GeoJournalAs you read this chapter, use your journal tonote facts about our planet. Include descrip-tive details about Earth’s physical features,its structure, and the forces of change thatshape its surface.

Chapter Overview Visit the Glencoe WorldGeography Web site at tx.geography.glencoe.comand click on Chapter Overviews—Chapter 2 topreview information about Planet Earth.

Planet Earth

A Geographic ViewPatterns of LifeAs we fly over the Sahara, we allstrain to see Lake Chad, which is between the northern deserts and the green portion of equatorialAfrica. Its level is a good indicatorof how wet or dry this boundaryregion, called the Sahel, has been.It was mostly cloudy during myfirst flight: a good sign for thoseliving below. On my secondflight, 17 months later, wecould see that the level hadrisen: The drought in the Sahelhad abated [ended] for themoment.

—Jay Apt, “Orbit: The Astronauts’ View of Home,”National Geographic, November 1996

An astronaut, seeing Earth from the blackness ofspace, described it as “piercingly beautiful.” From the vantage pointof space, the earth’s great beauty resembles a blue and white marble,with contrasts of water and land beneath huge swirls of white clouds.Together these features form the physical environment of the earth.In this section you will discover what humans know about the phys-ical nature of our planet, Earth.

Our Solar SystemEarth is part of our solar system, which is made up of the sun and

all of the countless objects that revolve around it. At our solar sys-tem’s center is the sun––a star, or ball of burning gases. About 109times wider than Earth, the sun’s enormous mass—the amount ofmatter it contains—creates a strong pull of gravity. This basic physicalforce keeps the earth and the other objects revolving around the sun.

Guide to ReadingConsider What You KnowWhat do you know about PlanetEarth? How does it compare to otherplanets and to other objects, such asstars and moons? How would youdescribe the surface of the earth?

Read to Find Out• Where is Earth located in our solar

system?

• How is Earth shaped?

• What is Earth’s structure?

• What types of landforms are found on Earth?

Terms to Know• hydrosphere

• lithosphere

• atmosphere

• biosphere

• continental shelf

Places to Locate• Australia

• Antarctica

• Europe

• Asia

• North America

• South America

• Africa

• Mount Everest

• Dead Sea

• Mariana Trench

C h a p t e r 2 33

Earth viewed from space

Lake Chad, northern Africa

34 U n i t 1

The Planets: Neighbors in SpaceExcept for the sun, spheres called planets are the

largest objects in our solar system. At least nineplanets exist, and each is in its own orbit around thesun. The diagram above shows that the planets varyin distance from the sun. Mercury, Venus, Earth,and Mars are the inner planets, or those nearest thesun. Earth, the third planet from the sun, is about 93million miles (about 150 million km) away. Farthestfrom the sun are the outer planets—Jupiter, Saturn,Uranus, Neptune, and Pluto.

The planets vary in size. Jupiter is the largest.Earth ranks fifth in size among the planets, anddistant Pluto is the smallest. All of the planetsexcept Mercury and Venus have moons, smallerspheres or satellites that orbit them. The number of

moons each planet has also differs. Earth has 1moon, and Saturn has at least 18 moons.

All of the planets except Pluto are grouped intotwo types—terrestrial planets and gas giant plan-ets. Mercury, Venus, Earth, and Mars are called terrestrial planets because they have solid, rockycrusts. Mercury and Venus are scalding hot, andMars is a cold, barren desert. Only Earth has liq-uid water at the surface and can support varietiesof life.

Farther from the sun are the gas giant planets—Jupiter, Saturn, Uranus, and Neptune. They are muchmore gaseous and less dense than the terrestrial plan-ets, even though they are larger in diameter. Each gasgiant planet is itself like a miniature solar system,with orbiting moons and thin, encircling rings. Only

Neptune

Uranus

Pluto

Saturn

Jupiter

MarsComet

Earth Moon

Mercury

Sun

VenusAsteroids

DIAGRAM STUDY

The Solar System

1. Interpreting Diagrams Which four planets are closest to the sun?

2. Applying Geography Skills What characteristics do the inner planetsshare? What characteristics do the outer planets share?

C h a p t e r 2 35

Saturn’s rings, however, are easily seen from Earthby telescope. Pluto, the exception among the plan-ets, is a ball of ice and rock.

Asteroids, Comets, and MeteoroidsIn addition to the planets, thousands of smaller

objects, including asteroids, comets, and mete-oroids, revolve around the sun. Asteroids aresmall, irregularly shaped, planetlike objects. Theyare found mainly between the orbits of Mars andJupiter in a region called the asteroid belt. A fewasteroids follow paths that cross the earth’s orbit.

Comets, made of icy dust particles and frozengases, look like bright balls with long, feathery tails.Their orbits are inclined at every possible angle to theearth’s orbit. They may approach from any direction.

Meteoroids are pieces of space debris—chunksof rock and iron. When they occasionally enterEarth’s gravitational field, friction usually burnsthem up before they reach the earth’s surface.Those that collide with Earth are called meteorites.Meteorite strikes, though rare, can significantlyaffect the landscape, leaving craters and causingother devastation. In 1908 a huge area of forest inthe remote Russian region of Siberia was flattenedand burned by a “mysterious fireball.” Scientiststheorize it was a meteorite or comet. As a writerdescribes the effects:

“ The heat incinerated herds of reindeerand charred tens of thousands of ever-greens across hundreds of square miles.For days, and for thousands of milesaround, the sky remained bright withan eerie orange glow––as far away aswestern Europe people were able to readnewspapers at night without a lamp.”Richard Stone, “The Last Great

Impact on Earth,” Discover,September 1996

Getting to Know EarthAlthough globes show our planet as a perfect

sphere, the earth is really a rounded object wideraround the center than from top to bottom. Earthhas a larger diameter at the Equator—about 7,930miles (12,760 km)—than from Pole to Pole, but the

difference is less than 1 percent. With a circumfer-ence of about 24,900 miles (40,070 km), Earth is thelargest of the inner planets.

