molten rock

o Viscous magma more likely to solidify within and clog up vent inhibits further rise of magma below and builds up pressure

Features of Extrusive Volcanism: Shield volcanoes

o Basaltic lavao Broad, domed structure with average surface slope of a few degreeso Wide base over 100km in diametero Small percentage of pyroclastic materialo Built by successive flows of basaltic lava, capable of flowing great distances

down gentle slopes, forming thin sheets of nearly uniform thicknesso Slopes slight near the summit because magma will readily run down slope,

but slope steeper further from summit so that lava can flow more easilyo Common in oceanic area like the Hawaiian Islands

Stratovolcanoeso Most violento Large and nearly symmetrical eg. Mayon volcano in Philippineso Inter-bedded lavas and pyroclastic deposits emitted from central vent

Extrude viscous lava for long periods, then ejects pyroclastic materialo Significant proportion made up of laharso Steep summits and more gently sloping flanks: concave slopes due to finer

pyroclasts found further away Cinder cones

o Pyroclastic material ejected into atmosphere then fall back to surface to accumulate around vent

o Small, steep-sided cones, may have large, bowl-shaped crater

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o Parasitic cones and occur in groupso Form within calderas of large volcanic mountains to represent final stages of

activity eg. Wizard Island in Crater lake, Oregon Basalt plateau

o Fissure eruption: extrusion of basaltic lava along extended fractureo Very fluid lava can remain molten long enough to flow 150km from source

creation of basalt plateau as basalt is resistant to erosion and the rock which basalt lava-flows overlie may not be so resistant basalt act as cap-rock

o Eg. Deccan Plateau of West India Calderas

o Circular depressions that exceed 1km in diametero Summit of volcanic structure collapses into partially emptied magma

chamber below

Volcanic Hazard Management: look at supplementary notes for more: Difficulties

o Authorities dislike the panic and disorders a large-scale evacuation entails and will be blamed for miscalculations

Eg. Mt Pelee reached climax in electoral period in 1902. While opposition urged evacuation, the governor demonstrated his solidarity with the citizens and his confidence in their future.

o Psychological barriers surrounding danger perception will be raised whenever predicted eruption fails to materialize

o More difficult to predict the end of eruptiono Authorities face problems with ensuring the comfort level and survival of

evacueeso ¾ of world’s most dangerous volcanoes in poor countries which spend

limited resources on famine relief and disease prevention Eg. Annihilation of the town of Armero with the eruption of Nevado

del Ruiz in 1985 due to the lack of disaster planning compounded by ineffective administration and conflicting advice from experts and non-experts

Vs. eg. Mt Pinatubo in Philippines in 1991: many lives saved due to successful communication between experts and the civil and military authorities

Vs. eg. Mt St Helens in 1980: evacuation, small numbers, better communications, danger more obviously understood

o Revival of extinct volcanoes eg. Mt Pinatubo in 1991

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4. Earthquake Hazards

Characteristics of Earthquakes: Faults

o Dip-slip faults (vertical displacement) Normal fault: hanging wall moves down relative to footwall Reverse fault: hanging wall moves up relative to footwall Results in fault scarp

o Strike-slip faults (horizontal displacement) Right-slip fault Left-slip fault

Seismic waveso Surface waves

Low frequency more likely to stimulate resonance in buildings Rayleigh waves: ripples Love waves: horizontal shearing of ground

o Body waves P-waves: parallel to travel direction S-waves: perpendicular to travel direction

Earthquake Measurement: Mercalli scale: measures earthquake intensity

o Variations in population densityo Building materials and methodso Distance from epicenter

Richter scale: measures earthquake magnitudeo Logarithmic

Effects of Earthquakes Intrinsic conditions: magnitude, type, location, depth Geologic conditions: distance from event, path of waves, types of soil Societal conditions: quality of construction, preparedness Tsunami

o Especially common for subduction earthquakeso Long period between waves: crests very high but troughs very lowo Eg. Indian Ocean tsunami

Landslideso Mountains formed along convergent plate boundaries which have steep

slopeso Soil layer on side of hill liquefies and flow as landslide

Liquefactiono Water-saturated sediment reorganized due to violent shaking

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o Sediment collapses, expelling water and causing the ground surface to subside

o Suspend grains in waterlogged soil so they lose contact and friction with other grains

o Liquefied sand layer can shoot to surface through cracks, forming sand boilso Eg. Mexico City earthquake

