Overview of Physical Geography in 24 Hours

Introducing Physical Geography

The study of Geography explores many aspects of the human and physical environment.

This chapter introduces the discipline of geography and explains its many branches. The

scope of physical geography covered in this text is introduced. Geographers are also very

interested in the relationship between humans and the earth. In this first chapter linkages

are made between physical geography and some very important environmental issues

facing humankind.

Another aim of this chapter is to introduce you to some of the major ideas that help to

organize the study of physical geography. These are the ideas of spheres (or realms),

scales, systems, and cycles. The idea of systems is especially important as it is the

framework for studying Earth surface processes that is used throughout the book.

Geography is the study of the changing patterns and processes taking place at the

Earth’s surface.

Geographers also study human activities that take place on the Earth and the

relationships between human activities and the natural world.

Geography has two approaches,: regional geography and systematic geography.

Regional geography examines the characteristics of particular places on the

Earth.

Systematic geography looks for principles that allow us to explain and predict

the patterns and processes that we observe on the Earth.

Systematic geography can be divided into human geography and physical

geography.

Human geography examines economic, social and behavioral processes while

physical geography examines natural processes.

Physical geography includes climatology, geomorphology, coastal and marine

geography, geography of soils, and biogeography.

Hazard assessment and water resources bring together both human and physical

geography by studying how humans affect and are affected by the natural world.

Geographers use specialized tools including maps, geographical information

systems (GIS), and remote sensing to allow them to portray information that

varies spatially on the Earth’s surface.

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A systems approach considers the interconnections and flows of material and

energy in natural systems.

Physical geography is also concerned with the relationships between humans and

their environments. Environmental change is caused by both natural and human

processes. Some important topics of global change that physical geographers are

studying are global climate change, the carbon cycle, biodiversity, pollution,

and extreme events.

The natural systems and processes that are studied in physical geography can be

considered to operate in four great spheres (or realms): the atmosphere, the

lithosphere, the hydrosphere, and the biosphere.

The life layer is the focus of physical geography. It is the shallow surface layer where

the four realms (or spheres) interact and where most life forms are found.

Processes operating in the four spheres are studied at different spatial scales or levels

of detail. These range from global, through continental and regional, to local and

individual scales.

The processes studied in physical geography also operate at a range of time scales;

some act over millions of years while others act over seconds.

The processes of the four realms interact in a very complex way to shape the life layer.

Viewing these interactions as systems allows us to unravel and understand that

complexity.

A system is a set of things that are somehow related or organized.

Most natural systems are flow systems in which matter or energy flow along

pathways interconnected in a structure.

All flow systems have a power source.

Open flow systems have inputs and outputs, while closed flow systems do not.

Cycles are closed matter flow systems. In a cycle, a fixed amount of material is

continually recirculated through a series of pathways or loops.

Feedback in a flow system occurs when the flow in one pathway affects the flow in

another. Positive feedback increases flow while negative feedback reduces it.

Negative feedback in a flow system tends to produce stability or equilibrium.

Time cycles are periodic changes in system flow rates that occur over periods ranging

from hours to millions of years.

Studying the systems of the life layer and their interactions leads to a better

understanding of the human habitat, environmental problems, and global change.

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Chapter 1

The Earth as a Rotating Planet

This chapter deals with the way solar radiation drives energy and matter flows in the

atmosphere and oceans and how these flows are linked to weather and climate. This

chapter introduces you to some basic ideas about the Earth, its rotation, and revolution.

The Earth is shaped as an oblate ellipsoid because the Earth's rotation causes it to

bulge slightly at the equator.

The Earth rotates in an eastward direction.

The Earth's rotation has three important environmental effects:

1. It imposes a daily, or diurnal, cycle of daylight, air temperature, air humidity, and

air motion.

2. It produces the Coriolis effect which deflects the flow of fluids (air and water) to

the left in the southern hemisphere and to the right in the northern hemisphere.

3. Tides result from the moon’s gravitational pull on the side of the Earth closest to

the moon, creating a rise and fall of ocean water as the Earth rotates.

The Geographic Grid provides a system for locating features on the Earth’s surface

using parallels of latitude and meridians of longitude.

Latitude is the angular distance of a point north or south of the equator. It increases

from a minimum of 0° at the equator to a maximum of 90° at the north and the south

poles. Lines of latitude are parallel to each other and describe circles that decrease in

circumference away from the equator.

Longitude is the angular distance of a point east or west of the prime meridian at

Greenwich, England. It increases to the west and the east away from the prime

meridian (0°) to a maximum of 180°. Lines of longitude are farthest apart at the

equator and converge at the poles. All circles described by meridians of longitude are

the same circumference.

A map projection is a system for changing the curved/spherical geographic grid to a

flat grid.

Map scale relates distance on a map to distance on the Earth’s surface.

The polar projection produces a map with true shapes of small areas.

The Mercator projection shows true compass direction on any straight line on the

map and is useful for showing the flow of winds and ocean currents as well as lines of

equal air temperature and pressure.

The Goode projection is an equal area projection useful for depicting geographical

features that occupy surface areas.

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The standard time system is based on twenty four time zones that keep time

according to standard meridians that are spaced 15° apart and represent a time

difference of one hour.

The international date line is located near 180° longitude. Crossing this line in a

westward direction requires the calendar to be advanced by one day.

The Earth revolves counterclockwise around the sun every 365¼ days in an elliptical

orbit.

The Earth is closest to the sun at perihelion (~ January 3) and farthest from the sun at

aphelion (~ July 4).

The Earth’s axis of rotation is tilted 23½° away from the perpendicular and its north

pole always points towards Polaris (the north star).

The axial tilt and the revolution of the Earth around the sun combine to produce the

progression of the seasons.

At an equinox, everywhere on Earth experiences a 12-hour day and a 12-hour night.

At a solstice, polar regions experience either a 24-hour day or a 24-hour night.

The maximum solar radiation is received at the subsolar point which crosses the

equator twice in the course of a year as it moves between the Tropic of Cancer to the

Tropic of Capricorn.

Two important facts about the Sun-Earth energy flow system are that:

1. half of the Earth is always receiving solar energy.

2. Not all places on the Earth’s surface receive the same amount of energy.

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Chapter 2

The Earth’s Global Energy Balance

This chapter focuses on solar radiation which flows through the atmosphere to the Earth’s

surface. This energy is responsible for driving the Earth’s physical and biological

systems.

The Earth’s energy balance is the balance between the flow of energy reaching the

Earth and the flow of energy leaving the Earth.

Solar energy is the driving force for most natural phenomena at the Earth’s surface.

Electromagnetic radiation is emitted from all objects as a collection of wavelengths

traveling away from the surface of an object.

Two principles that govern the emission of electromagnetic radiation are:

An inverse relationship exists between the temperature of an object and the range

of wavelengths that object emits as electromagnetic radiation.

Hot objects radiate more energy than cooler objects.

The sun is a star of average size with a surface temperature of 6000° C generated by

nuclear fusion.

The solar constant is the amount of energy received per square meter just outside the

Earth’s atmosphere. The value is 1370 watts per square meter (1370 W/m2).

The sun emits a large amount of energy, concentrated in the ultraviolet, visible, and

shortwave infrared wavelengths. This is called short wave radiation.

The Earth is much cooler than the sun. It therefore emits less energy and emits that

energy as longwave radiation.

Insolation, or incoming solar radiation, varies with the angle of the sun above the

horizon and daylength.

Locations between 23½° north and 23½° south of the equator experience two

insolation maxima per year, while locations poleward of these latitudes experience

only one insolation maximum.

Locations poleward of the arctic and Antarctic circles experience daily insolation

values of zero for part of the year.

Daily insolation values are greatest at the pole during the summer solstice.

The seasonal pattern of daily insolation is the basis for dividing the Earth into world

latitude zones which include equatorial, tropical, subtropical, midlatitude, subarctic

(subantarctic), arctic (antarctic), and north (south) polar zones.

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Although the Earth’s atmosphere extends to approximately 10,000 kilometers above

the Earth, ninety-seven percent of the atmosphere lies within 30 kilometers of the

Earth’s surface.

Pure dry air consists of seventy-eight percent nitrogen and twenty-one percent

oxygen by volume. Argon, CO2, and other trace gases make up the remaining one

percent.

CO2 is a very important gas due to its ability to absorb radiant heat and its role in

photosynthesis.

The ozone layer is found in the stratosphere, where it absorbs ultraviolet radiation

and shields the Earth from the stratosphere’s harmful effects.

