the earth's water -...

The Earth's Water

If you were able to ride a drop of water in a mountainstream, you would experience an incredible journey.You might spend parts of the journey tumbling

over rocks, plunging down waterfalls, creeping alongunderground passages, and emerging to rush headlong outto sea. But your journey would not end there. All waterkeeps moving. In this chapter, you will learn how watermoves and changes. You will also learn about differentbodies of water, such as rivers, lakes, and oceans.

ORGANIZE YOUR THOUGHTS

Air

R ivers and lakes Water Oceans

^ Underground

Goals for Learning

^•To describe the water cycle^•To compare fresh water and salt water

^-To explain how bodies of fresh water form

To describe the ocean floor and other ocean features

215

Lesson How Does the Earth's Water Move?

Objectives

After readingthis lesson, youshould be able to> describe the

movement ofwater throughthe water cycle.

> identify howwater runs offland.

> compare freshwater and saltwater.

Water cycleMovement of waterbetween theatmosphere and theearth's surface.

Three-fourths of the earth's surface is covered with water.Water is everywhere. Most of it is in the oceans. But it isalso in rivers, in lakes, in the air, and even in your own body.

The Water CycleEarth's water is in continuous motion. It moves from theatmosphere to the earth's surface and back to theatmosphere. This movement of water is called the watercycle. Study the diagram and notice the different formsthat water takes as it goes through a complete cycle.

The water cycle is powered by the sun. Heat from the sunevaporates surface water, and the water vapor rises intothe atmosphere. The rising water vapor cools andcondenses into clouds. Water droplets or ice crystals in theclouds grow larger, then fall to earth as precipitation.

Sun

Condensation Precipitation

216 Chapter 11 The Earth's Water

GroundwaterWater that sinks intothe ground.

RunoffWater that rum overthe earth's surfaceand flows intostreams.

Did You Know?

The Dead Sea inthe Middle Eastis almost ninetimes saltier thanany ocean.Nothing lives inthe Dead Seaexcept bacteria.

What happens after precipitation falls? Some of it sinksinto the ground and becomes groundwater. This watercollects in the spaces between rocks and moves slowlyunderground. Precipitation that does not sink into theground is called surface water. Some surface waterevaporates. But most of it becomes runoff—surface waterthat flows into streams.

Why doesn't all precipitation sink into the ground? Thereare three main reasons.

1. The ground may be saturated, or completely soaked,and unable to hold any more. It is like pouring wateron a sponge. Eventually the sponge fills and waterruns off it.

2. On a slope, the water may run off too quickly tosink in.

3. The ground may not have enough vegetation to stopthe water from flowing. Plants and their roots soakup water.

Salt Water and Fresh WaterEventually, surface water evaporates or reaches the oceans.If you have ever tasted ocean water, you know it is muchtoo salty to drink. Salt water also cannot be used forfarms and industry. Salt water kills most land plants andruins machinery. In Lesson 3, you will learn more aboutthe properties of salt water.

Like the water on land, ocean water evaporates and movesback into the atmosphere. Dissolved salts are left behind,however. So the water that condenses in the atmosphereand falls onto the land contains no salt. It is fresh water.

klf-Check• How does water move between the atmosphere and the

oceans?

2. What is runoff?

3. How are salt water and fresh water different?

The Earth's Water Chapter II 217

Lesson Where Is the Earth's Fresh Water?

Objectives

After readingthis lesson, youshould be able to

explain howgroundwatercollects andmoves.

- describe how ariver develops.

» compare lakes,ponds, andreservoirs.

PorousContaining manyspaces throughwhich air and watercan move.

Water tableTop of thegroundwater layer.

Fresh water is an important resource. Think of the manyways you use it every day, such as for drinking, washing, andcooking. Farms and industry, however, use 90 percent of thefresh water consumed in the United States. Fresh water canbe found in many places, both above and below the ground.

GroundwaterGroundwater starts as precipitation and runoff that soaksinto the earth. The water can sink into the ground becausemost soil is porous, or has spaces between its particles.Loose soil, such as sandy soil or soil with a lot of decayedplant material, is very porous. Beneath the soil, thebedrock may also be porous. Water trickles around brokenrock pieces and through cracks.

The diagram shows what happens as water continuesdownward. Eventually, water comes to a solid rock layerthrough which it cannot move. Groundwater collects ontop of the rock layer, filling the spaces above it. The top ofthis wet earth layer is the water table. Find the water tablein the diagram, and notice the depth of the well. If you drilla well down to the water table, water flows into the welland can be pumped to the surface. About half the drinkingwater in the United States comes from groundwater.

Groundwater above this line


SpringPlace wheregroundwater flowsnatually onto thesurface.

GeyserSpring at whichheated groundwateris blasted into the air.

Springs, Geysers, and CavesNotice in the diagram what happens when the water tablereaches the surface on a hillside. Groundwater flows out ofthe ground as a natural spring.