Water, Land, and AirThe surface of the earth is made up of water and

land. About 70 percent of our planet’s surface is water, which gives the planet the deep blueappearance that astronauts see from space. Oceans,lakes, rivers, and other bodies of water make up apart of the earth called the hydrosphere.

About 30 percent of the earth’s surface is land,including continents and islands. Land makes upa part of the earth called the lithosphere, theearth’s crust. The lithosphere also includes theocean basins, or the land beneath the oceans.

The air we breathe is part of Earth’s atmosphere,a layer of gases extending about 6,000 miles (9,700 km) above the planet’s surface. The atmos-phere is composed of 78 percent nitrogen, 21 percentoxygen, and small amounts of argon and other gases.

All people, animals, and plants live on the earth’ssurface, close to the earth’s surface, or in the atmos-phere. The part of the earth that supports life is thebiosphere. Life outside the biosphere, such as on aspace station orbiting Earth, exists only with theassistance of mechanical life-support systems.

LandformsLandforms are the natural features of the earth’s

surface. The diagram on pages 14–15 shows manyof the earth’s landforms, made up of physical fea-tures with a particular shape or elevation. Fourmajor landforms are mountains, hills, plateaus,and plains. Others include valleys, canyons, andbasins. Landforms often contain rivers, lakes, andstreams.

Underwater landforms are as diverse as thosefound on dry land. In some places the ocean floor isa flat plain. Other parts of the seabed feature moun-tain ranges, cliffs, valleys, and deep trenches.

Seen from space, Earth’s most visible landforms,however, are the seven large landmasses calledcontinents. Two continents, Australia and Antarc-tica, stand alone, while the others are joined insome way. Europe and Asia are actually partsof one huge landmass called Eurasia. A narrowneck of land called the Isthmus of Panama linksNorth America and South America. At the Sinai

Checking for Understanding1. Define hydrosphere, lithosphere,

atmosphere, biosphere, continen-tal shelf.

2. Main Ideas Copy the diagrambelow on your paper, filling ininformation about the structures of the solar system and the earth.

Critical Thinking3. Drawing Conclusions Recently

NASA has launched space probesto explore Mars. Why might Marshave been chosen for these explorations?

4. Making Inferences Think aboutEarth’s surface and list conditionsthat must be present in a spacestation in order to support life.

5. Comparing and Contrasting Howdo the inner planets differ fromthe outer planets?

Analyzing Diagrams6. Location Study the diagram of

the solar system on page 34. Howis the size of a planet’s orbit influ-enced by its distance from the sun?

36 U n i t 1

7. A Delicate Balance Thinkabout the ratio of waterand land on Earth. Write adescription of how Earth’sphysical features would bedifferent if the proportionswere reversed.

Applying Geography

Peninsula, the human-made SuezCanal separates Africa and Asia.

The part of a continent thatextends underwater is called acontinental shelf. Continentalshelves are narrow in some places and wide in oth-ers. They slope out from land for as much as 800miles (1,287 km) and descend gradually to a depthof about 660 feet (200 m) where a sharp drop marksthe beginning of the continental slope. This areadrops more sharply to the ocean floor.

Earth’s Heights and DepthsGreat contrasts exist in the heights and depths of

the earth’s surface. The highest point on Earth is inSouth Asia at the top of Mount Everest, which is

29,035 feet (8,852 m) above sea level. The lowestdry land point, at 1,349 feet (411 m) below sealevel, is the shore of the Dead Sea in SouthwestAsia. Earth’s deepest known depression lies underthe Pacific Ocean southwest of Guam in the Mariana Trench, a long, narrow, underwatercanyon about 35,827 feet (10,923 m) deep. Theseand other natural features have developed as theearth has changed over millions of years. In thenext section you will learn about the forces ofchange that have shaped the earth.

Planets

Structures

Solar System Earth

Planet Earth Parts of theatmosphere (left), lithosphere (center), and hydrosphere (right) form thebiosphere, the part of the Earth where life exists.

Human-Environment Interaction How does human activity impact the biosphere?

C h a p t e r 2 37

Guide to ReadingConsider What You KnowThrough observation you are familiarwith some features of the earth’ssurface. These features have beenshaped by forces above and beneaththe earth’s surface. How many ofthese forces can you name?

Read to Find Out• How do Earth’s layers contribute to

the planet’s physical characteristics?

• What internal forces operate toaffect Earth’s surface, the settingfor human life?

• What external forces affect Earth’ssurface?

Terms to Know• mantle

• continental drift

• magma

• plate tectonics

• subduction

• accretion

• spreading

• fold

• fault

• weathering

• erosion

• loess

• glacier

• moraine

Places to Locate• San Andreas Fault

• Ring of Fire

Forces of Change

A Geographic ViewA Planet in MotionScientists now know that the earth is dynamic and enormouslycomplex. At the [earth’s] surface ride more than a dozen huge, stifffragments, or plates. They move at a slower-than-snail’s pace—only inches a year—but coverthousands of miles over millionsof years. As they collide and separate, they change the face of the globe by deforming andrearranging its features. Theengine that propels [these]plates lies below: hot innerlayers that churn like thicksoup simmering in very slow motion.

––Keay Davidson and A.R. Williams, “Under Our Skin: Hot Theories on the Center of the Earth,” National Geographic, January 1996

Although we cannot look into the center of the earth,scientists have concluded from available evidence that it is adynamic interior of intense heat and pressure. Movements deepwithin the earth drive numerous changes that renew and enrich theearth’s surface. In this section you will learn about the earth’s struc-ture and the natural forces that continually act upon our planet.

Earth’s StructureFor hundreds of millions of years, the surface of the earth has

been in slow but constant motion. Some forces that change theearth, such as wind and water, occur on the earth’s surface. Others,such as volcanic eruptions and earthquakes, originate deep in theearth’s interior.

San Andreas Fault, California

38 U n i t 1

A Layered PlanetAs you see in the diagram above, the earth is

composed of three layers—the core, the mantle,and the crust. At the very center of the planet is asuper-hot but solid inner core. It lies about 4,000miles (6,430 km) below the surface of the earth.Scientists believe that the inner core is made up ofiron and nickel under enormous pressure.

