Prologue

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.

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.

Chapter 1The 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.

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.

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.

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 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.

Chapter 3Air 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.

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.

Chapter 4Atmospheric 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.

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.

Chapter 5Winds 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.

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.

Chapter 6Weather 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.

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.

Chapter 7Global 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

Precipitation maximum during the coolest period of the year

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

Wet equatorialTrade wind coasts

Tropical DesertsMidlatitude deserts and steppesMoist subtropicalMidlatitude west coastsArctic 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 equatorialMonsoon and trade-wind coastWet-dry tropicalDry tropical

Midlatitude climates occupy the polar front zone where warm and cold air masses conflict producing wave cyclones. This groups of climates includes:

Dry subtropicalMoist subtropicalMediterraneanMarine west-coastDry midlatitudeMoist 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 forestTundraIce 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

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:

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

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

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.

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

Chapter 8Biogeographic 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.

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.

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.

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.

Chapter 9Global 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 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.

Chapter 10Global 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.

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.

Chapter 11Earth 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.

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.

Chapter 12The 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.

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.

Chapter 13Volcanic 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.

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.

Chapter 14Weathering 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.

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.

Chapter 15Fresh 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.

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.

Chapter 16Landforms 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 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.

Chapter 17Landforms 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.

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.

Chapter 18Landforms 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.

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.

Chapter 19Glacial 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.

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.