Chapter 8: Weathering and Erosion

224

Weathering and Erosion

Erosion can be a devastatingproblem in many places in theworld, especially in hilly or

mountainous regions. Rock and sedi-ment tend to move downhill underthe influence of gravity. This mud-flow in California threatened livesand destroyed a house. In this chap-ter, you will learn how weatheringand erosion affect rocks. You also willlearn how soil develops from weath-ered rock.

What do you think?Science Journal Look at the picturebelow with a classmate. Discuss whatthis might be. Here’s a hint: Glaciersdon’t always flow in the same direction.Write your answer or best guess inyour Science Journal.

Science TEKS 6.6 A, C; 6.7 A, B

88

225

The Grand Canyon is 440 km long, up to 24 kmwide, and up to 1,800 m deep. The water of the

Colorado River carved the canyon out of rock by wearing away particles and carrying them away formillions of years. The process of wearing away rock

is called erosion.Over time, erosion has shaped and reshaped Earth’s surfacemany times. In this activity, you will explore how running water formed the Grand Canyon.

Model water erosion1. Fill a bread pan with packed sand and

form a smooth, even surface.

2. Place the bread pan in a plastic wash tub.Position one end of the wash tub in a sink under the faucet.

3. Place a brick or wood block under the end of the bread pan beneath the faucet.

4. Turn on the water to form a steady trickle of water falling into the pan and observe for 10 min.The wash tub should catch the eroded sand.

ObserveIn your Science Journal, draw a top view picture of the erosion pattern formedin the sand by the running water. Write a paragraph describing what the sandwould look like if you had left the water running overnight.

EXPLOREACTIVITY

225225225

Handbook ReferenceHandbook Reference

Making a Compare and Contrast Study Fold Make the follow-ing Foldable to help you see how weathering and erosion are similar and different.

1. Place a sheet of paper in front of you so the short side is atthe top. Fold the paper in half from top to bottom and thenunfold.

2. Fold in to the centerfold line to divide the paper into fourths.

3. Label the flaps Weathering and Erosion. Label the middleportion inside your Foldable Both. Before you read thechapter, write the definition of each on the front of the flaps.

4. As you read the chapter, write information you learn on the back of the two flaps.

FOLDABLESReading & StudySkills

FOLDABLESReading & Study Skills

Weathering

Erosion

Weathering and Soil Formation

S E C T I O N

Weathering Have you noticed potholes in roadways and broken concrete

in sidewalks and curbs? When a car rolls over a pothole in theroad in late winter or when you step over a broken sidewalk, youknow things aren’t as solid or permanent as they look. Holes inroads and broken sidewalks show that solid materials can bechanged by nature. Weathering is a natural process that causesrocks to change, breaks them down, and causes them to crum-ble. Freezing and thawing, oxygen in the air, and even plants andanimals can affect the stability of rock. These are some of thethings that cause rocks on Earth’s surface to weather, and insome cases, to become soils.

Mechanical Weathering When a sidewalk breaks apart, a large slab of concrete is bro-

ken into many small pieces. The concrete looks the same. It’sjust broken apart. This is similar to mechanical weathering.Mechanical weathering breaks rocks into smaller pieces with-out changing them chemically. The small pieces are identical incomposition to the original rock, as shown in Figure 1. Two ofthe many causes of mechanical weathering are ice wedging andliving organisms.

■ Identify processes that break rockapart.

■ Describe processes that chemicallychange rock.

■ Explain how soil evolves.

Vocabularyweatheringmechanical weatheringchemical weatheringsoiltopography

Soil forms when rocks break apartand change chemically. Soil is hometo many organisms and most plantsneed soil in order to grow.

Figure 1The forces of mechanical weath-ering break apart rocks. How doyou know that the smaller pieces ofgranite were produced by mechani-cal weathering?

226

Ice Wedging In some areas of the world, air temperaturedrops low enough to freeze water. Then, when the temperaturerises, the ice thaws. This freezing and thawing cycle breaks uprocks. How can this happen? When it rains or snow melts, waterseeps into cracks in rocks. If the temperature drops below freez-ing, ice crystals form. As the crystals grow, they take up morespace than the water did because ice is less dense than water. Thisexpansion exerts pressure on the rocks. With enoughforce, the rocks will crack further and eventually breakapart, as shown in Figure 2. Ice wedging also causes pot-holes to form in roadways.

Explain how ice wedging can break rock apart.

Plants and Animals Plants and animals also causemechanical weathering. As shown in Figure 3, plants cangrow in what seem to be the most inconvenient places.Their roots grow deep into cracks in rock where watercollects. As they grow, roots become thicker and longer,slowly exerting pressure and wedging rock apart.

Gophers and prairie dogs also weather rock—as doother animals that burrow in the ground. As they bur-row through sediment or soft sedimentary rock, animalsbreak rock apart. They also push some rock and sedi-ment to the surface where another kind of weathering,called chemical weathering, takes place more rapidly.

Figure 2Over time, freezing water can break apart rock.

Water seeps into cracks.The deeper the cracks are, thedeeper water can seep in.

The water freezes andexpands forcing the cracks toopen further.