Certain springs, called geysers, shoot water and steam intothe air. Geysers occur where groundwater lies close to hot

rock or magma. Pockets of groundwaterare heated and turned to steam. Thesteam rises, pushing the hot water aboveit. The steam and water erupt as ageyser. Geysers occur in Wyoming, NewZealand, and Iceland.

Groundwater creates some otherunusual features. For example,groundwater seeping through cracks inlimestone may dissolve the rock andform caves. Some caves are barely largeenough to crawl through. Others areimmense. The U.S. Capitol building, forexample, could fit in one of the caves ofCarlsbad Caverns in New Mexico.

The photo below shows what happens when the roof of acave collapses. A funnel-shaped sinkhole forms. Sinkholesmay fill with groundwater and rain to become ponds.

SinkholeCircular depressionon the surface causedby groundwaterdissolving rock.

The Earth's Water Chapter 11 219

Tributaries

TributaryRiver that joinsanother river of equalor greater size.

Drainage basinLand area that isdrained by a riverand its tributaries.

Rivers and Drainage BasinsMuch of the fresh water above ground flowsas rivers. Rivers begin as runoff moves overthe land, carving small paths in the ground.These paths get wider and deeper as watercontinues to flow through them. The pathsbecome tiny streams. They always flowdownhill because of the force of gravity. Asthe streams flow, they join and becomelarger rivers. These rivers then join and formeven larger rivers. Rivers that join otherrivers are called tributaries. Find some ofthe tributaries in the diagram to the left. Youcan see how water and sediment in the mostdistant tributaries end up in the main river.

The land area in which runoff drains into a river and itstributaries is a drainage basin. The map shows the threelargest drainage basins in the United States. Notice howrain that falls in Montana can eventually reach the Gulf ofMexico. Ridges that separate drainage basins are calleddivides. The Continental Divide runs along the RockyMountains. Rivers east of this divide flow into the AtlanticOcean or Gulf of Mexico. Rivers west of the divide flow intothe Pacific Ocean.

Mississippi Riverdrainage basin Mississippi River

Ridge that separatesdrainage basins.

Columbia Riverdrainage basin

ColoradoRiver

AtlanticOceanPacific

Ocean Colorado RiverdTaTnaqexbasin

Gulf ofMexico

220 Chapter IJ The Earth's Water

Did You Know?

Minnesota'slicense plates callthe state the"land of 10,000lakes." Actually,Minnesota hasmore than 15,000lakes. In fact,Minnesota means"sky-tintedwater" in theDakota Indianlanguage.

LakesSurface water does not always flow along a path. Someof it collects in depressions, or low areas. Water eventuallyfills the depressions, forming lakes. Even though someof the water evaporates, lakes continue to be fed byprecipitation, runoff, springs, and rivers.

Lakes are many different sizes. For example, some lakes inWisconsin are only a few meters deep. You can hearpeople talking from the opposite shore. The Great Lakes,on the other hand, are so wide that you cannot see acrossthem. Lake Superior is the largest freshwater lake in theworld. Its deepest point is about 400 meters. Many of thelakes in northern United States and Canada formed whenhuge sheets of moving ice gouged out depressions. Youwill learn more about this process in the next chapter.

Many cities storelarge supplies offresh water inartificial lakes calledreservoirs.Reservoirs are madeby constructingdams along rivers.As you can see inthe photo, waterbacks up behind thedam, turning part ofthe river into a lake.Reservoirs serveseveral purposes, asshown below.

ReservoirLake created byplacing a dam acrossa river.

Store water for home use, farming, and industryWater can be stored and piped to dry areas. Much ofsouthern California's water, for example, comes through acanal from Lake Havasu, a reservoir behind Parker Dam onthe Colorado River between California and Arizona.

Earth's Water Chapter 11 221

Control floodingDuring periods of heavy rain and runoff, the reservoir canfill up. Then the water can be released slowly and safelydownstream through gates in the dam.

Produce electricityWater moves through generators near the bottom of thedam. The moving water turns the blades of a turbine,which spins a magnet. When the magnet spins throughloops of wire, electricity is produced.

Self-CheckI What is a water table?

2. How can runoff in a mountain end up in the ocean 2,000kilometers away?

3. How are reservoirs useful?

0ENCEipa'/'i'ii

IFEWhat is yourwater budget?

A water budget describes the amount ofwater coming in and going out of an area. Bycreating your own personal water budget, youcan cut down on wasted water. Use theinformation in the table to find out howmuch water is used in your household in oneweek. Think of ways to save water to lowerthese numbers. Then try to set and keep aweekly limit on water use.

Daily

Washing handsShower

Bath

Flushing toilet

Brushing teeth

Washing dishes

0.5

20

30

1.5

0.5

12

Weekly

Doing laundry (1 load)

Washing car

Watering lawn (30 min)

30

20

240


Making a Well

Materialsv clear plastic

fishbowl

v gravel

v rulerv cardboard

tubev container of

waterV

PurposeTo make a model of a water table and a well

Procedure1. Copy the data table below on your paper.