Fireo Water lines that feed the fire hydrant may be brokeno Fuelled by winds eg. typhoon: 30 000 people incinerated by fire stormo Dependent on time when earthquake strikes eg. lunch time when people are

using fire to cook foodo Eg. Tokyo earthquake: fire the biggest cause of death

Property damageo Worsened by population explosion, industrialization, building on soft or

filled-in soilo Eg. 26th December earthquake in Bam, Iran: complete collapse of the Bam

Citadel; high death toll because people were trapped when their mud-brick homes collapsed – completely disintegrated and buried people in piles of earth, rather than trapping them in voids of air pockets between building slabs, as would happen in the collapse of a concrete building

Seicheo Resonance in water bodyo Tide-like rises and drops in water levels in lakeso Swimming pools especially prone eg. 1994 Northridge earthquake

Fluctuation in underground hydrologyo Water in wells rise and dip in wavelike patterno Water levels may remain permanently changed eg. 1999 Izmit earthquake

5. Classification of Rock Types

Igneous rocks: Solidification of molten magma from mantle

o As molten magma cools, crystallization of minerals cause rock to solidifyo Nature of rock determined by mineral content and rate of crystallization

Most common rock type Intrusive igneous rocks

o Formed under surface as material from mantle forced its way through crust, cooling and solidifying as it moved

o Large crystals due to slow rate of cooling coarse-grained eg. granite Extrusive igneous rocks

o Formed above surface as lava erupted from volcanoes cooled on contact with atmosphere or sea

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o Smaller crystals fine-grained eg. basalt

Sedimentary Rocks: Clastic

o Material moves deeper as more sediment deposited above it hotter and greater pressure diagenesis because sediment in an environment where it becomes unstable cause clasts to be bound together and turned into rock

o Compaction: sediment squeezed by pressure of overlying sediment, water lost as porosity and permeability rate reduced

o Cementation: minerals crystallize in pore spaces from circulating fluids, cementing grains together

o Eg. sand laid down, lithified to give sandstone Non-clastic

o Not the result of weathered rock laid downo Chemical

Evaporation of water leaving salts behind / precipitation of salts around central point eg. flint

o Biological Remains of marine organisms subjected to diagenetic processes on

bed of warm, calm seas eg. limestone

Metamorphic Rocks: Forces produced by movement of plates Mineral stable at atmospheric temperature and pressure but becomes unstable as

the pressure and temperature increases mineral composition changed Contact metamorphism

o Source of extreme heat eg. igneous intrusion changes balance of minerals within rock

o Water lost with increased temperature, and minerals recrystallised or broken down into constituent parts

o Eg. marble from limestoneo As distance from intrusion increases, degree of alteration decreases due to

the decrease in temperature and duration of processes Eg. mudstone can be slate, schist or gneiss

Regional metamorphismo Increase in temperature and pressure due to forces at plate boundarieso Great pressure as plates converge / great heat and pressure as melted crustal

material forces its way upward through continental crust

Comparison of Rock Types: Heat and pressure: sedimentary, metamorphic, igneous Hardness: sedimentary (once cement dissolves, clast left), igneous (crystals can

interlock to form much stronger bonds than cementation of loose particles),

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metamorphic (undergone lots of compression which results in realignment of minerals – most resistant)

6. Weathering

Geometry of Rock Breakup Block disintegration: breaking down of rocks into large blocks

o Well-developed bedding planes with joints intersecting at right angles eg. sedimentary rocks like limestone

o Selective weathering along these lines of weaknesses detachment of large, frequently angular boulders

Granular disintegration: breaking down of rocks into numerous small fragmentso Crystalline rocks made up of numerous small crystals particular mineral

less resistant and selectively weathered loosen entire rock structure coarse sand in which each grain consists of single mineral particle separated from others

o Eg. granite Exfoliation

o Outer layer receives most of the sun, so it expands and contracts the most and will peel off first

o Detachment of rocks in concentric slabs from underlying rock mass, leaving behind successively smaller spheroidal bodies

Frost shatteringo Disintegration of rock along new surfaces of breakage to produce highly

angular rocks with sharp corner edges Spheroidal weathering

o Rounded by weathering form an initial block shape because chemical weathering acts more rapidly and intensely on corners

Weathering Processes: Physical weathering

o Application of physical force to break up rock superficial weatheringo Pressure release: exertion of physical stresses