The ice melts. If the temper-ature falls below freezing again,the process will repeat itself.

Figure 3Tree roots can break rock apart.

SECTION 1 Weathering and Soil Formation 227

Elements in Kaolinite

56%Oxygen

13%Silicon

6%Aluminum

25%Hydrogen

Chemical Weathering Chemical weathering occurs when the chemical composi-

tion of rock changes. This kind of weathering is rapid in tropicalregions where it’s moist and warm most of the time. Becausedesert areas have little rainfall and polar regions have low tem-peratures, chemical weathering occurs slowly in these areas.Table 1 summarizes the rates of chemical weathering for differ-ent climates. Two important causes of chemical weathering arenatural acids and oxygen.

Why is chemical weathering rapid in the tropics?

Natural Acids Some rocks react chemically withnatural acids in the environment. When water mixeswith carbon dioxide in air or soil, for example, carbonicacid forms. Carbonic acid can change the chemicalcomposition of minerals in rocks, as shown in Figure 4.

Although carbonic acid is weak, it reacts chemicallywith many rocks. Vinegar reacts with the calcium car-bonate in chalk, dissolving it. In a similar way, whencarbonic acid comes in contact with rocks like lime-stone, dolomite, and marble, they dissolve. Other rocksalso weather when exposed to carbonic acid.

228 CHAPTER 8 Weathering and Erosion

Feldspar crystals reactwith carbonic acid.

The mineral kaoliniteis formed.

Table 1 Rates of Weathering

Climate Chemical Weathering

Hot and dry Slow

Hot and wet Fast

Cold and dry Slow

Cold and wet Slow

Figure 4Chemical weathering changesthe chemical composition of minerals and rocks. How iskaolinite different from feldspar?

61%Oxygen

23%Silicon

8%Aluminum

8%Potassium

Elements in Feldspar

Plant Acids Plant roots also pro-duce acid that reacts with rocks. Manyplants produce a substance called tan-nin. In solution, tannin forms tannicacid. This acid dissolves some mineralsin rocks. When minerals dissolve, theremaining rock is weakened, and it canbreak into smaller pieces. The nexttime you see moss or other plantsgrowing on rock, as shown in Figure 5,peel back the plant. You’ll likely see dis-coloration of the rock where plantacids are reacting chemically with someof the minerals in the rock.

Effect of Oxygen When you see rusty cars, reddish soil, orreddish stains on rock, you witness oxidation, the effects ofchemical changes caused by oxygen. When iron-containingmaterials such as steel are oxidized a chemical reaction causes thematerial to rust. Rocks chemically weather in a similar way.When some iron-containing minerals are exposed to oxygen,they can weather to minerals that are like rust. This leaves therock weakened, and it can break apart. As shown in Figure 6,some rocks also can be colored red or orange when iron-bearingminerals in them react with oxygen.


Rock Dissolving AcidsProcedure

WARNING: Do not removegoggles until activity, clean up,and handwashing is completed.1. Use an eyedropper to put

several drops of vinegar onpieces of chalk and lime-stone. Observe the resultswith a magnifying glass.

2. Put several drops of 5%hydrochloric acid on thechalk and limestone.Observe the results.

Analysis1. Describe the effect of the

hydrochloric acid and vine-gar on chalk and limestone.

2. Research what type of acidvinegar contains.

Figure 5Moss growing on rocks can causechemical weathering.

Even a tiny amount of ironin rock can combine with oxygenand form a reddish iron oxide.

The iron contained in metalobjects such as this truck also cancombine with oxygen and form areddish iron oxide called rust.

Figure 6Oxidation occurs in rocks and cars.


Parent Rock Slope of Land Climate Time Organisms

Table 2 Factors that Affect Soil Formation

Analyzing SoilsProcedure1. Obtain a sample of soil from

near your home.2. Spread the soil out over a

piece of newspaper.3. Carefully sort through the

soil. Separate out organicmatter from weathered rock.

4. Wash hands thoroughly afterworking with soils.

Analysis1. Besides the organic materi-

als and the remains ofweathered rock, what else is present in the soil?

2. Is some of the soil too fine-grained to tell if it is organicor weathered rock?

Soil Is soil merely dirt under your feet, or is it something more

important? Soil is a mixture of weathered rock, organic matter,water, and air that supports the growth of plant life. Organic mat-ter includes decomposed leaves, twigs, roots, and other material.Many factors affect soil formation.

Parent Rock As listed in Table 2, one factor affecting soil for-mation is the kind of parent rock that is being weathered. Forexample, where limestone is chemically weathered, clayey soil iscommon because clay is left behind when the limestone dissolves.In areas where sandstone is weathered, sandy soil forms.

The Slope of the Land The topography, or surface featuresof an area also influence the types of soils that develop. You’veprobably noticed that on steep hillsides, soil has little chance ofdeveloping. This is because rock fragments move downhill con-stantly. However, in lowlands where the land is flat, wind andwater deposit fine sediments that help form thick soils.