Depth of well Depth of water What do you see?

2. Slowly add gravel to the fish-bowl until you have a layerabout 4 centimeters deep.

3. Stand a cardboard tube uprightin the fishbowl. Add anotherlayer of gravel around the tubeuntil it stands up by itself. Thetube represents a well dug intothe ground.

4. Slowly add water to the bowluntil you have a layer of waterabout 2 centimeters deep at thebottom of the bowl.


5. From the top of the gravel, measure the depth to thebottom of the well. Then measure the depth to thetop of the water layer. Record your data.

6. Look inside the cardboard tube. Record what yousee.

7. Slowly add water to the bowl until you have a layerof water about 6 centimeters deep at the bottom ofthe bowl.

8. Repeat steps 5 and 6.

Questions1. How is the water in your model like groundwater?

2. In your model, what does the bottom of the fishbowlrepresent?

3. Which part of your model is like the water table?

4. Compare what you saw inside the well in steps 6and 8.

5. How deep must a well be dug in order to get water?

Explore FurtherUsing the central tube and nozzle from a spray bottle,how could you demonstrate what happens if too muchwater is pumped from wells? Try it.


Lesson

Objectives

After readingthis lesson, youshould be able to> identify two

properties ofocean water.

*• explain whatcauses oceanwaves andcurrents.

»• describe severalfeatures of theocean floor.

> identify threemajor groups ofocean life.

What Is in the Oceans?

SalinityThe saltiness ofwater.

Properties of Ocean WaterThe water in the oceans is salt water. The circle graphbelow shows why. Notice that 96.5 percent of ocean wateris pure water. But 3.5 percent is dissolved salts. Thatamount of salt makes a mouthful of ocean water saltierthan a mouthful of potato chips. Most of the salt issodium chloride—common table salt. Some sodiumchloride and other salts come from rocks of the oceanfloor. Salts also wash into the ocean from rivers.

Sodium chloride 27.2%Magnesium chloride 3.8%Magnesium sulfate 1.7%Calcium sulfate 1.3%Potassium sulfate 0.9%Calcium carbonate 0.1%Magnesium bromide 0.1%

Not all parts of theocean arc equally salty.The saltiness, or salinity,of ocean water varies.In warm, dry climates,ocean water evaporatesquickly. The salts thatremain make the salinity 1'200

greater than average. In L4001"rainy climates, or where 1-600 m

rivers and melting iceadd fresh water to theocean, the salinity is lessthan average.

Ocean water is warmestat the surface where thesun heats it. Notice inthe diagram to theright how the water 3,000 mtemperature decreases

O m

200m

400m -

600m

800 m

1,000m

in

Q.01Q

Warm surface water

Thermocline(temperature drops sharply)

25

20

15

10

Cold deep water

Ocean floor

I


ThermoclineLayer of the oceanbetween about 300and 800 meters, wherethe temperaturedrops sharply.

WaveUp and down motionof water caused byenergy movingthrough the water.

with depth. The temperature is fairly constant near thesurface because winds and waves keep the water well-mixed.However, in the thermocline, the temperature dropssharply. Below the thermocline, the temperature decreasesslowly. The bottom of the ocean is near freezing.

Ocean WavesWhen you think of the ocean, you probably picture waves.A wave is the regular up and down motion of watercaused by energy traveling through the water. A wave getsits energy from wind. When the wind blows, it pushes upthe water to start small waves. The waves become larger asthe wind blows longer and harder. Most ocean waves areabout three meters high. However, storms can producewaves as high as 30 meters—the height of a 10-storybuilding. No matter what the size, all waves have the partsdescribed below.

Crest(Highest point of the wave)

(Distance between two crests)

Trough(Lowest point of the wave)

Wave height(Distance from trough to crest)

Have you ever seen a leaf bob up and down on passingwaves? The waves move forward but the leaf does not.Although it looks like waves constantly push waterforward, the water generally stays in the same place. Onlythe waves move forward.

As a wave approaches the shore and shallow water, thewave rubs against the ocean floor. Friction slows thebottom of the wave, but the crest keeps moving at thesame speed. Therefore, the crest moves ahead of the rest ofthe wave. The wave tilts forward and tumbles over, orbreaks. After a wave breaks on shore, the water actuallymoves. It may be hurled against rocks or pushed up theslope of a beach.


CurrentA large stream ofwater flowing in theocean, in rivers, andin some large lakes.

Ocean CurrentsAlthough waves do not move water, currents do. Oceancurrents are large streams of water that move through theocean. Winds cause currents near the ocean surface.Therefore, currents tend to follow the major wind belts ofthe earth. Use the map to compare the trade winds withocean currents near the equator. Both the winds and thecurrents flow westward.