Human activity has increased the amount of gases such as chloroflourocarbons,

nitrous oxides, bromine oxides, and hydrogen oxides which are depleting the ozone

layer.

For every one percent decrease in global ozone, ultraviolet radiation may increase by

two percent.

Sensible heat is the quantity of heat held by an object that can be sensed by touch,

measured by a thermometer, and transferred by conduction from warmer to cooler

objects.

Latent heat is energy that is absorbed and stored when a substance changes state

from a liquid to a gas or a solid to a liquid. Latent heat is transferred when water

evaporates from a land or water surface and is important in moving large amounts of

energy from one region to another.

As solar radiation flows through the atmosphere, energy is scattered and absorbed

by gas molecules and dust particles in the air.

Clouds are a major factor in determining how much energy reaches the Earth’s

surface absorbing five to twenty percent and reflecting thirty to sixty percent of

insolation.

Albedo refers to the percentage of shortwave (SW) energy reflected by a surface.

The albedo of the Earth is twenty-nine to thirty-four percent.

CO2 and water vapor absorb incoming SW radiation and outgoing LW radiation from

the Earth. They re-emit this radiation in all directions with part of it returning to the

Earth’s surface in counterradiation, making the surface of the Earth warmer than it

would otherwise be.

Energy entering the Earth’s atmosphere is reflected by molecules, dust, clouds, and

the surface and absorbed by molecules, dust, and clouds leaving only forty-nine

percent of the incoming energy to be absorbed by the Earth’s land and water surfaces.

The energy entering the Earth’s system must be balanced by energy leaving the

Earth’s system. Energy leaves the Earth’s surface as longwave radiation as well as

through transfers of sensible heat and latent heat. Human changes to the Earth that

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affect albedo, cloud cover, or other aspects of the energy transfers may have an

impact on this balance.

Net radiation is the difference between all incoming and all outgoing radiation.

Although net radiation is zero for the Earth as a whole, it is positive between latitude

40° north and 40° south and negative poleward of these latitudes. As a result, global

and atmospheric circulation systems transport energy from lower to higher latitudes.

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Chapter 3

Air Temperature

This chapter focuses on the temperature of the air above the Earth’s surface. It examines

air temperature, its measurement, and the factors that cause it to vary through time and

space. This chapter also considers the topic of global warming.

Five important factors influencing air temperature are:

insolation

latitude

surface type

coastal versus interior location

elevation

Temperature is a measure of the sensible heat of a substance which changes as

energy flows across its surface.

When the net radiation of a surface – the balance between incoming shortwave and

outgoing longwave radiation – is positive, the surface temperature rises, and when net

radiation is negative, the surface temperature falls.

Heat energy can be transferred by conduction, by latent heat transfer, and by

convection.

The daily cycles of insolation and net radiation peak at solar noon, while the daily

cycle of air temperature peaks in the mid-afternoon.

Air temperature measured above an urban surface is usually higher than that over a

nearby rural surface.

The troposphere is the lower part of the atmosphere in which temperature declines

with altitude.

The environmental temperature lapse rate – the rate of temperature change with

altitude – averages 6.4° C per 1000 meters in the troposphere.

The troposphere is the zone in which most every-day weather phenomena (i.e.

clouds, storms, rainfall, snowfall) occur.

The top of the troposphere is known as the tropopause and is found about six

kilometers above the surface at the poles and sixteen kilometers above the surface at

the equator.

In the stratosphere, the absorption of ultraviolet radiation causes the temperature to

increase with altitude.

Daily air temperature cycles tend to be more pronounced in high elevation

environments.

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In a temperature inversion, the normal situation of air cooling with altitude is

reversed and air warms with altitude.

Yearly temperature range is greater in high latitude and continental locations and

less at equatorial and coastal locations.

The world patterns of isotherms are largely explained by latitude, coastal-interior

contrasts, and elevation.

Six important points about temperature patterns are:

Temperatures decrease from the equator to the poles.

Large landmasses in the subarctic and arctic develop centers of extremely low

temperatures in winter.

Temperatures in equatorial regions change little from January to July.

Isotherms make a large north-south shift from January to July over continents in

the midlatitude and subarctic zones.

Highlands are colder than surrounding lowlands.

Areas of perpetual ice and snow are intensely cold.

Five important points about temperature range are:

The annual temperature range increases with latitude.

The greatest ranges are in the subarctic and arctic zones of Asia and North

America.

Annual range is moderately large on land in the tropical zone.

Annual range in coastal areas is less than the range inland at the same latitude.

Small temperature ranges are found near oceans in the tropical zone.

Factors affecting global warming and cooling include:

greenhouse gases

tropospheric aerosols

cloud changes

land cover changes

changes in solar output

aerosols from volcanic activity

The warming effect of the greenhouse gases (carbon dioxide, methane, nitrous

oxides, ozone, and chloroflourocarbons) has exceeded the cooling effect of other

factors since about 1850.

Observations show both substantial annual variations in the average temperature of

the lower atmosphere and a pronounced trend toward warmer temperatures in recent

years.

Most scientists agree that the observed atmospheric warming trend is the result of

greenhouse gas emissions from human activities and that the warming trend will

continue into the future.

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Chapter 4

Atmospheric Moisture and Precipitation

This chapter considers the types and sources of moisture in the atmosphere. It examines

in detail the mechanisms by which atmospheric moisture becomes precipitation: in

particular, the important process of adiabatic cooling that occurs when air moves upward

in the atmosphere. This chapter also discusses the effect that human activities can have

on air quality.

Water exists in the atmosphere as water vapor, clouds, fog, and precipitation.

The movement of water between the land, the oceans, and the atmosphere is called the

hydrologic cycle.

Humidity refers to the amount of water vapor in the air.

The amount of water the air can hold depends on temperature. Warm air can hold

more moisture than cold air.

Specific humidity is the actual mass of water vapor per mass of air, usually stated in

grams of water vapor per kilogram of air. It is a measure of the amount of water vapor

that can be extracted from the atmosphere as precipitation.

The dew point temperature is the temperature at which relative humidity would be

100%. Condensation will occur if the temperature falls producing dew or frost.

Relative humidity is a measure of the amount of water vapor in the air expressed as a

percentage of the amount of water vapor the air can hold given its present temperature.

Precipitation results when a large mass of air is lifted and cooled to a temperature

below its dew point.

The adiabatic process causes heating or cooling solely by pressure change: air that

rises, expands, and cools as pressure decreases with altitude or air that descends,

encounters higher pressures, is compressed, and warms.

A parcel of air cooling without condensation cools at the dry adiabatic lapse rate of

10° C per 1000 meters (5.5° F per 1000 feet.).

Once air has cooled to its dew point, condensation releases latent heat, slowing the

rate of cooling to the wet adiabatic lapse rate which varies between 4° and 9° C per

1000 meters (2.2° and 4.9° F per 1000 feet) depending on the temperature and

pressure of the air and its moisture content.

A cloud is made up of water droplets or ice formed on tiny particles of matter called

condensation nuclei.

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Clouds are classified on the basis of height and form.

Clouds at ground level are called fog. Radiation fog forms when the temperature of the

air near the ground falls below the dew point. Advection fog occurs when warm moist

air is cooled below dew point as it moves over a cold surface.

Precipitation forms when either cloud droplets or ice crystals increase in size by

colliding with each other until they are heavy enough to fall.

Precipitation that occurs as a result of air being forced over a topographic barrier is

called orographic precipitation. Air that rises because it is warmer than the air around

it produces convectional precipitation, and air that is forced to rise over another air

mass produces cyclonic precipitation.

Thunderstorms are intense convectional storms associated with massive

cumulonimbus clouds. They may produce heavy rains, hail, thunder, lightening, and

intense downdrafts (microbursts) which may create hazards for humans.

Air pollutants are undesirable gases, aerosols, and particulates injected into the

atmosphere by human and natural causes.

The most important human source of pollutants is the combustion of fossil fuels for

the production of energy for transportation, heating, and industrial processes.

Urban air pollution produces smog and haze which reduce visibility and

illumination. Urban areas also experience more fog, cloudiness, and precipitation than

adjacent rural areas.

Acid deposition refers to acid rain and acidic dust particles produced by emissions of

sulfur dioxide and nitric oxide. Acid deposition is very damaging to natural

ecosystems.

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Chapter 5

Winds and Global Circulation

This chapter considers winds and ocean currents. It examines how unequal surface

heating and the rotation of the Earth generate global circulation systems in the

atmosphere and oceans.