Macro Rock that has been confined decompresses as load of rock and

regolith above it is removed by erosion Expansion generates stresses that will cause fractures if they

are greater than the strength of the rock Less serious: sheet joints More extreme: exfoliation then block disintegration Eg. granite

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Micro Release of strain energy sets up stress between minerals / in

the cement between grains granular disintegrationo Freeze-thaw

Macro Water penetrates joints and beddings freezes and expands

about 9%, exerting pressure on rock walls Closed system produced if freezing proceeds rapidly from

surface downwards seal in water contained within openings of rock maximum pressure easily exceeds tensile strength of rocks

Repeated stress widening of fissures block disintegration Micro

Water penetrates pore spaces between minerals especially for rocks of high permeability and porosity minerals may break along their boundaries granular disintegration

o Insolation weathering Disintegration of rocks caused by expansion and contraction through

solar heating and cooling Macro

Effects of diurnal heating confined to surface layers sharp thermal gradient surface of rocks expands more than rock at depth formation of stresses exfoliation in thin layers

Micro Dark-coloured minerals expand more than light-coloured ones

stress between minerals granular disintegration Rocks are bad conductors of heat and effect of heating can only

penetrate a few mm into rock exfoliation very small scale Blackwelder and Griggs experiment

After 244 simulated years of insolation weathering, no detectable disintegration

Only with some water content then breakdown is observedo Salt weathering

Coastal areas Water within rock saturated with salt through evaporation or

temperature change, some salt forced to crystallize and exert pressure salt crystal grows with cycles grains of sand dislodged

Arid environments High daytime temperatures, low and erratic rainfall, high

potential evaporation Water with contained salts drawn up by capillary action

water evaporates leaving salts behind as surface salt coatings or contained in rock pores and joints

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Thermal expansion of salts within pores / hydration of salts during high humidity / growth of salt crystals

Chemical weatheringo Alteration of chemical composition of rock mineralso Can occur at great depths since water can infiltrate and percolate regolitho Regolith produced predominantly fine-grainedo Solution

Highly alkaline and acidic water from soil moisture and groundwater more effective in dissolving rock

o Carbonation Weak carbonic acid from rainfall dissolves limestone as carbonate

minerals become bicarbonateso Hydration

Absorption of water into existing minerals Minerals may change colour / expand to weaken rock, show signs of

cracking, causing granular disintegrationo Hydrolysis

Reaction of mineral with water eg. feldspar reacts with water to produce potassium hydroxide (removed in solution) and alumino-silicic acid (which will break down into clay minerals)

o Oxidation Reaction of mineral with oxygen to form oxides / hydroxides eg. iron

Biological weatheringo Weathering by living organism or by-product of living organismo Root prising block disintegration eg. Angkorwat

Commonly seen in urban areas as roots prise through concrete to search for water, opening passageways that increases vulnerability to other forms of weathering

o Decay of organic matter produces variety of chemicals some organic acids dissolved as water passes through soil acidic chelation: breakdown of soil and eventual weathering of rock

Climate and Weathering: Humid tropics

o High precipitation and temperatureo Rapid chemical weathering

Van’t Hoff: speed of chemical reaction increase 2.5 times with each rise of 10 degree Celsius

Abundant soil moisture for chemical reaction to occur Dense vegetation and dead matter increases biochemical weathering Chemical weathering 4x more rapid than in temperate regions

o Physical weathering limited

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Masking effect of thick regolith of 30 – 60m comprising sand, clay and corestones: regolith removal less effective than regolith formation

Lack of exposures of solid rock Uniformly high temperature

Seasonally humid tropicso High summer temperatures and heavy seasonal rainfallo Chemical weathering more dominant than physical weathering

Less vegetation in savanna Formation of low domes and inselbergs as rapid stripping of regolith

partially exposes fresh rock of basal surface of weathering Hot arid

o High diurnal temperature and low precipitationo Physical weathering dominant through insolation and salt weathering

Shallow regolith: weathered material no time to accumulate before being removed

Temperateo Moderate temperature and rainfallo More physical than chemical weathering especially during wintero Shallow regolith: freezing rarely severe and does not penetrate to any great

depth Glacial

o Abundant snowfall and low temperatureo Physical more than chemical weathering

Freeze-thaw dominant process due to melt water Water from thawed snow contain much carbon dioxide as solubility

of carbon dioxide increased at low temperatures carbonation Peltier’s diagrams

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Strahkov’s diagram

Factors affecting Weathering: Rock strength and hardness

o Stronger and harder rocks more resistant to physical weatheringo Rock hardness

Hardness of mineral constituents Strength of cementation between minerals

o Rock age Harder if more time for cementation to take place Compression from young rocks above and from crustal movements

help to bind rock constituents more tightly Chemical composition

o Determines if minerals are susceptible to chemical changes and breakdowno Weathering of the rock itself or the cement