Climate Climate affects soil evolution, too. If rock weathersquickly, deep soils can develop rapidly. This is more likely to hap-pen in tropical regions where the climate is warm and moist. Cli-mate also affects the amount of organic material in soil. Soils indesert climates contain little organic material. However, in mild,humid climates, vegetation is lush and much organic material ispresent. When plants and animals die, decomposition by fungiand bacteria begins. The result is the formation of a dark-coloredmaterial called humus, as shown in the soil profile in Figure 7.Most of the organic matter in soil is humus. Humus helps soilhold water and provides nutrients that plants need to grow.

Time It takes time for rocks to weather. It can takethousands of years for some soils to form. As soilsdevelop, they become less like the rock from whichthey formed. In young soils, the parent rock deter-mines the soil characteristics. As weathering continues,however, the soil resembles the parent rock less andless. Thicker, well-developed soils often are found inareas where weathering has gone on undisturbed for along period of time. For this to happen, soil materialsmust not be eroded away and new sediment must notbe deposited over the land’s surface too quickly.

Organisms Organisms influence soil development.Lichens are small organisms that consist of an alga anda fungus that live together for mutual benefit. You mayhave seen lichens in the form of multicolored patchesgrowing on tree branches or cliff faces. Interestingly,lichens can grow directly on rock. As they grow, theytake nutrients from the rock that they are starting tobreak down, forming a thin soil. After a soil hasformed, many types of plants such as grasses and treescan grow.

The roots of these plants further break down the parent rock.Dead plant material such as leaves accumulates and adds organicmatter to the soil. Some plants contribute more organic matterto soil than others. For example, soil under grassy areas often isricher in organic matter than soil developing under forests. Thisis why some of the best farmland in the midwestern UnitedStates is where grasslands used to be.


Section Assessment

1. What are two ways that rocks are mechani-cally weathered?

2. Name two agents of chemical weathering.

3. How does carbonic acid weather rocks?

4. How does soil form? What factors areimportant?

5. Think Critically How could climate affectrates of mechanical weathering? Whatabout chemical weathering? How are thetwo kinds of weathering related?

6. Comparing and Contrasting Compare andcontrast mechanical weathering caused by icewedging with mechanical weathering caused by growing roots. For more help, refer to theScience Skill Handbook.

7. Communicating Write a descriptive poem in your Science Journal that explains differentways rocks are weathered. For more help,refer to the Science Skill Handbook.

Figure 7Soils contain layers that are cre-ated by weathering, the flow ofwater and chemicals, and theactivities of organisms. Whatpart do microorganisms play in soildevelopment?

Humus-rich surface layer

Intermediate layer

Slightly altered parent material

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Compare your conclusions with those ofother students in your class. For more help,refer to the Science Skill Handbook.

Classifying Soils


2. After classifying your soil sample, examine itunder a microscope. Draw the particles andany other materials that you see.

3. Wash your hands thoroughly after you are finished working with soils.

Conclude and Apply1. Determine the texture of your soil

sample.

2. Describe two characteristics of loam soil.

3. Describe two features of sandy loam soil.

4. Based on your observations with the stereomicroscope, what types of particlesand other materials did you see? Did you observe any evidence of the activities of organisms?

Not all soils are the same. Geologists and soilscientists classify soils based on the amounts

and kinds of particles they contain.

What You’ll InvestigateHow is soil texture determined?

Materialssoil samplestereomicroscope*hand lens*Alternate materials

Safety Precautions

Goals■ Classify a soil using an identification key.■ Observe soil with a stereomicroscope.

Procedure1. Place a small sample of moistened soil

between your fingers. Then follow the directions in the classification key below.a. Slide your fingers back and forth

past each other. If your sample feelsgritty, go to b. If it doesn’t feel gritty,go to c.

b. If you can mold the soil into a firm ball, it’ssandy loam soil. If you cannot mold it intoa firm ball, it’s sandy soil.

c. If your sample is sticky, go to d. If yoursample isn’t sticky, go to e.

d. If your sample can be molded into a long,thin ribbon, it’s clay soil. If your soil can’tbe molded into a long, thin ribbon it’s clay loam soil.

e. If your sample is smooth, it’s silty loamsoil. If it isn’t smooth, it’s loam soil.

Agents of Erosion Imagine looking over the rim of the Grand Canyon at the

winding Colorado River below or watching the sunset overUtah’s famous arches. Features such as these are spectacularexamples of Earth’s natural beauty, but how can canyons andarches form in solid rock? These features and many other naturallandforms are a result of erosion of Earth’s surface. Erosion is thewearing away and removal of rock or sediment. Erosion occursbecause gravity, ice, wind, and water sculpt Earth’s surface.

Gravity Gravity is a force that pulls every object toward every other

object. Gravity pulls everything on Earth toward its center. As aresult, water flows downhill and rocks tumble down slopes.When gravity alone causes rock or sediment to move down aslope, the erosion is called mass movement. Mass movementscan occur anywhere there are hills or mountains. One placewhere they often occur is near volcanoes, as shown in Figure 8.Creep, slump, rock slides, and mudflows are four types of massmovements, as seen in Figure 9.

■ Identify agents of erosion.■ Describe the effects of erosion.