The weight of air and the force of gravity pulling air towards the Earth create air

pressure. Air pressure is greatest at the Earth's surface and decreases with altitude.

Differences in pressure cause air to move horizontally. This air in motion is called

wind. Winds move from areas of high pressure to areas of low pressure.

Pressure differences between two places create pressure gradients and the resulting

pressure gradient force causes air to move from high pressure areas to low pressure

areas.

Land and sea breezes are examples of winds caused by pressure differences that

result from temperature differences over land and water surfaces.

Wind direction is measured by a wind vane, and wind speed is measured by an

anemometer.

The Coriolis effect is due to the Earth's rotation and causes objects in motion to

appear to be deflected off course. This apparent deflection is to the right in the

northern hemisphere and to the left in the southern hemisphere. The effect is absent at

the equator and increases as you move towards the poles.

Another force affecting the direction of wind is that of friction.

Air flow spirals into a low-pressure center and rises while the air descends and flows

out of a high pressure center.

The inward spiral at a low-pressure center is counterclockwise in the northern

hemisphere and clockwise in the southern hemisphere.

The outward spiral at a high-pressure center is clockwise in the northern

hemisphere and counterclockwise in the southern hemisphere.

Cyclones (low pressure centers) are associated with cloudy or rainy weather.

Anticyclones (high pressure centers) are associated with clear, dry weather.

At the equator, heating causes air to rise creating an area of low pressure called the

Intertropical Convergence zone (ITCZ).

At 30° latitude, air descends creating areas of high pressure in the subtropical high

pressure belt. Air moves out of these high pressure areas toward the equator creating

the Trade Winds. Winds also move toward the midlatitudes creating the Westerlies.

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The monsoon is a seasonally reversing wind pattern that brings heavy rains onto the

Asian subcontinent in summer and hot, dry conditions in the winter.

Winds at an altitude of five to seven kilometers above the Earth’s surface are

influenced by pressure gradient force and Coriolis force but not by the force of

friction. These winds are the geostrophic winds that flow parallel to isobars.

Rossby waves are large undulations in the flow of the upper air Westerlies along the

zone of contact between cold and warm air. They allow warm air to penetrate

northward and cold air to penetrate southward.

Jet streams are narrow bands of high velocity air that form primarily along the polar

front and above the Hadley cell in the subtropics.

The uppermost layer of ocean water is the warmest. Below this warm layer,

temperatures decline rapidly to around 0° and remains cold in a layer extending to the

ocean floor.

Ocean currents are persistent, mainly horizontal flows of ocean water set in motion

by the prevailing surface winds. Coreolis force causes the flows to be deflected about

45 ° from the direction of the wind.

Gyres are circular movements of water that are driven by the subtropical high pressure

cells.

El Niño occurs when warm water replaces the usual upwelling cold water that flows

along the South American coast. El Niño affects climate in other parts of the world.

Thermohaline circulation refers to slowly moving, deep ocean currents driven by the

sinking of cold, salty water in the northern Atlantic. This circulation is thought to play an

important role in the storage and release of CO2.

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Chapter 6

Weather Systems

This chapter examines the way in which atmospheric circulation processes generate the

daily variations in temperature, humidity, cloudiness, windiness, and precipitation that we

know as weather.

An air mass is a large body of air with a similar temperature, moisture, and lapse rate

characteristics over thousands of kilometers.

The air mass characteristics are acquired in source areas where the air remains for

some time allowing it to acquire the characteristics of the surface over which it rests.

Air masses are classified on the basis of the latitude and the surface type of the source

area. The main air mass classes are:

mT maritime tropical

mE maritime equatorial

cT continental tropical

mP maritime Polar

cP continental Polar

cA continental Arctic

cAA continental Antarctic

A front is a boundary between one air mass and another. The leading edge of cold air

advancing into an area is called a cold front. Warm air moving into an area of cold

air is called a warm front.

An occluded front develops when a cold front overtakes a warm front and forces

warm air aloft.

Cyclonic precipitation can occur when moist air is forced aloft and adiabatically

cooled in the convergent, upward flow of a cyclone.

An important weather system affecting middle and high latitudes is a traveling low

pressure system called a wave cyclone that develops along the polar front.

Wave cyclones move from west to east and the interaction of warm and cold fronts

within the cyclone often produces cyclonic storms.

A tornado is an intense low pressure system with very high wind speeds. Tornadoes

occur in association with thunderstorms that develop along cold fronts and with

hurricanes.

A weather system associated with tropical areas is the easterly wave, a low pressure

trough into which air converges and is lifted producing precipitation.

A polar outbreak occurs when cold polar air forces its way into very low latitudes,

bringing storms followed by cold, clear weather.

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Tropical cyclones, hurricanes, and typhoons are all names for powerful storms

which develop over warm ocean surfaces between 8° and 15° latitude, migrate

westward, and curve toward the poles.

Tropical cyclones often create tremendous damage due to high winds, high waves,

flooding, and heavy rains.

The atmospheric circulation transfers heat and moisture from equatorial regions

toward the Polar Regions by the Hadley cell circulation and Rossby waves.

The thermohaline circulation within the oceans is another important mechanism by

which heat is transferred from the equatorial to the polar regions of the Earth.

An important element of climatic change studies is the positive and negative

feedbacks between surface temperature and cloud cover.

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Chapter 7

Global Climates

This chapter brings together many of the ideas developed in previous chapters and

considers climate at the global scale. It introduces the processes and factors that produce

different climates on the Earth.

The climate of an area refers to the average weather conditions over a long period of

time based on measurements of temperature and precipitation.

Important principles to help you understand climate are:

Low latitude locations have warmer temperatures and smaller annual temperature

ranges than high latitude locations.

Continental locations tend to have much larger annual temperature ranges than

coastal locations at the same latitude.

Colder locations tend to have less precipitation than warm locations, because

warm air can hold more moisture than cold air.

The basic control on temperature is latitude, while the effect of a continental or

maritime location is an important secondary control.

Some generalizations about precipitation are:

Pressure systems and the global circulation are the major determinants of

precipitation patterns.

The equatorial region experiences high convectional precipitation because of

heating. These areas of high precipitation extend north and south along the east

sides of the continents because the trade winds bring moisture onto the land.

The subtropical high-pressure cells, characterized by dry, subsiding air, produce

arid and semiarid regions.

Mountain ranges produce wet areas where air masses are forced to rise over the

mountains creating an orographic effect.

Coastal mountains also act as barriers to moisture producing a rainshadow effect

on the lee side of the mountains.

Continental interiors tend to be dry because they are far away from the source

areas of moist air masses.

Three types of annual precipitation patterns are:

Uniformly distributed precipitation

Precipitation maximum during the warmest period of the year

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Precipitation maximum during the coolest period of the year

Seven global precipitation regions can be identified based on these principles:

Wet equatorial

Trade wind coasts

Tropical Deserts

Midlatitude deserts and steppes

Moist subtropical

Midlatitude west coasts

Arctic and Polar Deserts

Low latitude climates are dominated by cT, mT, and mE air masses. The position of

the ITC and the subtropical high pressure cell affect these climates throughout the

year. Weather disturbances include the easterly wave and tropical cyclones. This

group of climates includes:

Wet equatorial

Monsoon and trade-wind coast

Wet-dry tropical

Dry tropical

Midlatitude climates occupy the polar front zone where warm and cold air masses

conflict producing wave cyclones. This groups of climates includes:

Dry subtropical

Moist subtropical

Mediterranean

Marine west-coast

Dry midlatitude

Moist continental

High latitude climates are dominated by polar and arctic air masses. They are

sources areas of cP, mP, cA, and cAA air masses. Continental polar air meets cA air

along the arctic front zone. This group of climates includes:

Boreal forest

Tundra

Ice Sheet

Dry climates are those in which the potential total annual evaporation greatly

exceeds the annual precipitation amount.

Low latitude climates:

occupy the equatorial zone, much of the tropical zone, and some of the

subtropical zone

include climates that range from very wet to very dry

are influenced by the intertropical convergence zone, tropical easterly wind

systems, and the subtropical high pressure cells

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experience traveling low pressure systems such as the easterly wave and tropical

cyclones

The wet equatorial climate is characterized by:

dominance of the intertropical convergence zone (ITC)

mE and mT air masses

uniform, very warm temperatures in all seasons

ample precipitation, heaviest when the ITC is nearby

Monsoon and trade wind coastal climates are characterized by:

heavy rainfall with strong seasonal patterns

a larger temperature range than the wet equatorial climate

dominance of the ITC during the heavy rainfall period and the subtropical high

pressure system during the dry season

trade wind coast climates are a result of mT and mE air masses blowing onto

coastal areas bringing large amounts of moisture

the monsoon aspect of these climates is a result of the changing position of the

ITC and reversing pressure gradients

heavy rainfall is associated with the ITC and an airflow from ocean to land, while

the dry season is associated with airflow off the Asian continent to the ocean

The wet equatorial, monsoon, and trade wind coastal climates produce low latitude

rainforest with dense vegetation, numerous streams, and a great diversity of plant

and animal life.