Surrounding the inner core is a liquid outer core,about 1,400 miles (2,250 km) thick. A band ofmelted iron and nickel, the outer core begins about1,800 miles (2,900 km) below the surface of theearth. Temperatures there reach a scalding 8,500ºF(about 4,700ºC).

Next to the outer core is a thick layer of hot,dense rock called the mantle. The mantle consistsof silicon, aluminum, iron, magnesium, oxygen,and other elements. This mixture continually rises,

cools, sinks, warms up, and rises again, releasing80 percent of the heat generated from the earth’sinterior.

The outer layer is the crust, a rocky shell formingthe earth’s surface. This relatively thin layer of rockranges from about 2 miles (3.2 km) thick underoceans to about 75 miles (121 km) thick undermountains. The crust is broken into more than adozen great slabs of rock called plates that rest—ormore accurately, float—on a partially melted layerin the upper mantle. The plates carry the earth’soceans and continents. The map on page 39 showsthe boundaries of the various plates.

Plate MovementIf you had seen the earth from space 500 million

years ago, the planet probably would not havelooked at all like it does today. Many scientistsbelieve that most of the landmasses form-ing our present-day continents were once partof one gigantic supercontinent called Pangaea(pan•JEE•uh). Over millions of years, this super-continent has broken apart into smaller conti-nents. These continents in turn have drifted and,in some places, recombined. The theory that thecontinents were once joined and then slowlydrifted apart is called continental drift.

Many scientists theorize that plates movingslowly around the globe have produced Earth’slargest features—not only continents, but alsooceans and mountain ranges. Most of the time,plate movement is so gradual—only about 4 inches (10 cm) a year—that it cannot be felt. Asthey move, the plates may crash into each other,pull apart, or grind and slide past each other.Whatever their actions, plates are constantly chang-ing the face of the planet. They push up mountains,create volcanoes, and produce earthquakes. Whenthe plates spread apart, magma, or molten rock, ispushed up from the mantle and ridges are formed.When the plates bump together, one may slideunder another, forming a trench.

Scientists use the term plate tectonics to refer toall of these activities, which create many of theearth’s physical features. Many scientists estimatethat plate tectonics have been shaping the earth’ssurface for 2.5 to 4 billion years. According to somescientists, plate tectonics will have sculpted a wholenew look for our planet millions of years from now.

Crust

Mantle

Outercore

Inner core

DIAGRAMSTUDY

Inside the Earth

1. Interpreting Diagrams Which of Earth’s lay-ers is between the crust and the outer core?

2. Applying Geography Skills How do theplates of the earth’s crust shape the physicallandscape?

N

S

EW

NORTH AMERICANPLATE

PACIFIC PLATE

PHILIPPINEPLATE

ANTARCTIC PLATE

ANTARCTIC PLATE

NAZCAPLATE SOUTH AMERICAN

PLATE

SCOTIAPLATE

COCOSPLATE

JUAN DE FUCAPLATE

CARIBBEANPLATE

EURASIANPLATE

AFRICAN PLATE

INDIANPLATEARABIAN

PLATE

SOMALIPLATE

EURASIAN PLATE

AUSTRALIAN PLATE

MAP STUDY

Plates and Plate Movement

C h a p t e r 2 39

Scientists, however, have not yet determinedexactly what causes plate tectonics. They theorizethat heat rising from the earth’s core may createslow-moving currents within the mantle. Overmillions of years, these currents of molten rockmay shift the plates around, but the movementsare extremely slow and difficult to detect.

Internal Forces of ChangeThe surface of the earth has changed greatly

over time. Scientists believe that some of thesechanges come from internal forces associated withplate tectonics. One of these internal forces relatesto the slow movement of magma within the earth.Other internal forces involve movements that canfold, lift, bend, or break the rock along the earth’scrust.

Colliding and Spreading PlatesMountains are formed in areas where giant con-

tinental plates collide. For example, the Himalayaranges in South Asia were thrust upward when theIndian landmass rammed into Eurasia. Himalayanpeaks are still getting higher as the Indian land-mass continues to push against them.

Mountains also are created when a sea platecollides with a continental plate. In a processcalled subduction (suhb•DUHK•shuhn), theheavier sea plate dives beneath the lighter conti-nental plate. Plunging into the earth’s interior, thesea plate becomes molten material. Then, asmagma, it bursts through the crust to form vol-canic mountains. The Andes, for example, wereformed over millions of years as a result of theprocess of subduction.

2. Applying Geography Skills Identify anddescribe physical features that are the result of plate movement.

Find NGS online map resources @ www.nationalgeographic.com/maps

Miller projection

Plate boundary

Direction of plate movement

1. Interpreting Maps Which plates are movingtoward each other? Away from each other?

40 U n i t 1

In other cases where continental and sea platesmeet, a different process, known as accretion,occurs. During accretion (uh•KREE•shuhn),pieces of the earth’s crust come together slowly asthe sea plate slides under the continental plate.This plate movement levels off seamounts, under-water mountains with steep sides and sharppeaks, and piles up the resulting debris intrenches. Such a buildup can cause continents togrow outward. Most scientists believe that muchof western North America expanded outward overmore than 200 million years as a result of theprocess of accretion.

New land is also created where two sea platesconverge. In this process one plate moves underthe other, often forming an island chain at theboundary. Sea plates also can pull apart in aprocess known as spreading. The resulting rift, ordeep crack, allows magma from within the earth towell up between the plates. The magma hardens tobuild undersea volcanic mountains or ridges. Thisspreading activity occurs down the middle of theAtlantic Ocean’s floor, pushing Europe and NorthAmerica away from each other.

Folds and FaultsMoving plates sometimes squeeze the earth’s

surface until it buckles. This activity forms folds, orbends, in layers of rock. In other cases plates maygrind or slide past each other, creating cracks in theearth’s crust called faults. One famous fault is theSan Andreas Fault in California.