Vocabularyerosionmass movementcreepslumpdeflationabrasionrunoff

Erosion shapes Earth’s surface.

Erosion of Earth’s SurfaceS E C T I O N

Figure 8The town of Weed, California,was built on top of a landslidethat moved down the volcanoknown as Mount Shasta.

233

Yreka

Weed

MountShasta

Landslide

97

5

0 15 km

Figure 9

VISUALIZING MASS MOVEMENTS

When the relentless tug of gravitycauses a large chunk of soil or rock tomove downhill—either gradually or

with sudden speed—the result is what geol-ogists call a mass movement. Weathering andwater often contribute to mass movements.Several kinds are shown here.

SLUMP This cliff in North Dakotashows the effects of the mass move-ment known as slump. Slumping oftenoccurs after earthquakes or heavy andprolonged rains.

ROCK SLIDESWhen rocks breakfree from the side of a cliff or a mountain, theycrash down in whatis called a rock slide.Rock slides, like the one at the left inYosemite NationalPark, can occur withlittle warning.

CREEP Whensoil on a slopemoves very slowlydownhill, a massmovement calledcreep occurs. Someof the trees at righthave been gradu-ally bent becauseof creep’s pressureon their trunks.

A

B

C

MUDFLOWS A Japanese town shows the devastation that a fourth type of mass movement—a mudflow—can bring. When heavy moisture saturates sediments,mudflows can develop, sending a pasty mix of waterand sediment downhill over the ground’s surface.

D


Creep Creep is the name for aprocess in which sediments moveslowly downhill, as shown in Fig-ure 9A. Creep is common wherefreezing and thawing occur. As iceexpands in soil, it pushes sedi-ments up. Then as soil thaws, thesediments move farther down-slope. Figure 10 shows how smallparticles of sediment can creepdownslope. Over time, creep canmove large amounts of sediment,possibly causing damage to somestructures. Do you live in an areawhere you can see the results ofcreep?

Slump A slump occurs when a mass of rock or sedimentmoves downhill along a curved surface, as shown in Figure 9B.Slumps are most common in thick layers of loose sediment, butthey also form in sedimentary rock. Slumps frequently occur onslopes that have been undercut by erosion, such as those abovethe bases of cliffs that have been eroded by waves. Slumping ofthis kind is common along the coast of Southern Californiawhere it threatens to destroy houses and other buildings.

Rock Slides Can you imagine millions of cubic meters ofrock roaring down a mountain at speeds greater than 100 km/h?This can happen when a rock slide occurs. During a rock slidelayers of rock break loose from slopes and slide to the bottom.The rock layers often bounce and break apart during move-ment. This produces a huge, jumbled pile of rocks at the bottomof the slope, as you can see in Figure 9C. Rock slides can bedestructive, sometimes destroying entire villages or causing haz-ards on roads in mountainous areas.

Mudflows Where heavy rains or melting snow and ice satu-rate sediments, mudflows, as shown in Figure 9D, can develop. Amudflow is a mass of wet sediment that flows downhill over theground surface. Some mudflows can be thick and flow slowlydownhill at rates of a few meters per day. Other mudflows can bemuch more fluid and move down slope at speeds approaching70 km/h. This type of mudflow is common on some volcanoes.

What is the slowest of the four kinds of massmovement?

SECTION 2 Erosion of Earth’s Surface 235

Slumps and rock slidesoften occur when sedimentbecomes saturated by rain.Water between sedimentgrains helps lift up over-lying rock and sediment.This makes it easier for thesediment to overcome theforces holding it in place.Can you think of a waysome slopes might be pro-tected from slumps androck slides? Explain.

Soil or sediment

Creep

Top of soil when frozen

Expansion caused by freezing

Falling caused by thawing

Top of soil when thawed

Figure 10When soil freezes, particles arelifted. When it thaws, the parti-cles are pulled downhill by grav-ity. Eventually, large amounts ofsediment are moved by thisprocess.

Ice In some parts of the world, ice is an agent of erosion. In cold

regions, more snow might fall than melts. Over many years, thesnow can accumulate to form large, deep masses of ice called gla-ciers. When the ice in a glacier becomes thick enough, its ownweight causes it to flow downhill under the influence of gravity.As glaciers move over Earth’s surface, they erode materials fromsome areas and deposit sediment in other areas. Figure 11 showsthe two kinds of glaciers—continental glaciers and valley glaciers.

Today, continental glaciers in polar regions cover about tenpercent of Earth. These glaciers are so large and thick that theycan bury mountain ranges. Valley glaciers are much smaller andare located in high mountains where the average temperatureisn’t warm enough to melt the ice sheets. Continental and valleyglaciers move and cause erosion.

Glacial Erosion Glaciers can erode rock in two different ways.If the rock that the glacier is sliding over has cracks in it, the ice can pull out pieces of rock. This causes the rock to erodeslowly. The loose pieces of rock freeze into the bottom of the gla-cier and are dragged along as the glacier moves. As these differ-ent-sized fragments of rock are dragged over Earth’s surface, theyscratch the rock below like giant sheets of sandpaper. Thisscratching is the second way that glaciers can erode rock. Scratch-ing produces large grooves or smaller striations in the rockunderneath. The scratching also can wear rock into a fine powdercalled rock flour.