Currents carry warm water from the equator toward thepoles and bring cold water back toward the equator. In sodoing, currents affect climates on land by warming orcooling the coasts of continents. Both wind and landabsorb heat from warm ocean currents. The Gulf Streamis an ocean current that has a warming effect. Find theGulf Stream on the map. Notice that it carries warm waterfrom the tropics up along the east coast of North Americaand then across the Atlantic. The Gulf Stream givesWestern Europe mild summers and winters. For example,even though Great Britain is far north, warm winds fromthe Gulf Stream keep the temperatures mild.

NorthAmerica / GU|f

Stream f


Continental shelfPart of a continentthat extends from theshoreline out into theocean.

Continental slopeSteep slope betweenthe continental shelfand the deep oceanfloor.

Mid-ocean ridgeMountain chain onthe ocean floor.

SeamountUnderwatermountain that isusually a volcano.

TrenchDeep valley on theocean floor.

The Ocean FloorUntil the middle of this century, the ocean floor was agreat mystery. Today we have new measuring devices andthe ability to actually travel to the ocean floor. Thedrawing shows some of the features of the ocean floor.You will learn more about these features in Chapter 13.

Continental shelfThe continental shelf is part of the continent that extendsunderwater. The continental shelf slopes gently. Averagewater depth is 130 meters. Average width is 75 kilometers.

Continental slopeThe continental slope dips sharply to the ocean floor.

PlainsAbout one-half of the ocean floor consists of flat plains,where sediment constantly settles. Average depth is about4,000 meters.

Mid-ocean ridgeUnderwater mountain chains called mid-ocean ridgesextend for thousands of kilometers along the ocean floor,

SeamountSeamounts are underwater mountains. Many of these areactive or extinct volcanoes. A seamount that rises abovesea level forms an island.

TrenchTrenches are long, deep valleys. They are the deepestplaces on earth. Some are ten kilometers deep.


Plankton

Nekton

Benthos

Ocean LifeOcean environments support arich variety of living things.Scientists divide these forms oflife into three groups, based onhow and where they live. Lookat the cross section of oceanlife zones. Which zoneprovides most of the seafoodthat people eat?

Plankton are one group in theocean. This group includes tinyplants and animals that float ator near the ocean surface.Plankton are a source of foodfor larger animals.

Animals that swim freely arcclassified as nekton. Thisgroup includes the widestvariety of sea creatures, fromthe tiniest fish to the largestwhale.

Organisms that live on theocean floor are called benthos.

Some, such as corals, remain in one place their wholelives. Others, such as snails and crabs, crawl along theocean floor.

PlanktonTiny organisms thatlive at or near theocean surface.

NektonFree-swimmingocean animals.

BenthosOrganisms that liveon the ocean floor.

Self-Check1. What are two sources of salts in ocean water?2, What causes most ocean waves and currents?

What are three features of the ocean floor?4. What are three groups of ocean life and how do they differ?


Materialsv 2 small, clear

plastic soft-drink bottles

v tablespoon

v table saltv masking

tape* fine-tip

waterproofmarker

v drinkingstraw

v ruler

* modelingclay

N V E S T I G A T I O N

How Salt Water AffectsFloatingPurposeTo compare how objects float in salt water and fresh water


Trial

1

2

Fresh water Salt water

2. Fill two small soft-drink bottles about three-quartersfull of water. Stir in 6 tablespoons of salt in one bottle.

3. Label the salt water bottle S and the fresh water bottle F.4. Make a float meter. Put a 6-centimeter strip of tape

along one end of a drinking straw. Mark off1/2-centimeter lengths along the tape. Push apea-size ball of clay onto the end of the straw.

5. Drop the float meter into the fresh water. Count howmany markings on the tape stay above water. Recordyour observations.

6. Drop the float meter into the salt water. Count themarkings above water. Record your observations.

7. Repeat steps 5 and 6 for Trial 2.

Questions1. In which did the meter float higher, the salt water or

the fresh water?2, Based on your results, make a general statement about

how objects float in salt water cpa^ared to how theyfloat in fresh water.


SUMMARY

Water moves between the land,the atmosphere, and the ocean inthe water cycle.

The earth's water includes saltwater, which is too salty to drink,and fresh water, which does notcontain salt.

Water under the earth's surface iscalled groundwater.

Groundwater moves downwardin the ground and collects toform a soaked layer, the top ofwhich is called the water table.

Rivers and their tributaries drainrunoff from large areas of landcalled drainage basins.

Lakes form when water collects ina depression on land.

Reservoirs are lakes made whenpeople dam a river.

Ocean water is salt water becauseit contains dissolved salts.

The temperature of ocean waterdecreases with depth.

A wave is the up and downmotion of water caused byenergy from the wind.

Currents move ocean water fromplace to place. The currents arecaused by winds and follow thesame general pattern as theglobal winds.

Features of the ocean floorinclude the continental shelf, thecontinental slope, mid-oceanridges, trenches, hills, plains, andmountains.