Rock textureo Coarse-grained

Unstable constituents selective chemical weathering reduces rock’s coherence

Large voids between grains high primary permeability increases possibility of trapping water for chemical and frost weathering

o Fine-grained Numerous boundaries between tiny materials of fine-grained rocks

provide lines of weaknesses along which weathering agents can penetrate

Rock structureo Joints: surface area of rocks that can be weathered increases due to high

secondary permeability

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Topographyo Gentle slope: weathered material may accumulate in situ and prevent access

to fresh rock retards physical weathering Altitude

o Places above tree-line experience more effective freeze-thaw due to reduced insulation from vegetation and soil

o Too high an altitude decreases diurnal cycles Aspect: impacts the sunlight received and temperature of the place

Landforms: Scree slopes and block fields

o Freeze-thaw weathering causes angular fragments of rocks to fall and accumulate at bottom of steep slopes

o On gentler slopes, fragments remain Exfoliation domes Limestone pavements: clints and grykes due to chemical weathering along joints

7. Mass Movement

Factors affecting Mass Movement: Gravity, angle of slope and shear stress

o Shear stress: force trying to enact movemento Gravitational force stronger on steeper slopes, pulls down regolith faster,

thinner regolith Nature of slope and shear strength

o Shear strength: resistance slope offers against movemento Sands and gravel: friction between particleso Silt and clay: cohesive forces between particles which is influenced by

moisture availabilityo Rock slopes: solidification and crystallization create very strong chemical and

physical bonds Role of water

o Increases shear stress but decreases shear strength Reduce cohesion between particles due to pore pressure Lubricates contacts between soil particles minimize friction and

allows them to slip over each other more easily, reducing resistance to gravity

Increases weight of sediment Triggering mechanisms

o Earthquakes: vibration reduces internal strength of sediments by shaking material loose from supported position and reduces cohesion

o Undercutting of slopes by streams

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Classification and Types of Mass Movement: Carson and Kirby’s classification [draw diagram] Soil creep

o Gravity creep: soil particles set in motion when disturbed by plant growth or activity of soil fauna

As one particle moves, dislodges other particles and so on till initial disturbance absorbed

o Soil heave: downslope movement of regolith Expansion and contraction due to heating and cooling / wetting and

drying / frost action Solifluction

o Summer: layer above permafrost melts and becomes unstable permafrost prevents downward drainage of melt water water-logged soil slide

Fallo Steep slope where angle of friction greatly exceededo Weathering done sufficient damage to allow block of rock to detach and fall

under gravity until it rests where the slope angle is low enough Slide

o Triggering mechanism usually neededo Moving mass moves down a slide plane until it reaches bottom of the plane

where impact usually breaks it upo Slide plane lubricated or selectively weathered

Slumpo Rotational movement along curved slip planeo Moisture concentrated at baseo Steplike terrace bounded by curved, wall-like scarp left behind

Flowo Soil moisture content higho Velocity greatest at surface and decreases to nil at bottom of masso Slope materials with high proportion of fine particles (more than 35%) most

proneo Internal deformation under its own weight, dependent on clay becoming

saturated with water to a percentage greater than the liquid limit Avalanches

Human Activities and Mass Movement Undercutting

o Construction of roadways especially in mountainous terraino Landslides common in the Himalayas, especially during the monsoon season,

but increasing number of roads and settlements built along foot of slopes overlooking river valleys eg. Himachal Pradesh, India in 1995

Constructiono Clifftop buildings: increase gravitational force applied to slope

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o Eg. Holbeck Hall Hotel in Scarborough in 1993 but it was following a number of drier than average summers, and a particularly wet spring

Top layers of cliff made of unconsolidated material deposited by retreating glaciers

Bounder clay had dried and cracked, so water can easily penetrate Foot of cliff attacked by sea, so not as stable

Deforestationo Eg. Hong Kong expansion onto marginal land to accommodate demands of

population pressureo Reduces stability of slope due to removal of binding properties of roots, and

exposes soil to direct impact of downpours

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