Figure 11Glaciers form in cold regions.

Continental glaciersare located near thepoles in Antarctica andGreenland.

Valley glaciersare found at highelevations on manycontinents.

Research Visit the Glencoe Science Web site attx.science.glencoe.comfor more information aboutglacial erosion and deposi-tion. Communicate to yourclass what you learned.

Continental GlacierValley Glacier

http://tx.science.glencoe.com

Effects of Glacial Erosion Glacial erosion of rock can be apowerful force shaping Earth’s surface. In mountains, valley gla-ciers can remove rock from the mountaintops to form largebowls, called cirques (SURKS), and steep peaks. When a glaciermoves into a stream valley, it erodes rock along the valley sides,producing a wider, U-shaped valley. These features are shown inFigure 12. Continental glaciers also shape Earth’s surface. Theseglaciers can scour large lakes and completely remove rock layersfrom the land’s surface.

Glacial Deposition Glaciers also can deposit sediments.When stagnant glacier ice melts or when ice melts at the bot-tom of a flowing glacier or along its edges, the sediment theice was carrying gets left behind on Earth’s surface. This sedi-ment, deposited directly from glacier ice, is called till. Till is amixture of different-sized particles, ranging from clay to largeboulders.

As you can imagine, a lot ofmelting occurs around glaciers,especially during summer. Somuch water can be produced thatrivers often flow away from theglacier. These rivers carry anddeposit sediment. Sand and graveldeposits laid down by these rivers,shown in Figure 13, are calledoutwash. Unlike till, outwash usu-ally consists of particles that areall about the same size.

Figure 12Many high-altitude areas owe their distinctiveappearance to glacial erosion.

Glaciers can widen valleys giving them aU-shaped profile.

Mountain glaciers can carve bowl-shapeddepressions called cirques.

Figure 13This valley in New Zealand hasbeen filled with outwash. Howcould you distinguish outwashfrom till?


Wind If you’ve had sand blow into your eyes, you’ve experienced

wind as an agent of erosion. When wind blows across loose sediments like silt and sand, it lifts and carries it. As shown inFigure 14, wind often leaves behind particles too heavy to move.This erosion of the land by wind is called deflation. Deflationcan lower the land’s surface by several meters.

Wind that is carrying sediment can wear down, or abrade,other rocks just as a sandblasting machine would do. Abrasionis a form of erosion that can make pits in rocks and producesmooth, polished surfaces. Abrasion is common in some desertsand in some cold regions with strong winds.

How does abrasion occur?

When wind blows around some irregular feature on Earth’ssurface, such as a rock or clump of vegetation, it slows down.This causes sand carried by the wind to be deposited. If thissand deposit continues to grow, a sand dune like that shown inFigure 15A might form. Sand dunes move when wind carriessand up one side of the dune and it avalanches down the other,as shown in Figure 15B.

Sometimes, wind carries only fine sediment called silt. Whenthis sediment is deposited, an accumulation of silt called loess(LOOS) can blanket Earth’s surface. Loess is as fine as talcumpowder. Loess often is deposited downwind of some largedeserts and near glacial streams.


Figure 14In a desert, where small particleshave been carried away by wind,larger sediments called desertpavement remain behind.

Figure 15Wind transportation of sand creates sand dunes.

Sand dunes do not remain inone location—they migrate.

Dune movement

As wind blows over a sand dune, sand blows upthe windward side and tumbles down the other side.In this way, a sand dune migrates across the land.

Water You probably have seen muddy water streaming

down a street after a heavy rain. You might evenhave taken off your shoes and waded through thewater. Water that flows over Earth’s surface is calledrunoff. Runoff is an important agent of erosion.This is especially true if the water is moving fast.The more speed water has, the more material it cancarry with it. Water can flow over Earth’s surface inseveral different ways, as you will soon discover.

Sheet Flow As raindrops fall to Earth, they break up clumpsof soil and loosen small grains of sediment. If these raindropsare falling on a sloped land surface, a thin sheet of water mightbegin to move downhill. You have observed something similar ifyou’ve ever washed a car and seen sheets of water flowing overthe hood, as shown in Figure 16. When water flows downhill asa thin sheet, it is called sheet flow. This thin sheet of water cancarry loose sediment grains with it, and cause erosion of theland. This erosion is called sheet erosion.

If you’ve ever traveled through parts ofyour state where there are farms, you

might have seen bare, recently cultivatedfields. Perhaps the soil was prepared forplanting a crop of corn, oats, or soybeans.Do you think sheet erosion can visibly affectthe soil in farm fields?

Identifying the ProblemThe top layer of most soils is much

darker than layers beneath it because it con-tains more organic matter. This layer is thefirst to be removed from a slope by sheetflow. How does the photo show evidence ofsheet erosion?

Solving the Problem1. Observe the photo and write a descrip-

tion of it in your Science Journal.

Can evidence of sheet erosion be seen in a farm field?

2. Infer why some areas of the field aredarker colored than others are. Wheredo you think the highest point(s) are inthis field?