Ocean life includes floatingplankton, free-swimming nekton,and ocean floor-dwelling benthos.

benthos, 229continental shelf, 228continental slope, 228current, 227divide, 220drainage basin, 220geyser, 219groundwater, 217mid-ocean ridge, 228nekton, 229plankton, 229porous, 218

reservoir, 221runoff, 217salinity, 225seamount, 228sinkhole, 219spring, 219thermocline, 226trench, 228tributary, 220water cycle, 216water table, 218wave, 226


R E V I E W

ORD

'continental shelf

[Drainage basin

pgeyser

'^mid-ocean ridge

'/^salinity

trench

tributary

\ table

Vocabulary ReviewNumber your paper from 1 to 8. Then choose a word orwords from the Word Bank that best complete eachsentence. Write the answer on your paper.

1. A river that flows into another river is a

2. The land area in which runoff flows into a river and itstributaries is a .

3. Underground water forms a soaked layer, the top ofwhich is the .

4. Heated groundwater blasts out of the ground at a

5. A deep valley on the ocean floor is called a .

6. A mountain chain on the ocean floor is called a

7. Water with more salt has greaterless salt.

than water with

8. The extends from the shoreline into the ocean.

Concept ReviewNumber your paper from 1 to 9. Choose the answer thatbest completes each sentence. Write the letter of the answeron your paper.

1. Water moves from the ocean to the atmosphere by .a. evaporation b. condensation c. precipitation

2. are sources of fresh water.a. Rivers b. Ocean ridges c. Ocean currents

3. Porous rock contains many .a. nekton b. salts c. spaces

4. As you go deeper in ocean water, the temperature .a. gets warmer b. gets colder c. stays the same

5. An island can form from a on the ocean floor.a. trench b. seamount c. continental slope

232 Chapter II The Earth's Water

6. The top of a wave is the __ .a. trough b. wave height c. crest

7. Ocean currents are caused by .a. waves b. tides c. winds

8. A fish swimming in the ocean is classified as _ _.a. nekton b. benthos c. plankton

9. When salt water evaporates, the salt .a. also evaporatesb. stays in the remaining salt waterc. changes the water into fresh water

Number your paper from 10 to 15. Then match each oceanfeature below with a letter in the diagram. Write theanswers on your paper.

10. trench11. continental slope12. mid-ocean ridge13. island14. plain15. continental shelf

Critical ThinkingWrite the answer to each of the following questions.

1. Suppose many houses are built in an area. Each househas a well. It has been a dry summer, and after a fewmonths, water no longer comes up through the wells.What has happened?

2. One way to make salt water fit to drink is to heat the saltwater and collect the water vapor. How is this methodlike the water cycle? Suggest a way to collect the watervapor.

Make sure you have the same number of answers onyour paper as there are items on the test.


Weathering andErosion

This picture shows one of the beautiful sights ofCanyonlands National Park in Utah. It tookmillions of years for wind and running water to

carve the opening of Angel Arch. Though the river thatran through the arch has long since dried up, rainwaterand wind continue to shape this landscape. In fact, waterand wind constantly shape the land where you live, too. Inthis chapter, you will find out how the same forces thatwash dirt off a driveway create the most breathtakingscenery in the world.

ORGANIZE YOUR THOUGHTS

Weathering

Goals for Learning

To define weathering

To identify different kinds of weathering

To describe how water, wind. Ice, and gravity causeerosion

To give examples of several eroded landforms

To describe how deposited landforms develop

235

Lesson What Is Weathering?

Objectives

After readingthis lesson, youshould be able to> define

weathering.^give examples

of mechanicaland chemicalweathering.

+• identifydifferent soillayers.

WeatheringThe breaking downof rocks on the earth'ssurface.

MechanicalweatheringThe breaking apartof rocks withoutchanging theirmineral composition.

Rocks Break DownThe earth is constantly changing. Even a hard materiallike rock changes over time. How have these tombstoneschanged? Over the years, these slabs of limestone havebroken down so that it is difficult to read the carvings.

The breaking down ofrocks on the earth'ssurface is weathering.Weathering occurswhen the rock isexposed to air, water, orliving things. All thesefactors help to breakrocks apart.

Mechanical WeatheringIn mechanical weathering, rocks break into smaller piecesbut their chemical makeup stays the same. The photoshows one way that rocks break. This tree started growingin soil that collected in a crack of the rock. As the treegrew, its roots pushed against the rock and split it. You

might see this kind ofmechanical weathering in asidewalk near a tree. Thegrowing roots often lift andcrumble the sidewalk.

236 Chapter 12 Weathering and Erosion

Freezing water wedges rocks apartand makes smaller pieces.

ChemicalweatheringThe breaking apart ofrocks caused by achange in theirchemical makeup.

Mechanical weathering alsooccurs as water freezes in thecracks of rocks. When waterfreezes, it expands. As thefreezing water expands, itpushes the rock apart, asshown in the drawing. The icemay melt, and the water mayrefreeze. Each time the waterfreezes, the cracks get bigger.Finally, the rock breaks apart.