Products and resources of the rainforest include lumber, drugs, rubber, and foods

such as cassava, yams, taro, bananas, plantain, and coconuts.

The wet-dry tropical climate is characterized by:

a warm climate but with a more marked temperature range

during the high sun season, proximity to the ITC brings heavy rains

during the cooler period, the subtropical high pressure cell produces very dry

conditions

vegetation adapts to the seasonality of rainfall and is described as rain-green

because it enters a dormant period during the dry season and leafs out and blooms

in the rainy season

dramatic variations in rainfall are reflected in streamflow which varies from very

low flows to flood-like conditions

agriculture experiences periodic drought

The dry tropical climate:

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is dominated by the Subtropical High Pressure Cell

experiences very low precipitation and intense daytime heating under

predominantly clear skies

includes many of the world's great deserts

semi-arid areas on the edges of the desert may have a short wet season. These

steppe areas are transitional from the desert to the wet-dry tropical climate

Midlatitude and high latitude climates as a group:

occupy the midlatitude zone, part of the subtropical zone, and extend poleward

into the subarctic latitude zone

are located mainly in the northern hemisphere

are affected by the poleward portion of the subtropical high pressure cells, the

westerly wind belt, and the conflict between warm and cold air masses that occurs

along the polar front zone

The dry subtropical climate:

is a poleward extension of the dry tropical climate but shows a greater annual

temperature range due to its higher latitude

experiences a cool or cold season influenced by invasions of air from higher

latitudes

receives occasional precipitation from midlatitude cyclones

is divided into arid and semi-arid subtypes

has more abundant vegetation than the dry tropical climate due to lower

temperatures and slightly more precipitation

supports plants and animals that have adapted their life cycles to take advantage of

infrequent periods of rain

supports agriculture only under irrigation

The moist subtropical climate:

is created by warm, moist air flowing out of the subtropical high pressure cells

onto the eastern sides of the continents

has abundant summer rainfall, mainly convectional with an occasional tropical

cyclone

in Southeast Asia experiences a strong monsoon effect

receives winter precipitation from wave cyclones while the storm tracks are in

their most southerly position

abundant yearly precipitation provides ample water for urban and industrial

development but may cause flooding

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supports broadleaf deciduous and evergreen forests and some needle-leaf and pine

forests

soils are depleted by high rainfall as nutrients are washed out of the soil

The Mediterranean climate:

experiences a very dry summer due to migration of the subtropical high-pressure

cell into the area.

winter is dominated by rainfall provided by CP air masses and cyclonic storms

has a moderate temperature range

is limited to narrow coastal zones

is a pleasant climate for humans although there are problems with water

availability in summer and plants must adapt to the dry summer period

The marine west coast climate:

experiences mild temperatures with a small temperature range for its latitude

is a moist climate with a winter precipitation maximum due to frequent cyclonic

storms; in summer the northward movement of the subtropical high pressure cell

reduces precipitation

supports needle-leaf forests in the wet mountainous areas of Pacific North America

and deciduous trees in the relatively drier areas in Europe

The dry midlatitude climate:

influences the interior regions of North America and Eurasia

in some areas, the rainshadow effect blocks maritime air masses so drier

continental air masses dominate

summer rainfall is largely convectional associated with occasional maritime air

masses

has a strong annual temperature range with warm to hot summers and cold to very

cold winters

includes arid and semi-arid environments ranging from cold desert to steppes

soils have a high natural fertility and support a vegetation of short grass prairie

with moisture being the limiting factor

experienced serious land degradation during the drought years of the 1930s after

vast areas of land in this climate type were broken for agriculture

The moist continental climate:

is found in central and eastern North America and Eurasia

exhibits large seasonal temperature variation as well as strong day-to-day variation

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receives ample precipitation peaking in the summer with mT air masses while the

winter is dominated by cP and cA air masses

East Asia experiences a monsoon effect which increases summer precipitation

supports a native vegetation of deciduous forest which grades into tall grass prairie

toward continental interiors

soils show some leaching and acidity due to an abundance of precipitation

supports large scale agriculture

The high latitude climates:

are located in the westerly wind belt

are influenced by mP air masses conflicting with cP and cA air masses and wave

cyclones which develop along the arctic-front zone

experience higher summer precipitation brought in by mT air masses

The boreal forest climate:

has long, bitterly cold winters and short cool summers

experiences a very large annual temperature range as a result of its continental

location

is a source region for cP air masses, and invasions of cA air masses are common

has low total annual precipitation with a summer precipitation maximum produced

by maritime air masses

supports a native vegetation of needle leaf trees which supply the pulpwood and

lumber industry; agriculture is practiced along milder coastal areas

The tundra climate:

is found along arctic coastal areas

experiences long severe winters dominated by cP, mP, and cA air masses

has a smaller temperature range than expected for its latitude due to the

moderating effect of the nearby ocean

vegetation consists of grasses, sedges, lichens, and some shrubs; species diversity

is low but the number of individuals is high

soils are poorly developed and are underlain with permafrost

Permafrost is permanently frozen ground overlain by an active layer that thaws

during the summer.

The tundra climate is cold enough to create continuous permafrost, frozen ground

with few gaps or interruptions. Discontinuous permafrost occurs in patches and is

found in the boreal forest climate.

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The ice sheet climate:

is the source region of arctic and antarctic air masses

occurs on the ice sheets of Greenland and Antarctica and over the Arctic ocean ice

experiences the lowest mean annual temperature; no month has a mean

temperature above freezing

has very low precipitation which doesn’t completely melt away because the air is

too cold

is a very harsh environment devoid of soils and vegetation, and the few species of

animals found in this climate are marine oriented

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Chapter 8

Biogeographic Processes

Biogeography explores the distribution of plants and animals on the Earth. This chapter

examines how organisms live in ecosystems and the cycling of energy and matter through

ecosystems.

This chapter also explores ecological biogeography by examining factors that determine

the spatial distributions of organisms in time and space. We look at processes such as

evolution, dispersal, and extinction of species through time.

Ecology studies the interaction between life forms and their environments.

An ecosystem is defined as a group of organisms and the environment with which

they interact. These systems import and export matter and energy.

The food web, or food chain, refers to the flow of energy from one level to another in

an ecosystem.

Primary producers are plants and animals that are able to create carbohydrates from

carbon dioxide and water and light energy through the process of photosynthesis.

In the food web, consumers feed on the primary producers or on other consumers and

transfer energy through different levels in this manner.

Decomposers (microorganisms and bacteria) feed on decaying organic matter at all

levels in the food web.

Solar energy is absorbed initially by the primary producers and stored as chemical

energy which is digested by consumers. Only ten to fifty percent of the energy at any

level is passed on to the next level; consequently, the amount of organic matter and

consumers must decrease with each level.

Photosynthesis is a biochemical reaction which results in the production of

carbohydrates and oxygen using water, carbon dioxide, and light energy. A simplified

chemical reaction is:

H2O + CO2 + light energy = —CHOH— + O2

In the respiration process carbohydrate is broken down and combined with oxygen to

create carbon dioxide, water, and chemical energy. A simplified chemical reaction is

—CHOH— + O2 = CO2 + H2O + chemical energy

Photosynthesis is dependent on light and heat. Photosynthesis only occurs when light

is available, so longer days produce more plant growth. Photosynthesis also increases

with temperature to about 20°C and then levels off.

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Net photosythesis is measured as the carbohydrate remaining after respiration takes

up carbohydrate to feed the plant. Net photosynthesis increases with temperature until

approximately 18° C, after which it declines as the rate of respiration increases faster

than the rate of photosynthesis.

Net primary production is the annual amount of useful energy produced by an

ecosystem. It is controlled by light intensity and duration, temperature, and water

availability. Net primary production is measured as biomass, the dry weight of

organic matter per unit area within an ecosystem.

Biomass is an important source of renewable energy. Using biomass involves

releasing solar energy that has been stored in plant tissue through photosynthesis.