3. Make a generalization about the darkerareas of the field.

Problem-Solving Activity


Figure 16Water flows over the hood of acar as a thin sheet. How is thissimilar to sheet flow on Earth’s surface?

Rills and Gullies Where a sheet of water flows aroundobstacles and becomes deeper, rills can form. Rills are smallchannels cut into the sediment at Earth’s surface. These channelscarry more sediment than can be moved by sheet flow. In somecases, a network of rills can form on a slope after just one heavyrain. Large amounts of sediment can be picked up and carriedaway by rills.

As runoff continues to flow through the rills, more sedimenterodes and the channel widens and deepens. When the channelsget to be about 0.5 m across, they are called gullies, as shown inFigure 17.

Streams Gullies often connect to stream channels. Streamscan be so small that you could jump to the other side or largeenough for huge river barges to transport products along theircourse. Most streams have water flowing through them continu-ally, but some have water only during part of the year.

In mountainous and hilly regions, as in Figure 18, streamsflow down steep slopes. These streams have a lot of energy and

often cut into the rock beneaththeir valleys. This type of streamtypically has white-water rapidsand may have waterfalls. Asstreams move out of the moun-tains and onto flatter land, theybegin to flow more smoothly. Thestreams might snake back andforth across their valley, erodingand depositing sediments alongtheir sides. All streams eventuallymust flow into the ocean or a largelake. The level of water in the oceanor lake determines how deeply ariver can erode.

Research Visit the Glencoe Science Web site attx.science.glencoe.comfor more information abouthow running water shapesEarth’s surface. Communicateto your class what you learn.

Figure 17Gullies often form on vegetation-free slopes.

Figure 18Streams that flow down steepslopes such as this one inYosemite National Park oftenhave whitewater rapids andwaterfalls.

240


Shaping Earth’s Surface If you did theExplore activity at the beginning of thechapter, you saw a small model of erosionby a stream. You might not think aboutthem much, but streams are the mostimportant agent of erosion on Earth. Theyshape more of Earth’s surface than ice, wind,or gravity. Over long periods of time, watermoving in a stream can have enough powerto cut large canyons into solid rock. Manystreams together can sculpt the land over awide region, forming valleys and leavingsome rock as hills. Streams also shape theland by depositing sediment. Rivers candeposit sand bars along their course, andcan build up sheets of sand across their val-leys. When rivers enter oceans or lakes, thewater slows and sediment is deposited. This can form large accu-mulations of sediment called deltas, as in Figure 19. The city ofNew Orleans is built on the delta formed by the Mississippi River.

Effects of Erosion As you’ve learned, all agents of erosion change Earth’s surface.

Rock and sediment are removed from some areas only to bedeposited somewhere else. Where material is removed, canyons,valleys, and mountain bowls can form. Where sediment accumu-lates, deltas, sand bars, sand dunes, and other features make upthe land.


Section Assessment

1. List four agents of erosion. Which of theseis the fastest agent of erosion? The slow-est? Explain your answers.

2. How does deflation differ from abrasion?

3. How does a cirque form?

4. When do streams deposit sediments?When do they erode them?

5. Think Critically Why might a river thatwas eroding and depositing sedimentalong its sides start to cut into Earth toform a canyon?

6. Recognizing Cause and Effect Why might ariver start filling its valley with sediment? Formore help, refer to the Science Skill Handbook.

7. Solving One-Step Equations If wind is erod-ing an area at a rate of 2 mm per year anddepositing it in a smaller area at a rate of 7 mmper year, how much lower will the first area be in meters after 2 thousand years? How muchhigher will the second area be? For more help,refer to the Math Skill Handbook.

Figure 19A triangular area of sedimentnear the mouth of a river is calleda delta. Ancient deltas that arenow dry land are often excellentplaces to grow crops.

Possible Materialsblocks of wood pails (2)*books 1,000 mL beaker paint trays (2) triple-beam balancesoil calculatorgrass sod watchwater*Alternate materials

Goals■ Design an experiment to measure

soil loss from grass-covered soil andfrom soil without grass cover.

■ Calculate the percent of soil losswith and without grass cover.

During urban highway construction, surface mining, forest harvesting, or agricul-tural cultivation, surface vegetation can be removed from soil. These practices

expose soil to water and wind. Does vegetation significantly reduce soil erosion?

Recognize the ProblemHow much does vegetation reduce soil erosion?

Form a HypothesisBased on what you’ve read and observed, hypothesize about how much less soil willbe eroded from a sodded field than from bare soil.

Measuring Soil Erosion


Safety Precautions

Wash your hands thoroughlywhen you are through work-ing with soils.

Test Your Hypothesis

Analyze Your Data

Draw Conclusions

would be the results of the treat-ment for the uncovered soil sample.

Do1. Make sure your teacher approves

your plan before you start.

2. Carry out the experiment step bystep as planned.

3. While doing the experiment, recordyour observations and complete thedata table in your Science Journal.

Plan 1. As a group, agree upon the hypothe-

sis and decide how you will test it.Identify which results will falsify orconfirm the hypothesis.