Chemical WeatheringIn chemical weathering,changes occur in the chemicalmakeup of rocks. Newminerals might be added toor taken away from the rock.The minerals might bechanged into new substances.

For example, in a process called oxidation, oxygen fromthe air or water combines with the iron in rocks. As aresult, a new substance called iron oxide, or rust, forms.Iron oxides stain rocks various shades of yellow, orange,red, or brown. How are these rocks like the rusty old canin the photo?

OxidationProcess in whichminerals combinewith oxygen to formnew substances.

Weathering and Erosion Chapter 12 237

SoilMixture of tiny piecesof weathered rockand the remains ofplants and animals.

Top soilTop layer of soil, richwith remains ofplants and animals.

Chemical weathering also occurs when water changesminerals in the rocks. For example, water changes themineral feldspar, which is part of many rocks. The waterchanges feldspar to clay and washes it away. Without thefeldspar to hold the other minerals together, the rock fallsapart.

The limestone cave shownhere is the result ofchemical weathering. Rainand groundwatercombine with carbondioxide in the air to formcarbonic acid. This is thesame acid found incarbonated soft drinks. Ascarbonic acid tricklesthrough the ground, itdissolves calcite—themain mineral inlimestone. As more andmore limestone isdissolved, small holesbecome huge caves.

Weathering Forms SoilWhen rock has weathered for a long time, soil maydevelop. Soil is a mixture of tiny pieces of weathered rockand the remains of plants and animals. The makeup ofsoil depends on the types of rock particles and remainsthat are found in it.

As soil develops, it forms layers. Fully developed soil hasthree layers. Compare the drawing and photo on the nextpage as you read about soil layers.

Most soil you see is topsoil. This layer has the greatestamount of oxygen and decayed organic matter. Theorganic matter helps the soil hold moisture.


SubsoilLayer of soil directlybelow the topsail

Directly below the topsoil is the subsoil. It containsminerals that were washed down from the topsoil. Manyof these minerals are iron oxides and give the subsoil ayellowish or reddish color. Plant roots grow down into thesubsoil to get minerals and water.

The next layer contains chunks of partially weatheredrock. Near the bottom of this layer, rock fragments sitdirectly on solid rock.

Self-CheckWrite the word that best completes each sentence.1. Rusting is an example of __ _ weathering.'2. When carbonic acid dissolves a large amount of limestone

that is underground, a __ may form.T Freezing water and growing roots break down rocks by

weathering.4. The layer of soil that contains a lot of decayed plants is

the

Weathering find Krosion Chapter 12 239

Chemical Weathering

Materialsv safety

goggles

v paper towels

S hand lens

v

*r

5 limestonechips orpieces ofchalk

clear plastic12-oz cup

v 1 cup vinegar

v strainer

v water

PurposeTo model and observe how chemical weathering occurs


Observations of Weathering

appearance oflimestonebeforeweathering

appearance oflimestoneafterweathering

2. Put on your safety goggles.

3. Use the hand lens to look at the surfaces of thelimestone chips. In the data table, describe theirappearance. Check especially for jagged surfaces.

4. Place the chips in the cup. Pour enough vinegarinto the cup to cover the chips. Let the chips sitovernight.

5. Pour the vinegar and limestone chips through astrainer over a sink. Run water over the chips torinse off the vinegar.


6. Place the limestone chips on paper towels. Use thehand lens to look at the limestone surfaces. In thedata table, describe any changes you see.

Questions1. How did the surfaces of the limestone change?

2. Vinegar is an acid. What did the vinegar do tochange the appearance of the limestone?

Explore Further1. Repeat this experiment, leaving different groups of

limestone chips in vinegar for varying lengths oftime. For example, you might soak one group ofchips for a day, one for two days, one for three days,and so on for a week. Can you predict how eachgroup of rock chips will look? Find the mass of eachgroup before and after soaking.

2. Repeat this experiment, using different kinds ofrocks. For example, try samples of sandstone, granite,and marble. Which rocks are most resistant to thiskind of chemical weathering? Which are leastresistant?


Lesson How Does Water Shape the Land?

Objectives

After readingthis lesson, youshould be able to*• explain how

rivers erodethe land.

*• describe howriver valleysform.

*• identifyshorelinefeatures causedby waves.

ErosionThe wearing awayand moving ofweathered rockand soil

ErosionAfter rock has been loosened by weathering, it is wornaway and moved to another place. The wearing away andmoving of weathered rock and soil is called erosion. Themain agents, or causes, of erosion are rivers, waves,moving ice, wind, and gravity.

Erosion by RiversRivers and the waterthat flows into themchange more of thelandscape than anyother agent of erosion.After rain falls to theearth, the water flowsdownhill. The waterpushes soil and rockfragments as it moves.These solid particlesare sediments. The v/ater and sediments flow into smallgullies, which lead to rivers.