Energy can be obtained by burning firewood, or through intermediate products such as

charcoal, methane gas, and alcohol.

Biochemical cycles are the pathways of particular nutrients or materials through the

Earth's ecosystem.

The macronutrients hydrogen, carbon, and oxygen account for 99.5% of all living

matter.

The Carbon Cycle

most carbon lies in storage pools as carbonate sediments

only 0.2% is available as CO2 or as decaying biomass in active pools.

carbon exists as carbon dioxide in the atmosphere and oceans, carbohydrate in

organic matter, hydrocarbon compounds in rock, and as mineral carbonate

compounds.

CO2 is added to the Earth system by volcanic eruptions and by industry: it is taken

out of the Earth system by plants in photosynthesis and by phytoplankton in the

oceans.

The Oxygen Cycle: oxygen is added to the Earth system by volcanic activity and is

lost to the system through organic respiration, mineral oxidation, industrial and natural

combustion, and dissolved in ocean water.

The Nitrogen Cycle

the atmosphere is a large storage pool of nitrogen.

nitrogen can only be utilized through nitrogen fixation and is lost to the biosphere

through denitrification.

human influence has increased the amount of nitrogen in the biosphere through the

use of nitrogen fertilizers and fuel combustion.

Sedimentary cycles involve many macronutrients such as calcium, magnesium, iron,

potassium, sodium, and phosphorus which move from the land surface to the ocean

and subsequently return to land surfaces by tectonic uplift. Storage pools include sea

water, sediments, and sedimentary rocks. Eventually these macronutrients are

released into the Earth system through weathering.

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Ecosystems are strongly influenced by landforms and soils.

Habitat refers to the preferences of a species for a particular location including such

factors as conditions of slope, water drainage, and soil type.

Ecological niche refers to the functional role played by an organism, as well as the

physical space it inhabits. Many species may occupy the same habitat, but only a few

will ever share the same ecological niche.

A community is an assemblage of organisms that live in a particular habitat and

interact with each other.

The most important environmental factors influencing the location of species are

moisture and temperature.

Species have a variety of adaptations to help them cope with the abundance or scarcity

of water. Xerophytes are plants adapted to dry conditions.

Temperature affects physiological processes in plants. Plants have a temperature

range within which they can survive as well as optimum temperatures for each of their

functions.

Climatic factors of moisture, temperature, light, and wind are important in determining

plant distributions. Bioclimatic frontiers are boundaries that mark the limits of the

potential distribution of a species.

Geomorphic factors influencing ecosystems include slope steepness and slope aspect.

Edaphic or soil factors are also important in differentiating habitat.

Species interactions also determine the distribution patterns of plants and animals.

Interactions may be positive or negative. These include:

Competition between species occurs when two species require the same resource

that is in short supply.

Negative interactions include predation, parasitism, herbivory, and allelopathy.

Symbiosis includes three types of positive interaction between species:

commensalism, protocooperation, and mutualism.

Ecological succession is a development sequence in which plant communities, or

seres, succeed one another as they progress to a stable climax, the most complex

community of organisms possible in an area.

Succession starts with pioneer species that can survive in harsh conditions. These

pioneers moderate the harsh conditions and gradually other species move in.

Disturbances that can interrupt the sequence include fires, insects, disease, and

human activities such as cutting and clearing.

Four key historical biogeography processes that affect the distribution of species are

evolution, speciation, extinction, and dispersal.

Patterns of distributions include endemic species, which are found in one region and

no where else, cosmopolitan species which are found widely, and disjunction, in

which closely related species are found in widely separated regions.

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Biodiversity is determined by the variety of the Earth’s environments, as well as the

processes of evolution, dispersal, and extinction through geologic time. Human

activity on Earth is rapidly decreasing biodiversity by contributing to extinctions

through dispersing competing organisms, hunting, fire, habitat alteration, and

fragmentation.

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Chapter 9

Global Biogeography This chapter focuses on global patterns of vegetation and the characteristics of the

vegetation found in the biomes of the Earth.

Natural vegetation is the plant cover that would establish itself in an area without

human interference. Many areas of the Earth have been modified by humans, but

large areas of natural vegetation still exist in more inaccessible areas.

The life form of a plant refers to its physical structure, size, and shape. Lifeforms

include trees, shrubs, lianas, herbs, and lichens.

Biomes are the largest recognizable subdivision of terrestrial ecosystems. These

include the forest, savanna, grassland, desert, and tundra biomes.

The Forest Biome includes six major types of forest:

The low latitude rainforest is found in the equatorial and tropical zone which

experiences continuously warm temperatures with consistently abundant rainfall.

These ideal conditions produce a forest of tall, closely set trees with a multilayered

canopy and the largest diversity of species of any lifezone.

The monsoon forest grows in a wet-dry tropical climate. The stress of the dry

season results in a deciduous forest that sheds its leaves. The monsoon forest has

an open canopy allowing more development in the lower forest layers.

The subtropical evergreen forest is associated with the moist subtropical climate.

The native vegetation consists of broadleaf and needleleaf evergreen trees although

little natural forest remains due to agricultural development.

The midlatitude deciduous forest has a tall dense canopy in summer but sheds its

leaves in winter in response to the cold temperatures.

The needleleaf forest consists of a few species of tall cone-shaped mostly

evergreen coniferous trees. These trees create a continuous deep shade at ground

level which inhibits the growth of shrubs and herbs. The needleleaf forest is

associated with the boreal forest climate and the high elevations of mountainous

areas.

The schlerophyll forest develops in the Mediterranean climate. The trees have

adapted to the dry, hot summers by producing small, hard, thick leaves that

minimize water loss.

The Savanna Biome is a product of the tropical wet-dry climate. Vegetation changes

from woodland to thorntree grassland with increasing dryness. Adaptations to dryness

include deciduous habit and small leaves or thorns.

The Grassland Biome is found in the midlatitude and subtropical zones which have

well developed winter and summer seasons. The biome includes both tall-grass prairie

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and steppe. Steppe vegetation grows in the semi-arid subtype of the dry continental

climate.

The desert biome includes both desert and semi-desert subtypes. The area of semi-

desert ranges from the tropical to the midlatitude zone. The vegetation includes sparse

xerophytic shrubs and in some areas thorny trees and shrubs are adapted to a long hot

dry season with a short wet season. Desert vegetation ranges from small, hard leafed

or spiny shrubs, succulent plants, and hard grasses to many areas with no vegetation

covered by shifting sands and salt flats.

The tundra biome is found at high latitudes and high elevations. Plants include low

herbs, dwarf shrubs, sedges, grasses, mosses, and lichens. At high latitudes plant

growth is influenced by long winters with little light and short cool summers with very

long days. Permafrost underlies the surface and restricts drainage and root

development.

As elevation increases, temperatures decrease and precipitation increases, leading to a

sequence of vegetation zones or life zones related to altitude.

Climate changes with latitude and longitude are reflected in changes in vegetation.

These changes are gradual.

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Chapter 10

Global Soils

This chapter looks at the life layer where physical and biological processes interact. It

examines soil characteristics, the processes of soil formation, and the global pattern of the

distribution of soil that develops as the interface between the lithosphere, the biosphere,

and the atmosphere.

Soil is a complex mixture of solids, liquids, and gases. It contains mineral material

derived from the parent material and organic matter derived from living plants and

other organisms in the soil.

The major soil-forming factors are parent material, climate, vegetation, and time.

Soil characteristics develop over very long periods of time.

Soil color is the most obvious characteristic of soil. Color may be inherited from

parent material, but it is often a result of soil forming processes.

Soil texture refers to the proportion of soil particles that fall into each of three size

grades: sand, silt, and clay.

Soil colloids are the smallest particles in soils. They are important because they hold

plant nutrients in the soil in the form of ions.

Soil pH can range from acid to alkaline.

Soil structure refers to the way in which the soil grains are bound together by colloids

into peds.

Chemical and organic processes in soils change primary minerals into secondary

minerals such as oxides and clay minerals.

The nature of the clay minerals determines a soil’s base status, which, in turn, affects

its fertility or ability to retain nutrients.

The storage capacity of a soil is the maximum amount of water that it can retain by

capillary tension when it is allowed to drain. The wilting point is the water storage

level below which plants cannot access water. The difference between the storage

capacity and the wilting point is the available water capacity for plants which varies

with soil texture.

Water stored within a soil is depleted by evaporation and transpiration until it is

recharged by precipitation.