2. List the steps you will need to take to test your hypothesis. Describeexactly what you will do in each step.

3. Prepare a data table in your Science Journal to record your observations.

4. Read over the entire experiment tomake sure all steps are in logicalorder, and that you have all neces-sary materials.

5. Identify all constants and variablesand the control of the experiment.A control is a standard for comparingthe results of an experiment. Onepossible control for this experiment

3. What was your control in this experiment? Why?

4. Which were the variables you keptconstant? Which did you vary?

1. Compare the percent of soil lossfrom each soil sample.

2. Compare your results with those ofother groups.

1. Did the results support your hypoth-esis? Explain.

2. Infer what effect other types ofplants would have in reducing soilerosion. Do you think that grass isbetter or worse than most otherplants at reducing erosion?

ACTIVITY 243

Write a letter to the editor of a newspaper.In your letter, summarize what you learnedin your experiment about the effect of plantcover on soil erosion.

(A) Mass of (B) Mass of % of Soil LossSoil at Start Eroded Soil (B/A) � 100

Covered Soil Sample

Uncovered Soil Sample

Vegetation and Erosion

Parts ofIndia’s TajMahal areturning yellow frompollutants.

Acid rainhas notbeen kind tothis Mayanfigure.244

SCIENCEAND

HISTORYSCIENCE

CAN CHANGETHE COURSEOF HISTORY!

The Taj Mahal in India, the Acropolis in

Greece, and the Colosseum in Italy,

have stood for centuries. They’ve sur-

vived wars, souvenir-hunters, and natural

weathering from wind and rain. But now,

something far worse threatens their existence—

acid rain. Over the last few decades, this form

of pollution has eaten away at some of history’s

greatest monuments.

Acid rain leads to health and environmen-

tal risks. It also harms human-made structures.

Most of these structures are made of sand-

stone, limestone, and marble. Acid rain causes

the calcium in these stones to form calcium

sulfate, or gypsum. Gypsum’s powdery little

blotches are sometimes called “marble can-

cer.” When it rains, the gypsum washes away,

along with some of the surface of the monu-

ment. In many cases, acidic soot falls into the

cracks of monuments. When rainwater seeps

into the cracks, acidic water is formed, which

further damages the structure.

Acid rain isdestroyingsome of theworld’smostfamous monuments

In Agra, India, the smooth, white marble

mausoleum called the Taj Mahal has stood

since the seventeenth century. But acid rain

is making the surface of the building yellow

and flaky. The pollution is caused by hun-

dreds of factories surrounding Agra that emit

damaging chemicals.

What moisture, molds, and the roots of

vegetation couldn’t do in 1,500 years, acid

rain is doing in decades. It is destroying the

Mayan ruins of Mexico. Pollution is causing

statues to crumble and paintings on walls to

flake off. The culprits are oil burning refiner-

ies and exhaust from tour buses.

Acid rain is a huge problem affecting

national monuments and treasures in just

about every urban location in the world.

These include the Capitol building in Wash-

ington, D.C., churches in Germany,

and stained-glass windows in Sweden.

Because of pollution, many corroding statues

displayed outdoors have been brought inside

museums. In London, acid rain has forced

workers to repair and replace so much of

Westminster Abbey that the structure is

becoming a mere copy of the original.

Throughout the world, acid rain has

weathered many structures more in the last

20 years than in the 2,000 years before. This

is one reason some steps have been taken in

Europe and the United States to reduce

emissions from the burning of fossil fuels. If

these laws don’t work, many irreplaceable art

treasures may be gone forever.

CONNECTIONS Identify Which monuments and buildings representthe United States? Brainstorm a list with your class. Then choose a monument and, using your school’s media center or the Glencoe ScienceWeb site, learn more about it. Is acid rain affecting it in any way?

For more information, visittx.science.glencoe.com

Greece’s Parthenon is slowly being eatenaway by acid rain.

Greece’s Parthenon is slowly being eatenaway by acid rain.


Identify common charac-teristics of weathering anderosion and write them on

the middle section of your Foldable.

After You ReadFOLDABLESReading & StudySkills

FOLDABLESReading & Study Skills

Study GuideChapter XXXX Study GuideChapter 88

246 CHAPTER STUDY GUIDE

Section 2 Erosion of Earth’s Surface1. Erosion is the wearing away and removal of

rock. In the photo below, what evidence doyou see that erosion has occurred?

2. Agents of erosion include gravity, ice, wind,and water. Which agent of erosion is respon-sible for this unusual structure?

3. All agents of erosion move rock and sedi-ment. When energy of motion decreases,sediment is deposited.

4. Erosion and deposition determine theshape of the land.

Section 1 Weathering and Soil Formation

1. Weathering includes processes that breakdown rock.

2. During mechanical weathering, physicalprocesses break rock into smaller pieces.

3. During chemical weathering, the chemicalcomposition of rocks is changed. Whatcauses the reddish color of these rocks?

4. Soil evolves over time from weathered rock.Parent rock, topography, climate, andorganisms affect soil formation. Do youthink a thick soil layer could form on this surface? Why or why not?