The process of faulting occurs when the foldedland cannot be bent any further. Then the earth’scrust cracks and breaks into huge blocks. Theblocks move along the faults in different direc-tions, grinding against each other. The resultingtension may release a series of small jumps, felt asminor tremors on the earth’s surface.

EarthquakesSudden, violent movements of plates along a

fault line are known as earthquakes. These shakingactivities dramatically change the surface of theland and the floor of the ocean. For example, dur-ing a severe earthquake in Alaska in 1964, a portionof the ground lurched upward 38 feet (11.6 m).

Earthquakes often occur where different platesmeet one another. Tension builds up along fault

SUBDUCTION

Magma

VolcanoOcean

Mountains ACCRETION

Continent

Ocean floor

Continent

Trench

DIAGRAM STUDY

Forces of Change

lines as the plates stick. The straineventually becomes so intense that therocks suddenly snap and shift. Thismovement releases stored-up energy along thefault. The ground then trembles as shock wavessurge through it away from the area where the rockssnapped apart.

In recent years disastrous earthquakes haveoccurred in Kobe, Japan, and in Los Angeles and SanFrancisco, California. These cities are located alongthe Ring of Fire, one of the most earthquake-proneareas on the planet. The Ring of Fire is a zone ofearthquake and volcanic activity surrounding thePacific Ocean. It marks the boundary where theplates that cradle the Pacific meet the plates that holdthe continents surrounding the Pacific.

Volcanic EruptionsVolcanoes are mountains formed by lava or by

magma that breaks through the earth’s crust. Vol-canoes often rise along plate boundaries whereone plate plunges beneath another. This kind ofvolcanic formation occurs, for example, along theRing of Fire. In such a process the rocky plate

SPREADING FAULTING

Magma

Ocean floor

Fault

Fault

Ridge

Rift valley

C h a p t e r 2 41

melts as it dives downward into the hot mantle. Ifthe molten rock is too thick, its flow is blocked andpressure builds. A cloud of ash and gas may thenspew forth, creating a funnel through which thered-hot magma rushes to the surface. There thelava flow may eventually form a large volcaniccone topped by a crater, a bowl-shaped depressionat a volcano’s mouth.

Volcanoes also arise in areas away from plateboundaries. Some areas deep in the earth are hotterthan others, and magma often blasts through thesurface as volcanoes. As a moving plate passes overthese hot spots, molten rock flowing out of theearth’s surface may create volcanic island chains. Anexample of this type of formation is the HawaiianIslands in the Pacific Ocean. At various hot spots,molten rock may also heat underground water,causing hot springs or geysers. YellowstoneNational Park in Wyoming has many spectaculargeysers formed by this process, such as Old Faithful,which regularly sends water and steam into the air.

1. Interpreting Diagrams How does the process of accre-tion create deep trenches in the earth’s surface?

2. Applying Geography Skills How do you think humansettlement is affected by the process of faulting?

42 U n i t 1

External Forces of ChangeExternal forces, such as wind and water, also

change the earth’s surface. They have transformedthe planet’s appearance over millions of years andcontinue to do so today. Wind and water move-ments involve two processes: weathering and ero-sion. Weathering is the process that breaks downrocks on the earth’s surface into smaller pieces.Erosion is the wearing away of the earth’s surfaceby wind, glaciers, and moving water.

WeatheringThe earth is changed by two basic kinds of

weathering—physical weathering and chemicalweathering. Physical weathering occurs whenlarge masses of rock are physically broken downinto smaller pieces. For example, water seepsinto the cracks in a rock and freezes, thenexpands and causes the rock to split.

The process of chemical weathering changesthe chemical makeup of rocks, transforming their

minerals or combining them with new elements.For example, water mixed with carbon dioxidefrom the air easily dissolves certain rocks, such aslimestone. Many of the world’s caves have beenand continue to be formed by this process.

Wind ErosionWind erosion involves the movement of dust,

sand, and soil from one place to another. Plantshelp protect the land from wind erosion; however,in dry places where people have cut down treesand plants, winds pick up large amounts of soiland blow it away. Serious wind erosion devas-tated the Great Plains in the central United Statesduring the 1930s. Fierce winds swept up dry,overworked soil from exposed farmland and car-ried it away in dust storms.

Wind erosion, however, also provides benefits.The dust carried by wind often forms largedeposits of mineral-rich soil. These deposits pro-vide fertile farmland in various parts of the world.China’s Yellow River basin, for example, is thicklycovered with loess (LEHS), a fertile, yellow-graysoil deposited by wind.

Glacial ErosionAnother cause of erosion is glaciers, or large

bodies of ice that slowly move across the earth’ssurface. Glaciers form over a long period of time aslayers of snow press together and turn to ice. Theirgreat weight causes them to move slowly downhillor spread outward. As they move, glaciers pick uprocks and soil in their paths. Glacial movementschange the landscape, destroying forests, carvingout valleys, altering the courses of rivers, andwearing down mountaintops.

When glaciers melt and recede, in some placesthey leave behind large piles of rocks and debriscalled moraines. Some moraines form long ridgesof land, while others form dams that hold waterback and create glacial lakes.

There are two types of glaciers—sheet glaciersand mountain glaciers. Sheet glaciers are flat,broad sheets of ice. Today sheet glaciers covermost of Greenland and all of Antarctica. Theyadvance a few feet each winter and recede duringthe summer. Large blocks of ice often break offfrom the coastal edges of sheet glaciers to becomeicebergs floating in the ocean.

Water erosion in the Grand Canyon

Mountain glaciers are more common than sheetglaciers. They are located in high mountainvalleys where the climate is cold, such as in theRocky Mountains and Cascade Range of NorthAmerica. As they move downhill, mountain glac-iers gouge out round, U-shaped valleys. As theseglaciers melt, rock and soil are deposited in newlocations.