The soil water balance describes the inter-relationship between water need (potential

evapotranspiration), water use (actual evapotranspiration), precipitation, and storage

in the soil water zone.

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A soil water budget is a numerical accounting, usually done on a monthly basis, of

the soil water balance components. It determines the amount of water plants need for

optimal growth.

Most soils have distinct horizontal layers, known as soil horizons, that develop by the

processes of enrichment, removal, translocation, and transformation.

Soil forming processes include:

Enrichment - the addition of organic and mineral material to the soil by

sedimentation and biological activity.

Leaching - the removal of dissolved material in percolating soil water, and by

erosion.

Translocation - the removal of material from upper horizons (eluviation) and its

accumulation in lower horizons (illuviation).

Humification - an important transformation process in which organic material is

decomposed to humus.

Soil classification systems are used to study the distribution of soils and their

relationship to a variety of environmental factors.

The U.S. Comprehensive Soil Classification System groups the soils of the world into

11 soil orders that are distinguished primarily by the presence of diagnostic horizons.

Seven soil orders (Oxisols, Ultisols, Vertisols, Alfisols, Spodosols, Mollisols, and

Aridisols) have well-developed horizons and can often be associated with particular

climatic regimes.

One soil order, Histosols, includes soils with a large proportion of organic matter.

Three soil orders (Entisols, Inceptisols, and Andisols) have poorly developed

horizons or are capable of further mineral alteration.

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Chapter 11

Earth Materials

This chapter deals with the systems and cycles of the solid Earth. This chapter discusses

the basic materials of the solid Earth – rock and minerals – and some principals of their

formation. These are linked to the cycle of rock change which describes how different

rock types develop as Earth materials are cycled and recycled through geologic time.

The elements oxygen and silicon account for about seventy-five percent of the

Earth’s crust, while the metallic elements iron, aluminum, and the base elements

account for most of the rest.

The elements of the crust are combined in inorganic chemical compounds called

minerals.

These minerals are mixed together in various proportions to form many kinds of rock.

The rocks of the Earth’s crust are grouped into three major classes: igneous,

sedimentary, and metamorphic rocks.

Igneous rocks form when molten material from the Earth’s interior cools and

solidifies in the crust.

Magma cooled slowly below the surface forms coarse-textured intrusive igneous

rocks. Lava cooled rapidly at the surface forms fine-textured extrusive igneous rocks.

Igneous rocks consist mainly of silicate minerals containing silicon, oxygen, and

metallic elements. The kind of metallic elements present determines the mineral

density.

Less dense felsic minerals dominate the igneous rocks of the upper crust, while more

dense mafic and ultramafic minerals dominate those of the lower crust.

Silicate minerals undergo chemical changes called mineral alteration when exposed

to air and water at the Earth’s surface.

Most clay minerals are produced by mineral alteration.

Weathering, or the breakdown of rocks into smaller particles known collectively as

sediment, occurs through both mineral alteration and physical disintegration.

Layers of mineral sediment and organic matter accumulate in oceans and low-lying

land areas to be compacted and hardened into sedimentary rocks. Different types of

sediment produce different kinds of sedimentary rock.

Igneous and sedimentary rocks can be altered by heat and pressure to form

metamorphic rocks.

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The cycle of rock change describes the circulation of rock material between the

Earth’s interior and the crust. This is a very slow process powered by the heat of

radioactive decay deep within the Earth.

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Chapter 12

The Lithosphere and Plate Tectonics

This chapter introduces the grand cycle of plate tectonics. This cycle explains how the

continents and ocean basins of the Earth’s surface slowly change as forces deep within

the Earth cause vast tectonic plates to converge, collide, split, and separate.

The solid Earth has a layered structure: a dense, metallic core surrounded by a

mantle of ultramafic rock which is covered by a crust with mafic rocks exposed on

the ocean floor and felsic rocks exposed on the continents.

Geologists use the term lithosphere to refer to the Earth’s outer layer of rigid, brittle

rocks which extend through the crust to the upper mantle. Beneath the lithosphere lies

the asthenosphere: the layer of soft, plastic rock in the mantle.

The history of the Earth can be subdivided into various time intervals using the

geologic time scale. Most of the landscape features of the Earth’s surface developed

during the Cenozoic Era which began sixty-five million years ago.

The major relief features of the Earth’s surface are the continents and ocean basins.

The continents contain young, dynamic alpine belts and old, stable continental

shields.

The ocean basins consist of extensive, smooth abyssal plains marked by long, narrow

midoceanic ridges.

Shallow continental shelves are found beneath the ocean next to continental shields,

while deep oceanic trenches are found adjacent to alpine belts.

The two basic tectonic processes are compression and extension.

The lithosphere is broken into six great plates and several lesser plates that move

relative to one another.

Spreading boundaries exist where plates move apart, converging boundaries where

they collide, and transform boundaries where they move past one another.

Spreading boundaries are marked by midoceanic ridges on the ocean floor and rift

valleys on continents. New ocean crust is formed along spreading boundaries.

Subduction occurs where continents meet the ocean floor along a converging

boundary and the denser rock of the ocean floor plunges beneath the continent. An

oceanic trench marks the subduction zone.

Subduction along continental margins leads to the formation of island arcs and

alpine belts as subducted ocean crust melts and rises to the surface in volcanoes, and

sediment from the ocean floor is folded and faulted as it accumulates on the

continental margin.

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Plate movement is thought to be driven by convection currents in the plastic rock of

the asthenosphere.

The plate tectonics cycle ties together major relief features, volcanic and Earthquake

activity, and patterns of rock age and type at the Earth’s surface.

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Chapter 13

Volcanic and Tectonic Landforms

This chapter examines some of the landforms produced by the internal Earth processes of

volcanic activity, the folding and faulting of crustal rocks, as well as Earthquake activity.

The surface forms of the Earth’s crust are known as landforms; geomorphology is

the study of the processes which produce these features.

Initial landforms are formed by volcanic and tectonic activity, while sequential

landforms are formed by the agents of denudation such as running water, waves,

wind, and glacier ice.

Lava eruptions at the Earth’s surface form volcanoes and lava flows.

Stratovolcanoes form from the eruption of thick, gassy, felsic lavas and are most

common along the converging plate boundaries of the Pacific rim. They have steep

sides and often have explosive eruptions that form calderas. Stratovolcanoes may

also emit a glowing cloud of white-hot gases and fine ash that travels very rapidly,

searing everything in its path.

Broadly rounded shield volcanoes form over hotspots and along midoceanic ridges

where more fluid, less gassy basaltic lavas erupt. These lavas can form vast flood

basalts when they erupt on continents.

People who live near active volcanoes have frequently experienced the loss of life

and property associated with these environmental hazards. Scientific monitoring

of gases emitted from the volcano as well as monitoring minor Earthquakes and

tilting have improved scientists ability to predict periods of volcanic activity.

Tectonic movements can apply both compressional and extensional forces to rock.

Compression along converging plate boundaries initially causes folding which

produces anticlines and synclines. Sustained compression can lead to overturned

folds and, with enough force, overthrust faulting may occur.

A fault occurs when the brittle rocks of the Earth’s crust move along a plane,

breakage, or fault plane.

Extension along spreading boundaries generates normal faults with upthrown and

downdropped blocks that can be as large as mountain ranges and rift valleys. A

narrow block dropped down between two normal faults is a graben, and a narrow

block elevated between two normal faults is a horst.

Transcurrent faults occur where rock masses move horizontally past each other.

In a reverse fault, one side rides up over the other, and the Earth’s crust is shortened.

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Earthquakes occur when tectonic forces cause rock to suddenly fracture and move,

shaking the ground in the vicinity of the fracture

Submarine Earthquakes can produce sea waves known as tsunami.

The Richter scale is a logarithmic scale used to measure the energy released by an

Earthquake.

Most severe Earthquakes occur along lithospheric plate boundaries.

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Chapter 14

Weathering and Mass Wasting

This chapter begins the study of the matter and energy flows that shape the surface of the

Earth. This chapter examines how rock material is broken down by weathering and how

weathered material moves downhill under the influence gravity.

A variety of weathering processes cause rock to break down into smaller particles

and to decompose chemically at the Earth’s surface.

Physical weathering is the disintegration of rock into smaller fragments of the same

mineral composition by processes such as frost action, salt-crystal growth, and

unloading.

Chemical weathering is the decomposition of rock resulting from mineral alteration

processes such as hydrolysis, oxidation, and acid solution.

Over much of the land surface, the underlying bedrock is covered by a layer of

weathered material called regolith.