As water flows in a river, it erodes the banks and riverbed,which is the bottom of the river. Compare the eroding

power of a river to a hose. Theforce of water from the hose caneasily dig up soil and move itacross a lawn. A jet of water mayeven chip away at a sidewalk.Similarly, river water erodes theland. Sand and stones in theriver scrape against the banksand riverbed, causing moreerosion. The fast-moving waterand sediment in the streamshown here have worn theseboulders smooth.


Did You Know?Niagara Fallsmoves upriverabout one meterper year aseroded limestonetumbles from thetop of the fallsto the base ofthe falls.

Erosion and River ValleysAs you run your finger through sand, your finger carvesout a little valley. As a river erodes the land, it also carvesout a valley. Some valleys are narrow with steep walls.These are called canyons. Other valleys are wide andshallow. The shape of the valley largely depends on howold it is. Rivers and their valleys go through three stages:youth, maturity, and old age. Study the diagrams as youread about the life of a river.

V-shaped valley

Meanders Oxbow lake

A young river is narrow and fast. Its swift waters rapidlycut down through rock, carving out a V-shaped valley.The river covers all or most of the valley floor. The fastwaters have a lot of energy and can push boulders alongthe river's path. Rapids and waterfalls are common. TheYellowstone River and the Niagara River are examples ofyoung rivers.


MeanderLooping curve in ariver.

As erosion continues, a river becomes mature. At thisstage, the boulders and rocky ledges that cause rapids andwaterfalls have been eroded away. The slope, or angle, ofthe river is less steep, so the river does not flow as fast. Itcan move pebbles, sand, and mud, but not boulders. Thevalley of a mature river is much wider than the river itself.The Ohio River and Missouri River are mature rivers.

The diagram shows another characteristic of a matureriver. All rivers have curves. Mature rivers usually havemore of them. The water flows faster and pushes harderagainst the outside of each curve and erodes that bankfaster. Water slows down on the inside of each curve,allowing sediment to settle and build up. As you can see,this process creates large, looping bends called meanders.

Flow of water

As meanders grow, thecourse of the river changes.

Flood plainLand that a rivercovers when itoverflows its banks.

Oxbow lakeC-shaped body ofwater formed whena meander is cutoff from the rest ofthe river.

As a mature river's meanders grow, its flood plain alsogrows. A flood plain is the low, flat area that a river coverswhen it overflows its banks. Flood waters leave behindrich soil and nutrients on flood plains.

The valleys of old rivers are broad and flat. By this time,the river has eroded its way down to near sea level. Oldrivers tend to have enlarged meanders and more of them.As a meander continues to grow, it forms almost acomplete circle. During a flood, the river may breakthrough its banks and flow straighten The meander is cutoff and becomes a C-shaped oxbow lake.


DepositionThe dropping oferoded sediment.

DeltaFan-shaped area ofland formed whensediment is depositedwhere a river emptiesinto a lake or anocean.

River DepositsSediments that are carried by the agents of erosion areeventually dropped in a process called deposition. Forexample, when a river slows down, it may drop, ordeposit, its sediment. Heavier sediments, such as stones,

drop out first. As the river slowsdown further, lighter sediments,such as sand and clay particles,drop out.

A river slows down considerablyas it empties into a lake or anocean. The sediment settles out.Eventually, the sediment buildsup above the water level andforms a fan-shaped area of landcalled a delta. A delta providesrich farmland. Much of Egypt'sfarmland, for example, is locatedon the fertile Nile River delta.

Notice the fan-shaped land in the drawing below. Thisfeature is an alluvial fan. How do you think it formed? Analluvial fan is similar to a delta. It forms at the base of amountain where a mountain stream meets level land.

Alluvial fanFan-shaped area ofland deposited wherea mountain streammoves onto flat land.


Wave ErosionWaves in an ocean or a large lake change the shorelinethrough erosion and deposition. As waves pound theshoreline, they hurl not only water but also bits of rockand sand against the coast. These materials chip away atthe rocky shore. Waves also force water into cracks inrocks along the shoreline. With each wave, the waterpresses against the sides of the cracks. The cracks getbigger, and pieces of rock split off.

The erosion described above formed the cliffs, towers, andother rocky shapes shown here. During storms, wavesreach higher on the cliffs and carve the steep sides. Archesform when waves erode through a sea cliff. If the top ofthe arch collapses, a tower of rock called a sea stack is leftstanding.


Self-Check

Wave DepositionBeaches are areas where waves have deposited sand,pebbles, or shells. Some of this beach material is sedimentfrom nearby eroded rocks. Other beach material issediment carried to the sea by rivers. Currents near theshore carry sediment to different parts of the shoreline. Aswaves break on shore, the sediment is pushed onto thebeach.

Currents along the shore also carry sand away from abeach, forming a spit, or finger of sand, sticking out intothe ocean. Waves and currents may carry sand away fromthe beach and deposit a long sand bar offshore.