Water ErosionFast-moving water—rain, rivers, streams, and

oceans—is the most significant cause of erosion.Water erosion begins when springwater and rain-water flow off the land downhill in streams. As thewater flows, it cuts into the land, wearing away thesoil and rock. The resulting sediment—small parti-cles of soil, sand, and gravel—acts like sandpaper,grinding away the surface of rocks along thestream’s path. Over time, the eroding action ofwater forms first a gully and then a V-shaped val-ley. Sometimes valleys are eroded even further toform valleys with high, steep walls, called canyons.The Grand Canyon of the Colorado River is a goodexample of the eroding power of water. Peter Millerdescribes the canyon’s various layers as he hikedfrom the canyon rim to its floor:

C h a p t e r 2 43

Checking for Understanding1. Define mantle, continental

drift, magma, plate tectonics,subduction, accretion, spreading,fold, fault, weathering, erosion,loess, glacier, moraine.

2. Main Ideas Copy the organizerbelow, and fill in informationabout forces that shape Earth’sfeatures. Then choose one forcethat shapes the earth’s surfaceand explain this process.

Critical Thinking3. Predicting Consequences Based

on your understanding of platetectonics, what changes wouldyou predict to the earth’s appear-ance millions of years from now?

4. Making Inferences Think aboutwater erosion associated withrivers and streams. Based on that information, what would you expect the areas where riversempty into the oceans to be like?Explain.

5. Drawing Conclusions In whatways can erosion be both benefi-cial and harmful to agriculturalcommunities?

Analyzing Maps6. Region Study the map of plates

and plate movement on page 39.Which plates are responsible for the earthquakes that haveoccurred in California in theUnited States?

Forces of Change

Composition (layers)

Internal forces

External forces

7. Consequences of Earth’sForces Review how internalforces shape the surface ofthe earth. Now imaginethat the mantle ceased to circulate molten rock.Write a description of howland formation on the sur-face of the earth would bedifferent.

Applying Geography

“ [A]s we hiked down the twistingKaibab Trail to the Colorado River atthe bottom of the canyon, we were hop-ing . . . to share a . . . great adventure.The canyon opened up before us with allthe drama we had imagined. After wedropped below the ponderosa pine forestsof the South Rim, we descended narrowridgelines past crumbling cliffs of whitesandstone, red limestone, chocolatesandstone, and maroon shale, descend-ing 4,700 feet to the river.”Peter Miller, “John Wesley

Powell: Vision for the West,”National Geographic, April 1994

In addition to streams and rivers, oceans play animportant role in water erosion. Pounding wavescontinually erode coastal cliffs, wear rocks intosandy beaches, and move sand away to other coastalareas. Violent storms speed up this process. In thenext section, you will learn about the oceans andother water features of our planet and how theyaffect the earth’s surface.

ViewpointCASE STUDY on the Environment

InvasiveSpecies

A Global Concern:Until a few centuries ago, plantsand animals living in one part ofthe world rarely mixed with those on other continents. This changed,however, as new ways of trans-port allowed more and more people to travel the planet. Aturning point was the ColumbianExchange, the transfer of plants,animals, and diseases betweenEurope and the Americas thatbegan with the voyages ofColumbus. Since then, the biologi-cal exchange has become global.As a result, thousands of “alien”plants and animals have beenintroduced into areas where thespecies do not occur naturally.Some introduced species are ben-eficial, while others are invasiveand pose environmental threats.

InvasiveSpecies

A Global Concern:

44 U n i t 1

If you live near a wetland,you’ve probably seen the purple or pinkish flowers of the purple loosestrife

(left). Native to Europe, purpleloosestrife was brought to theUnited States as a garden plant.It soon spread to wetlands,where it now threatens nativeplants. Invasive species such aspurple loosestrife are a globalproblem.

Sometimes invasive speciesarrive in new environments byaccident—as seeds or as eggsor tiny insects hidden in pack-ing materials, in food or soil, orin the ballast water of visitingships. Without natural preda-tors in their new habitats,alien species can multiply atalarming rates and crowd outnative species.These invadersalso can cause widespreadenvironmental damage. Thegypsy moth, for example, wasaccidentally introduced to theUnited States in the late 1800s.The insect, during its caterpillarstage, feeds on the leaves ofseveral kinds of trees. Gypsymoth caterpillars have de-stroyed huge swaths of Amer-ican forests.

Plants andanimals are alsointroduced intonew environ-ments intention-ally. For example,an insect might beimported to com-bat a crop pest.Some introducedspecies have turned out to begreat successes. Wheat, forexample, originated in theMiddle East and is now grownon almost every continent.Spanish explorers broughthorses to the Americas.

Yet for every helpful intro-duction, there have been manydestructive ones. In 1859British settlers released twodozen rabbits in Australia. Nowmillions of rabbits roam thecontinent.They devour cropsand grasslands, creating desertsin the hot, dry climate. Waterhyacinth, another invasivespecies, went from SouthAmerica to Africa for use inornamental ponds. The fast-growing plant soon spread tolakes and rivers, forming densemats that interfere with boatingand fishing.

Some scientists argue that alienspecies are serious threats tobiodiversity. These peoplewant laws to limit trade and to

halt the spread of plants andanimals from place to place.Many biologists think using for-eign species to control pests istoo risky. Scientists cite numer-ous examples of introducedspecies that have damagedtheir new environments.

Some farmers and economistspoint out that in a worldwhere trade and transportationbring countries together, aglobal redistribution of speciesis inevitable. Such people, fear-ing financial losses, opposelaws that limit trade. Farmersgenerally support importingforeign species to controlpests.These biological controlscause less environmental dam-age than the use of pesticides.

A gypsy moth caterpillarkills trees by eating theirleaves.

Workers (below) cut invasive waterhyacinths clogging waters in India.Yet some imported plants are ben-eficial. In Turkey (right), women har-vest wheat to feed their families. ▼

▼

What’s Your Point of View?

Should international trade berestricted to limit the risks ofspreading invasive speciesaround the world?

Earth’s Water

A Geographic ViewUnder the Arctic OceanIn a world that’s been almost com-pletely mapped, it’s easy to forget why cartographers used to put monstersin the blank spots. Today we got areminder. The submarine captain hadwarned us that we were in unchartedwaters. . . . Yet the first days of ourcruise through this ice-covered ocean,Earth’s least explored frontier, were . . .smooth. . . . Even when we passedover a mile-high mountain that no one on the planet knew existed, the reaction was one of quiet enthusiasm—“Neat.”