Regolith is the source of sediment carried by wind, water, and glacial ice, and the

parent material for soil development.

Frost action is one of the most important physical weathering processes in cold

climates. It occurs when water freezes in joints in the rock, and the expansion of the

water during repeated freezing forces the joints to enlarge.

Salt crystal weathering operates extensively in dry climates and is the result of the

growth of salt crystals in rock pores. Groundwater moves to the surface through

capillary action and evaporates, leaving the salts behind producing grain by grain

breakup of sandstone.

Unloading is a form of physical weathering that occurs when the removal of

overlying layers causes the rock to expand, cracking in layers parallel to the surface

that break away from the rock in sheets.

Physical weathering can also occur when rocks are subject to intense heating and

cooling and through the growth of plant roots that can wedge rocks apart.

The chemical weathering processes of hydrolosis and oxidation change rock

minerals into clay minerals and oxides.

Acid action is most commonly due to the work of weak solutions of carbonic acid

dissolving certain rocks, particularly limestone and marble.

Mass wasting is the spontaneous downhill movement of soil, regolith, and rock on

slopes under the influence of gravity.

Regolith and soil are more susceptible to mass wasting than bedrock.

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Soil creep is the gradual downhill movement of particles as they are rearranged by

wetting and drying, freezing, and thawing and other processes.

Water-saturated regolith can move quickly down a slope in an Earthflow.

Mudflows and debris floods can develop when intense rains fall on exposed soil

surfaces.

A large mass of bedrock which breaks free from a slope can slide rapidly downhill as

a landslide. The rock mass usually disintegrates as it moves.

Earthflows, mudslides, and landslides can be induced both by natural processes and

by human activities. They can have a major impact on the environment and present a

serious hazard to humans. Human activities that extract mineral resources involve

scarification of the land surface.

The periglacial system refers to the distinctive landforms and geomorphological

processes of arctic and alpine tundra environments which have a strong annual

temperature cycle and extremely cold winters.

Much of the tundra environment has a layer of permafrost (perennially frozen

ground) beneath a surface active layer (seasonally thawed ground). Permafrost is

classified as continuous, discontinuous, sub-sea, and alpine. Continuous permafrost

may reach up to 450 meters in depth.

Ground ice in permafrost can occur as ice wedges and pingos.

Intense frost action can generate patterned ground features such as ice-wedge

polygons and stone polygons.

Summer thawing of the active layer can produce saturated soils that flow downhill to

form solifluction lobes and terraces.

Human activity which disturbs the surface cover of the permafrost may lead to thermal

erosion, in which the permafrost melts to greater depths leading to the development of

depressions and ground subsidence known as thermokarst.

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Chapter 15

Fresh Water of the Continents

This chapter examines what is probably the single most important environmental agent

acting at the Earth’s surface: water. The circulation of water within the Earth’s surface

system is described by the hydrologic cycle. This chapter examines two parts of that

cycle: water at the land surface and water underground.

Water on the continents represents on three percent of the hydrosphere’s total water.

Fresh water in lakes and rivers accounts for less than one percent of the Earth’s water.

This fresh water is derived from precipitation over the continents generated in the

operation of the global hydrologic cycle.

The soil layer plays an important role determining if precipitation will be directed to

the atmosphere by evapotranspiration, to groundwater by percolation, or to streams

and rivers as overland flow.

Groundwater refers to water beneath the surface that fully saturates the pore spaces

in bedrock, regolith, or soil.

The water table marks the upper surface of the groundwater zone where the pore

spaces in rock and regolith are completely filled or saturated with water.

Groundwater moves slowly underground to eventually return to the surface by

seepage into streams, rivers, ponds, and lakes.

An aquifer is a layer of rock or sediment that holds abundant free flowing ground

water.

The movement of groundwater through highly soluble limestone rock produces

distinctive karst landscapes.

Human use of groundwater resources have resulted in depletion of groundwater

resources, ground subsidence, and groundwater contamination.

Runoff includes both overland flow and flow in stream and river channels.

Streams and rivers are organized into drainage systems in which the channels collect

overland flow from slopes and seepage from groundwater and transfer this water

downstream to larger channels. Each stream has a drainage basin, which includes

all the land around the stream that supplies water to the stream. The drainage basin is

bound by drainage divides, which mark the ridges between adjacent drainage basins.

Discharge, or the flow rate of water past a location on a channel, increases

downstream through drainage networks as flow from tributary streams is collected.

A hydrograph is a plot showing the variation in the discharge of a stream over time.

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The lag time between peak discharge and peak precipitation reflects the time required

for water to move down slopes and through progressively larger stream channels.

In urban areas, the high proportion of impervious surfaces and the storm sewers that

move water to stream channels very rapidly act to reduce the lag time and increase

the peak discharge levels of urban streams.

In humid regions, annual stream hydrographs show discharge peaks from individual

rainfall events superimposed on base flow derived from groundwater.

Floods occur when discharge exceeds the capacity of a river channel and water flows

over the low-lying floodplain adjacent to the channel.

Lakes are important elements of the drainage system because they are sources of

fresh water and are used for recreation and hydroelectric power generation.

Lakes without a surface outlet often become saline as they lose water primarily

through evaporation. Saline lakes are characteristic of arid climates.

Humans living in desert areas have relied on exotic rivers for their water supplies.

Many of these areas have experienced problems from salinization and water logging

as a result of irrigation.

Human activities can cause pollution of both surface and underground water.

Our industrialized society demands huge quantities of fresh surface water. The

increasing environmental impacts of these high levels of demand necessitate very careful

planning for future water developments.

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Chapter 16

Landforms Made by Running Water

This chapter focuses on running water as a land-forming agent. It examines the processes

by which running water moves sediments and shapes landforms at the Earth’s surface.

Landforms produced by running water dominate most of the Earth’s terrestrial

environments.

Water is one of the four active agents of denudation (the others being wind, waves,

and glacial ice) that erode, transport, and deposit sediments at the Earth’s surface to

produce erosional and depositional landforms.

The term fluvial is applied to the processes and landforms associated with the action

of running water.

Fluvial processes can erode and transport soil particles from slopes and uplands

causing soil erosion.

Rates of soil erosion and soil formation are in equilibrium on the slopes of most

natural landscapes. This is known as the geologic norm. Disturbance of this

equilibrium by human activity or natural catastrophes can lead to accelerated erosion.

Some eroded soil particles are deposited immediately at the base of slopes to form

colluvium, while others enter streams and are carried downstream before being

deposited as alluvium along valley floors.

A stream can erode material from its bed and banks. Soft materials can be effectively

eroded by hydraulic action, while hard bedrock materials can only be eroded by

abrasion.

The stream load or sediment carried by a stream is transported in three ways: as

dissolved load, suspended load, and bedload. Suspended load is usually the largest

of these components.

Stream capacity to carry solid sediment is dependent on stream flow velocity, which,

in turn, is dependent on channel gradient.

Streams tend to a graded condition over time such that the channel gradient and

stream capacity are adjusted to move the average amount of water and sediment

supplied by slopes.

Indicators of a graded condition are the development of a floodplain and a smooth

stream profile.

Grade is maintained as landscapes are eroded toward base level and, in tectonically

stable areas, erosion can eventually lead to the formation of a peneplain.

Floodplain development involves lateral channel shifting by bank erosion on the

outside of channel bends and deposition of alluvium in point bars on the inside of

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channel bends. It results in valley widening and the development of alluvial

meanders.

Large rivers with low gradients and wide floodplains are called alluvial rivers.

Meandering or lateral shifting of alluvial rivers produces cutoff meanders, oxbow

lakes, and other distinctive landforms.

Tectonic and environmental changes can cause aggradation and degradation in

alluvial rivers and lead to the formation of alluvial terraces and entrenched

meanders.

Fluvial processes are very effective in shaping desert landforms because of the sparse

vegetation cover.

Some of the more distinctive fluvial landforms of arid regions are alluvial fans,

pediments, and playas.

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Chapter 17

Landforms and Rock Structure

This chapter considers how rock properties influence the landforms and drainage patterns

produced by fluvial denudation.

The Earth’s crust contains a variety of rock types which differ in their resistance to

denudation. More resistant rock tends to form uplands and ridges, while weaker rock

forms lowlands and valleys.

Rock layers can be tilted, folded, and fractured by tectonic forces to produce a variety

of rock structures. The tilt and orientation of rock layers and fractures are described by

their strike and dip.