Study GuideChapter XXXX Study GuideChapter 88

CHAPTER STUDY GUIDE 247

Vocabulary Wordsa. abrasion g. mechanicalb. chemical weathering weatheringc. creep h. runoffd. deflation i. slumpe. erosion j. soilf. mass movement k. topography

l. weathering

Using Vocabulary

Use each of the following pairs of terms in asentence.

1. chemical weathering, mechanical weathering

2. erosion, weathering

3. deflation, runoff

4. mass movement, weathering

5. soil, abrasion

6. soil, erosion

7. mass movement, mechanical weathering

8. weathering, chemical weathering

9. creep, slump

10. topography, runoff

Read the chapters before you go over them inclass. Being familiar with the material beforeyour teacher explains it gives you a better understanding and provides you with a goodopportunity to ask questions.

Study Tip

Erosional Agent Evidence of Erosion Evidence of Deposition

Gravity material piled at bottom of slopes

Ice cirques, striations, U-shaped valleys

Wind sand dunes, loess

Surface Water rills, gullies,stream valleys

Erosion and Deposition

Fill in the following table, which compares erosion and deposition by different agents.

curved slup scars, leaningfence posts, steep cliffs

till, outwashoutwash

desert pavementpitted rocks

bars, deltas

Assessment & ReviewChapter 1515 Assessment & ReviewChapter 88

Choose the word or phrase that best answersthe question.

1. Which of the following agents of erosionforms U-shaped valleys?A) gravity C) ice B) surface water D) wind

2. In which of these places is chemical weath-ering most rapid?A) deserts C) polar regions B) mountains D) tropical regions

3. Which of the following forms when carbondioxide combines with water?A) calcium carbonate C) tannic acid B) carbonic acid D) dripstone

4. Which process causes rocks to weather to areddish color?A) oxidation C) carbon dioxideB) deflation D) frost action

5. Which type of mass movement occurswhen sediments slowly move downhillbecause of freezing and thawing?A) creep C) slump B) rock slide D) mudflow

6. Which of the following helps form cirquesand U-shaped valleys?A) rill erosion C) deflation B) ice wedging D) till

7. What is windblown, fine sediment called?A) till C) loess B) outwash D) delta

8. Which of the following refers to water thatflows over Earth’s surface?A) runoffB) slumpC) chemical weatheringD) till

9. Which of the following is an example ofchemical weathering?A) Plant roots grow in cracks in rock and

break the rock apart.B) Freezing and thawing of water widens

cracks in rocks.C) Wind blows sand into rock, scratching

the rock.D) Oxygen causes iron-bearing minerals in

rock to break down.

10. Which one of the following erosional agentscreates desert pavement?A) wind C) water B) gravity D) ice

11. Explain why mass movement is more com-mon after a heavy rainfall.

12. How does climate affect the development ofsoils?

13. How could somemass movement beprevented?

14. Would chemicalweathering berapid in Antarc-tica?

15. Why do caves formonly in certaintypes of rock?

16. Recognizing Cause and Effect Explain howwater creates stream valleys.

17. Forming Hypotheses Form hypothesesabout how deeply water could erode andabout how deeply glaciers could erode.

248 CHAPTER ASSESSMENT

AssessmentChapter 1515

Geologists measured the amount ofcumulative precipitation and the amountof land movement along highway 50 inCalifornia to see if there was a relationshipbetween them. TEKS 6.2 C; 6.4 B

Study the graph and answer the following questions.

1. According to this information, atwhich precipitation level did soilmovement begin?A) about 125 cm C) about 75 cmB) about 50 cm D) about 100 cm

2. Based on this information, which ofthe following is a reasonable conclu-sion to make about the relationshipbetween precipitation and movement?F) As precipitation decreases,

movement increases.G) As movement decreases, precipita-

tion increases.H) There is almost no movement until

precipitation reaches a certain level.J) There is no relationship.

AssessmentChapter 88

CHAPTER ASSESSMENT 249

18. Recognizing Cause and Effect Explain howvalley glaciers create U-shaped valleys.

19. Classifying Classify the following by theagent that deposits each: sand dune, delta,till, and loess.

20. Concept Mapping Complete the conceptmap showing the different types of mass movements.

21. Poster Use photographs from old maga-zines to make a poster that illustrates differ-ent kinds of weathering and erosion.Display your poster in your classroom.

22. Model Use polystyrene, cardboard and clayto make a model of a glacier. Include a riverof meltwater leading away from the glacier.Use markers to label the areas of erosionand deposition. Show and label areas wheretill and outwash sediments could be found.Display your model in your classroom.

10/1/97 1/1/98 4/1/98 7/1/98

25

50

75

100

125

150

175

200

Date

Tota

l Cha

nge

(cm

)

Total Precipitation and Movement

Precipitation

Movement

Go to the Glencoe Science Web site at tx.science.glencoe.com or use theGlencoe Science CD-ROM for additionalchapter assessment.

TECHNOLOGY

Soil particlesmove slowly

downhill.

Wet sediment flows

downhill.

Slump Rock slide

Mass Movement

TAKS Practice


Chapter 8: Weathering and Erosion

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