—Glenn Hodges, “The New Cold War: Stalking Arctic Climate Change by Submarine,” National Geographic, March 2000

A submarine crew investigating the Arctic Oceancan still experience the thrill of exploring uncharted territory—one ofEarth’s last frontiers. Although humans live mostly on land, water isimportant to our lives, and all living things need water to survive.Rivers, lakes, and oceans contain water in liquid form. The atmo-sphere holds water vapor, or water in the form of a gas. Glaciers andice sheets are water in solid form. In this section you will learn aboutEarth’s water and its importance to human life.

The Water CycleAs you recall, oceans, lakes, rivers, and other bodies of water make

up a part of the earth called the hydrosphere. Almost all of thehydrosphere is salt water found in the oceans, seas, and a few largesaltwater lakes. The remainder is freshwater found in lakes, rivers,and springs.

46 U n i t 1

Guide to ReadingConsider What You KnowThink of the ways you use water every day. Where does that watercome from? Do you think it willalways be available?

Read to Find Out• How does the amount of water on

Earth remain fairly constant?

• How is the water that makes up70 percent of Earth’s surface distributed?

• Why is freshwater important tohumans?

Terms to Know• water cycle

• evaporation

• condensation

• precipitation

• desalination

• groundwater

• aquifer

Places to Locate• Pacific Ocean

• Atlantic Ocean

• Indian Ocean

• Arctic Ocean

Navy submarine in the Arctic

C h a p t e r 2 47

The total amount of water on the earth does notchange, but the earth’s water is constantly mov-ing—from the oceans to the air to the ground andfinally back to the oceans. The water cycle is thename given to this regular movement of water.The diagram above shows the major parts of thewater cycle.

The sun drives the cycle by evaporating waterfrom the surfaces of oceans, lakes, and streams.Evaporation is the changing of liquid water intovapor, or gas. The sun’s heat causes evaporation.Water vapor rising from the oceans, other bodies ofwater, and plants is gathered in the air. The amountof water vapor the air holds depends on its temper-ature. Warm, less dense air holds more water vaporthan does cool air.

When warm air cools, it cannot retain all of itswater vapor, so the excess water vapor changesinto liquid water—a process called condensation.Tiny droplets of water come together to formclouds. When clouds gather more water than theycan hold, they release moisture, which falls to theearth as precipitation—rain, snow, or sleet,depending on the air temperature and wind con-ditions. This precipitation sinks into the groundand collects in streams and lakes to return to the

oceans. Soon most of it evaporates, and the cyclebegins again.

The amount of water that evaporates is approxi-mately the same amount that falls back to theearth. This amount varies little from year to year.Thus, the total volume of water in the water cycleis more or less constant.

Bodies of Salt WaterSeen from space, the earth’s oceans and seas are

more prominent than the landmasses. As men-tioned earlier, about 70 percent of the earth’s surfaceis water, but almost all of it is salt water. Freshwatermakes up only a small percentage of Earth’s water.

OceansAbout 97 percent of the earth’s water consists of a

huge, continuous body of water that circles theplanet. Geographers divide this enormous expanseinto four oceans: the Pacific, the Atlantic, theIndian, and the Arctic. The Pacific, the largest of theoceans, covers more area than all the earth’s landcombined. The Pacific Ocean is also deep enough insome places to cover Mount Everest, the world’shighest mountain, with more than 1 mile (1.6 km) to

Clouds

Precipitation(snow, sleet,

hail, rain)

Evaporationfrom ocean

Evaporationfrom lakes

and streams

Groundwater to rivers and oceans

Surfacerunoff

Condensation

DIAGRAM STUDY

The Water Cycle

1. Interpreting Diagrams Inwhat part of the cycle doesliquid become vapor?

2. Applying GeographySkills How would life onEarth be different if therewere no precipitation?

48 U n i t 1

spare. The immense size of Earth’s oceans continuesto inspire awe and fascination in humans.

“ It is useless to speculate at great lengthabout why the sea has such a hold on us.Its mystery, its seeming infinity must bepart of the reason. . . . And along with itsvastness comes a visible, enduring wild-ness. We can raze the Amazonian rainforest if we like; we can settle the Alaskantundra or the Arabian desert given theright economic incentives. We cannot, tothe same extent, tame the ocean. . . .”Robert Kunzig, The Restless

Sea: Exploring the World Beneaththe Waves, 1999

Seas, Gulfs, and BaysSeas, gulfs, and bays are bodies of salt water

smaller than oceans. These bodies of water are oftenpartially enclosed by land. The Mediterranean Sea,one of the world’s largest seas, is almost entirelyencircled by southern Europe, northern Africa, andsouthwestern Asia. The Gulf of Mexico is nearlyencircled by the coasts of the United States andMexico. Scientists have identified 66 separate seas,gulfs, and bays, and many smaller divisions.

Economics

Ocean Water to Drinking WaterAlthough 97 percent of the world’s water is found

in oceans, the water is too salty for drinking, farming,or manufacturing. Today efforts focus on ways tomeet the world’s increasing need for freshwater, suchas turning ocean water into freshwater by removingthe salt. This process, known as desalination, is stillin the early stages of development. Because desalina-tion is expensive, only a small amount of freshwateris obtained this way. Even so, certain countries in

Southwest Asia and North Africa use desalinationbecause other freshwater sources are scarce.

Bodies of FreshwaterOnly about 3 percent of the earth’s total water

supply is freshwater, and most is not available forhuman consumption. More than 2 percent of Earth’stotal water supply is frozen in glaciers and ice caps.The Antarctic ice cap, for example, contains morefreshwater than the rest of the world’s regions com-bined. Another 0.5 percent is found beneath theearth’s surface. Lakes, streams, and rivers containfar less than 1 percent of the earth’s water.

Student Web Activity Visit the Glencoe World GeographyWeb site at tx.geography.glencoe.com and click on Student WebActivities––Chapter 2 for an activity about the earth’s oceans.