In areas with horizontal strata and an arid climate, fluvial denudation produces vertical

cliffs of resistant rock separated by gentler slopes of less resistant rock. These slopes

surround flat-topped plateaus, mesas, and buttes capped by resistant rock.

Different rock types and structures tend to produce different drainage patterns or

stream network characteristics. Drainage patterns have some interesting, systematic

geometric properties.

Areas of horizontal strata usually have broadly branching, dendritic drainage

patterns.

On gently dipping strata along coastal plains, cuestas form in more resistant rock

while lowland valleys develop in the less resistant rock. The development of

consequent streams across the cuestas and subsequent streams along the lowland

valleys produces a trellis drainage pattern.

Fluvial denudation of a sedimentary dome produces an annular drainage pattern and

a circular pattern of hogbacks of more resistant rock separated by lowlands of less

resistant rock.

Linear fold belts of anticlines and synclines are eroded into ridge-and-valley

landscapes, with the more resistant strata forming ridges and the less resistant strata

forming valleys. A trellis drainage pattern is typical of these landscapes.

A rock face produced by faulting can persist as a fault-line scarp, while a landscape is

worn down by denudation. A subsequent stream often marks the zone of weakness

along a fault plane.

Tightly folded metamorphic rocks tend to erode to ridge-and-valley landscapes that

are less rugged than those developed in folded sedimentary rock. The resistant

metamorphic rocks of slate and schist form the hill belts, while the less resistant

marble forms the valleys.

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A monadnock is an isolated projection of intrusive igneous rock surrounded by an

eroded plain. Dendritic drainage patterns develop on these eroded batholiths.

Radial drainage patterns develop in the early stage of erosion of stratovolcanoes. The

advanced stage of erosion produces volcanic necks and radial dikes of resistant

igneous rock.

The erosion of shield volcanoes results in landscapes of steep slopes and sharp ridges.

Radial consequent streams cut deep canyons into the sides of the extinct volcanoes.

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Chapter 18

Landforms Made by Waves and Wind

This chapter examines the processes and landforms that result when wind energy moves

surface materials, either directly as wind or indirectly as waves. Both waves and wind are

driven by the rotation and unequal surface heating of the Earth. Unlike fluvial action and

mass wasting, wind and waves can move material uphill against the force of gravity.

Shoreline refers to the line of contact between water and land, while coastline refers

to the zone of influence of coastal processes.

Wave action is the most important agent shaping coastal landforms. Waves

approaching a shoreline are slowed by the drag of the bottom, become steeper, and

eventually collapse to form breakers.

The energy of breakers expended along a coastline causes erosion and transportation

of shoreline materials.

Weak or soft shoreline materials are eroded rapidly by wave action to form marine

scarps, while more resistant materials are eroded slowly to form marine cliffs.

Beaches are thick wedge-shaped accumulations of sand shaped by the swash and

backwash of water along the shoreline.

Waves striking a shoreline at an oblique angle cause littoral drift: the movement of

sediment along the shore by the processes of beach drift and longshore drift.

The sands transported by littoral drift can form spits, bars, and pocket beaches along

a coast.

Variations in sediment transport along a coast can lead to progradation (building out)

and retrogradation (cutting back) of the shoreline.

Tides are regular fluctuations in sea level caused by the gravitational forces of the sun

and moon.

Tides drive ebb and flood currents which redistribute fine sediments within bays and

estuaries.

Coastlines of submergence result from partial drowning of the coast by a rise in sea

level or sinking of the land. Ria coasts and fjord coasts are deeply embayed with bold

relief.

Coastlines of emergence, where submarine deposits become exposed, usually have

gentle relief and slope gently toward the ocean. Repeated crustal uplift produces

barrier island coasts while rapid emergence can produce raised shorelines and

marine terraces.

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Coastlines created when new land is built out into the ocean include delta and volcano

coasts as well as coral reef coasts.

Global sea level has fluctuated substantially in the past and is presently rising slowly.

The present rise may be due to the melting of glacier ice and thermal expansion of the

oceans caused by global warming.

Wind action is capable of moving dry, fine, loose sediments that are not protected by

a vegetation cover and is an effective landforming agent in deserts, semiarid regions,

and along coasts.

Wind erosion includes abrasion and deflation. The wind drives loose sand and dust

particles against exposed surfaces in a process of abrasion.

The removal of loose particles from the ground surface by wind is called deflation.

Deflation can produce blowouts, desert pavements, and dust storms. Clay and silt

particles are lifted high into the air and carried long distances, while sand particles are

lifted only with moderately strong winds and are only carried one to two meters above

the ground.

Sand dunes form where there is an abundant source of sand available for movement

by wind.

Variations in wind conditions, vegetation cover, and sand abundance produce a wide

variety of dune types. Some of the more important are:

Barchan dunes - individual, crescent-shaped dunes with arms pointing downwind

Transverse dunes - wave-like dunes with a crest aligned perpendicular to the

wind direction

Parabolic dunes - crescent-shaped dunes with the dune crest bowed outward in a

downwind direction.

Longitudinal dunes – long, narrow ridges oriented parallel with the prevailing

wind direction

Loess is a sheet-like deposit of wind-transported silt. Extensive loess deposits in North

America were derived from fresh glacial deposits exposed at the end of the last ice

age.

Human disturbance of the vegetation cover in semiarid regions can expose the soil to

induced deflation or wind erosion.

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Chapter 19

Glacial Landforms and the Ice Age

This chapter examines the role of glacial ice as a denudation and landforming agent.

Glacial ice had a major impact on the landscapes of midlatitude and subarctic regions

during the past Ice Age and still covers many high latitude and high elevation areas of the

Earth.

Glaciers are natural bodies of land ice that have, or have had in the past, the ability to

flow. They form where winter snowfall exceeds summer ablation over long periods of

time.

Glaciers erode the land surface by plucking and abrasion. Eroded material is

incorporated into the glacier, transported, and eventually deposited when the ice melts.

Alpine glaciers form in cirques in high mountain locations and often flow down pre-

existing stream valleys carving them into U-shaped glacial troughs.

Snow builds up in the zone of accumulation found at the upper end of the glacier

where layers of snow undergo compaction and recrystalization to produce firn.

Beneath the surface of the glacier, the ice acts as a plastic substance and will flow

slowly. Glaciers also move by basal sliding.

Glaciated landscapes tend to be very rugged. Freeze-thaw weathering and glacial

erosion produce cirques, arêtes, and horn peaks.

The erosional capacity of glaciers also produces large quantities of depositional

materials. Piles of unsorted debris that form along the end and sides of glaciers are

moraines.

Ice sheets are accumulations of ice that cover large areas and extend over major

topographic features. Greenland and Antarctica are sites of present-day ice sheets.

Ice shelves are extensions of ice sheets that float on ocean water. Icebergs are pieces

of ice that break free from ice shelves and glaciers to float in the ocean.

Continental ice sheets expand and contract during an ice age, causing alternating

periods of glaciation, deglaciation, and interglaciation.

During the Late-Cenozoic Ice Age, extending over the past two to three million years,

continental ice sheets have grown and melted up to thirty times.

Much of North America and Europe, as well as parts of Asia and South America, were

covered with ice during the most recent episode of ice sheet expansion, the Wisconsin

Glaciation.

The erosive action of alpine glaciers and ice sheets produces grooved, scratched, and

polished surfaces on more resistant rock, and strips away regolith and weaker rock.

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Glacial drift refers to all those sediments that are deposited by glaciers. Unstratified

drift deposited directly from glaciers is called till. Those sediments derived from

glaciers, but modified by transportation by meltwater, are called stratified drift.

Some of the more common landforms made up of till deposits are moraines, till

plains, and drumlins.

Stratified drift features include outwash plains, which form where braided meltwater

streams issuing from glaciers deposit sediment over a wide area. Sediment deposited

by meltwater streams flowing in ice tunnels beneath a glacier form ridge-like eskers.

Kames are stratified drift deposits that originate as deltas in meltwater lakes near

glacier margins.

Three possible causes of the Late-Cenozoic Ice Age are:

a change in continent positions due to plate tectonic activity

an increase in the number and severity of volcanic eruptions

a reduction in solar energy output

Cycles of glaciation appear to be related to cyclic changes in the Earth’s axial tilt and

distance from the sun.

Global warming has the potential to change both ablation and snowfall on the Earth’s

ice sheets. The net effect of these changes on global sea level is uncertain.

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Overview of Physical Geography in 24 Hours

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Transcript of Overview of Physical Geography in 24 Hours