Kuwait Towers These towers—Kuwait’s mostfamous landmark—have restaurants, an observa-tion deck, and reservoirs storing 2 million gallons(7.6 million l) of desalinated water.

Human-Environment Interaction Why are scientists looking for ways to increase the amountof freshwater?

C h a p t e r 2 49

Checking for Understanding1. Define water cycle, evaporation,

condensation, precipitation,desalination, groundwater,aquifer.

2. Main Ideas Use a web like theone below to organize informa-tion about Earth’s water features.

Critical Thinking3. Drawing Conclusions Use your

knowledge of the water cycle toexplain how droughts might occur.

4. Making Inferences Why might salt water someday provide water for drinking, farming, and manufacturing?

5. Identifying Cause and Effect Howis drinking water contaminatedby hazardous substances releasedon land or into rivers and lakes?

Analyzing Diagrams6. Physical Geography Look at the

diagram of the water cycle on page 47. What source of watersupplies wells and springs?

7. Importance of Rivers Manylarge urban areas developedin river basins. Write adescription of how a riveror rivers contributed to yourcommunity’s development.

Applying Geography

Lakes, Streams, and RiversA lake is a body of water completely surrounded

by land. Most lakes contain freshwater, althoughsome, such as Southwest Asia’s Dead Sea andUtah’s Great Salt Lake, are saltwater remnants ofancient seas. Most lakes are found where glacialmovement has cut deep valleys and built up damsof glacial soil and rock that held back melting ice-water. North America has thousands of glacial lakes.

Flowing water forms streams and rivers. Melt-water, an overflowing lake, or a spring may be thesource, or beginning, of a stream. Streams maycombine to form a river, a larger stream of highervolume that follows a channel along a particularcourse. When rivers join, the major river systemsthat result may flow for thousands of miles. Rain,runoff, and water from tributaries or branchesswell rivers as they flow toward a lake, gulf, sea, orocean. The place where the river empties intoanother body of water is its mouth.

Although lakes, streams, and rivers hold only asmall part of the earth’s water, they meet impor-tant needs. Most large urban areas began as settle-ments along the shores of lakes and rivers, wherepeople would have a constant supply of water.

GroundwaterGroundwater, freshwater which lies beneath the

earth’s surface, comes from rain and melted snowthat filter through the soil and from water thatseeps into the ground from lakes and rivers. Wells

Earth’s Water Features

and springs tap into groundwater and are impor-tant sources of freshwater for people in many ruralareas and in some cities. An underground porousrock layer often saturated with water in the formof streams is called an aquifer (A•kwuh•fuhr).Aquifers and groundwater are important sourcesof freshwater.

Freshwater The Snake River in southern Idahoprovides vital freshwater for agriculture.

Human-Environment Interaction How have fresh-water sources affected the development of humansettlements?

Comparing and ContrastingDo you have a friend or relative who lives in a different state?

You may have talked about the similarities and differencesin the places where you each live. If so, you have compared andcontrasted those states.

50 U n i t 1

The Glencoe Skillbuilder Interactive Workbook,

Level 2 provides instruction andpractice in key social studies skills.

Learning the SkillComparing and contrasting

help you identify similarities anddifferences between two or morethings. Understanding similaritiesand differences can help youmake better judgments aboutthose things. When you compare,you look for similarities. Thingsthat share at least one commonquality can be compared. Whenyou contrast, you look for differ-ences. Things that differ fromone another in at least one waycan be contrasted.

To compare and contrast,apply the following steps:

• Decide which items you willcompare and contrast. In theexample above, you mightcompare and contrast thestates’ populations or theirclimates.

• Determine which character-istics you will use to compareand contrast items. In theexample, you might decide tofocus on the population sizesor the average temperaturesof the two states.

• Identify the similarities anddifferences in these charac-teristics. In the example, youmight compare populationsand contrast climates.

• If possible, identify causes forthe similarities and differences.For example, a state may havea larger population because ithas a warmer climate.

Practicing the SkillStudy the chart above to

answer the following questions.

1. What characteristics are usedto compare and contrast thecountries in the chart?

2. Which are the two smallestcountries in population size?Which country is the largest?

3. How do Argentina and Japandiffer in their populationsizes? How do they differ intheir governments?

4. How are Kenya, Canada, and Australia similar in thelanguages spoken?

5. Which two countries can youinfer probably share similarcultural characteristics?Explain your answer.

Country

Argentina

Australia

Canada

France

Japan

Kenya

Spanish

English

English, French

French

Japanese

English, Swahili

37.5

19.4

31

59.2

127.1

29.8

Republic

Parliamentarydemocracy

Parliamentarydemocracy

Republic

Constitutionalmonarchy

Republic

Languages

2001

Population

(millions)

Government

Source: Population Reference Bureau, 2001

With a partner, select four U.S.cities to research. Decide on atleast three characteristics, suchas population or land area, tocompare and contrast. Collect theinformation for each of the threecharacteristics for each city. Designand draw a chart using your infor-mation. Develop three questionsbased on the chart. Exchange yourwork with another pair of studentsto answer the questions.

C h a p t e r 2 51

Key Points• Planet Earth is located in our solar system.

• The hydrosphere (water), lithosphere (land),and atmosphere (air) make Earth’s biospheresuitable for plant and animal life.

• Great contrasts exist in the heights anddepths of the earth’s surface.

Organizing Your NotesUse a web like the one below tohelp you organize informationabout Planet Earth.

Terms to Know• mantle• continental drift• magma• plate tectonics• subduction• accretion• spreading• fold• fault• weathering• erosion• loess• glacier• moraine

Key Points• Planet Earth is composed of three layers––the

core, the mantle, and the crust.

• Plates that move slowly around the globe pro-duced Earth’s largest features—continents,oceans, and mountain ranges.

• Mountains and islands are created by internalforces called subduction and accretion.

• Internal forces, such as earthquakes and volca-noes, also shape the surface of the earth.

• External forces, such as weathering and wind,glacial, and water erosion, also shape the sur-face of the earth.

Organizing Your NotesCreate a series of flowcharts like the ones below to showwhich forces or physicalprocesses shape Earth’s variousand distinctive landforms.

Terms to Know• hydrosphere• lithosphere• atmosphere• biosphere• continental shelf

Terms to Know• water cycle• evaporation• condensation• precipitation• desalination• groundwater• aquifer

Key Points• The amount of water on Earth remains fairly

constant and moves in the water cycle.

• Water makes up 70 percent of the earth’s surface.

• Earth’s water features are classified as saltwater or freshwater.

• Freshwater is necessary to sustain human life.

Organizing Your NotesCreate an outline using the format below to organize thissection’s notes.

SECTION 1 Planet Earth (pp. 33–36)

SECTION 2 Forces of Change (pp. 37–43)

SECTION 3 Earth’s Water (pp. 46–49)

SUMMARY & STUDY GUIDE

Planet Earth

Effects of Earth’s Forces

Landform Shaped by Force(s)

Earth’s WaterI. The Water Cycle

A. Evaporation1.2.

6. Describe the forms that water takes through-out the water cycle.

Critical Thinking1. Comparing and Contrasting How do

internal and external forces of change affectEarth’s surface differently?

2. Making Inferences How might the rela-tionship among climate, vegetation, soil,and geology affect distribution of plants and animals in different regions?

3. Finding and Summarizing the MainIdea Copy the graphic organizer below,and write the main idea of each section inthe outer ovals and the main idea of thechapter in the center oval. Write a summaryusing appropriate vocabulary.

Reviewing Key TermsWrite the key term that best completes each ofthe following sentences. Refer to the Terms toKnow in the Summary & Study Guide on page 51.

1. Four parts of the earth’s surface are__________, __________, __________, and__________.

2. Underwater trenches are created through theprocess of __________.

3. The theory that continents are slowly movingis called __________.

4. An __________ is an underground porous rocklayer often saturated with water.

5. A __________ is a bend in layers of rock.6. Water vapor changes into liquid through

__________.7. __________ is molten rock within the earth.8. Moisture that falls from the

clouds is __________.9. A __________ is a break in the

earth’s crust.10. The activity of the earth’s mov-

ing plates is called __________.11. Liquid water changes into

water vapor through the pro-cess of __________.

12. __________ wears away theearth’s surface.

Reviewing FactsSECTION 1

1. What are two types of planets?

2. What are the four major typesof landforms found on Earth?

SECTION 23. Describe the earth’s layers.

4. What produced some of Earth’slargest landforms?

SECTION 35. What process keeps the amount

of Earth’s water constant?

Locating PlacesThe Earth: Physical Geography

Match the letters on the map with the physical features of Earth. Write your answers on a sheet of paper.

1. Rocky Mountains2. Isthmus of Panama3. Gulf of Mexico4. Andes

5. Himalaya6. Ural Mountains7. Arctic Ocean

8. Mediterranean Sea9. Bay of Bengal

10. Europe

AB

C

D

EF

GH

I

J

0˚

60˚E60˚W 120˚E120˚W

0˚

30˚N

30˚S

60˚S

60˚N

EQUATOR

TROPIC OF

TROPIC OFCAPRICORN

CANCER

Winkel Tripel projection2,000

2,0000

0

mi.

km

N

ASSESSMENT & ACTIVITIES

52 U n i t 1

C h a p t e r X 53

Self-Check Quiz Visit the Glencoe WorldGeography Web site at tx.geography.glencoe.comand click on Self-Check Quizzes—Chapter 2 toprepare for the Chapter Test.

Thinking Like a GeographerThink about the ways in which geographers classifythe earth’s physical features. As a geographer, inwhat other ways might you classify those features?What would your reasons be for classifying fea-tures differently? Identify different classificationsystems that may be helpful to geographers.

Problem-Solving ActivityProblem-Solution Proposal Using the Inter-net and library resources, learn more about aspecific location where freshwater is scarce andthe reasons for the scarcity. Then write a pro-posal identifying the problem and explainingpossible solutions for the scarcity. Choose onesolution, and give reasons why you think it is thebest alternative. Include charts, graphs, or otherdata that will help readers understand the basisfor your conclusions.

GeoJournalDescriptive Writing Use the journal entries youwrote in your GeoJournal as you read this chapter.Pick one feature, structure, or force of change,and write an expanded description of it. Paint aword picture using concrete, specific details. Con-sider sights, sounds, smells, and textures associ-ated with the feature. Be sure to organize yourdescription in an order that guides your readerthrough your composition.

Technology ActivityUsing the Internet for Research

Choose one of the earth’s features, such asmountains, lakes, or rivers. Use the Internet tofind specific examples of that feature in differentparts of the world. Note the measurement foreach and the continent where it is located. Orga-nize your findings in a chart. If, for example, youchoose rivers as a feature to research, you mightinclude headings such as longest river, widestriver, swiftest river, highest volume river, and soon. Then write a summary, ranking the conti-nents by the sizes of their features.

0

5

10

15

20

25

30

C h a p t e r 2 53

Look at the chart title to see whatkind of information is presented. Forquestion 1, find out what categories

are being compared. Identify which eruption causedthe greatest number of deaths. Then, identify in whichcentury it occurred. Question 2 asks you to infercauses. Note the chart’s title, which gives a clue tothe answer.

Use the information in the chart below toanswer the questions. If you have troubleanswering the questions, use the process of elimination to narrow your choices.

1. Based on the information shown in thechart, in which century did the deadliesteruption occur?

A seventeenthB eighteenthC nineteenthD twentieth

2. The chart probably contains data fromthe past 300+ years because

F more volcanoes erupted then.G more information is available.H no volcanoes erupted before 1631.J eruptions are getting closer together.

Year Volcano Location Deaths (est.)

1631 Mt. Vesuvius Italy 4,000

1783 Laki Iceland 9,350

1883 Krakatau Indonesia 36,000

1902 Mt. Pelée Martinique 28,000

1980 Mt. St. Helens United States 57

1991 Mt. Pinatubo Philippines 800

Notable Volcanic Eruptions