1. What is erosion?

2. How does a young river differ from an old river?3. From where does the sand on a beach come?

CENCEECXZXQIH

BFEI

How do peoplecause toomuch erosion?

: •*'

People's actions sometimes cause too mucherosion, which becomes harmful to theenvironment and to people. One way peopleincrease erosion is through the use of off-road vehicles (ORVs). Dirt bikes, dunebuggies, four-wheel drives, and all-terrainvehicles can be fun. But their overuse hasdamaged the land. The photo illustrates thisproblem. The hillside used to be covered withgrass. The roots of grass and other plantshold soil in place and catch runoff. Withinweeks, the use of ORVs dug up the vegetationand created ruts. Rainwater followed theseruts and formed deep gullies. The soil nowerodes quickly from the hill. Areas have beenset aside for ORVs, but users often ventureinto "closed" areas.


Materials

» 2 paint trays

S soil

» grass seeds

» sprinklingcan

v water

* paper towels

V E S T I G A T I O N

Comparing Erosion

PurposeTo find out if vegetation affects the amount of erosionthat occurs on a hillside


Trial 1

Trial 2

Trial 3

Trial 4

Trial 5

Bare soil Soil with grass

2. Spread a layer of soil in each paint tray to a depth ofabout 5 cm. Cover only the part of the tray thatslopes downward to create a hillside. Wet or pack thesoil a little if necessary so that it stays on the hillside.

3. Plant a handful of grass seeds in the soil of one tray.Water the seeds every day for about a week, until thegrass grows a few centimeters.

4. Place the trays side by side on paper towels. Sprinklewater over each tray for 5 seconds. Sprinkle towardthe top of the tray so that the water can run down


the hill. Record your observationsabout how the water runs downeach hill and how much erosionoccurs.

5. Repeat the sprinkling4 more times. Recordyour observations.

Questions1. What differences did you observe between the two

hillsides regarding how the water flowed?

2. What differences did you observe between the twohillsides regarding erosion?

3. What differences did you observe between the twohillsides regarding the color of the water at thebottom of the trays?

4. How can you use the results of this experiment toprevent unwanted erosion in areas near your home?

Explore FurtherDo certain kinds of vegetation reduce erosion betterthan others? Redesign the experiment to find out.


Lesson How Does Ice Shape the Land?

Objectives

After readingthis lesson, youshould be able to("define two

types ofglaciers.

*• explain howglaciers erodethe land.

*• describefeatures causedby glaciers.

GlacierA huge moving bodyof ice.

Glaciers—Moving IceIn cold climates, water falls as snow. This snow can buildup into thick layers. If the snow does not melt, increasingpressure causes the snow below to form solid ice. Yearafter year, more ice builds up. Eventually, a glacier mayform. A glacier is a huge sheet of ice that covers a large area.

Glaciers only form where average temperatures stay belowfreezing. So they are found only in mountain regions andnear the poles. Glaciers in mountain regions are calledalpine glaciers. They move slowly downhill. Notice howthis alpine glacier extends down the valley. Ice sheets thatcover broad areas of land are called continental glaciers.Continental glaciers cover most of Antarctica andGreenland.


CirqueBowl-like basin in amountain carved outby an alpine glacier.

Hornfagged pyramid-shaped peak formedby the intersection ofthree or more cirques.

Erosion by GlaciersBecause of their great size,glaciers move large amounts ofsediment. As glaciers move, theypick up loose rocks and soil.These materials freeze onto thebottom and sides of the glacier.They act like grinding and cuttingtools as the glacier continues tomove. The photo shows howrocks in the bottom of a glaciercut long grooves in the surfacerock. Small rocks in the ice actlike sandpaper, smoothing andshaping the bedrock.

"*v» .̂

How Alpine Glaciers Shape the LandAlpine glaciers begin in the upper reaches of mountainvalleys. At the beginning of the glacier, the ice carves outbowl-shaped basins called cirques. Several cirques aroundthe top of the mountain form a pyramid-shaped peakcalled a horn. TbC'&tatterhorn in Switzerland, shownbelow, is one of the most famous horns.

A horn forms where severalcirques come together.


Did You Know?The majesticfjords of Norwayand Alaska areU-shaped glacialvalleys that arepartly filled byocean waters.

MoraineRidge of sedimentdeposited by aglacier.

Before it was covered with a glacier, a valley may havebeen shaped by a river. The valley would have a typicalV shape. As the glacier moves downhill, it gouges out thevalley like a giant ice cream scoop. As a result, theV-shaped valley becomes a U-shaped valley. Use yourfinger to trace a U across the valley in the drawing below.

The glacier moves downhill until it reaches temperatureswarm enough to melt. As the ice melts, it deposits itssediment. The sediment forms ridges called moraines.Moraines are among the "footprints," or evidence, of aglacier, telling us it was here.


How Continental Glaciers Shape the LandContinental glaciers are up to three kilometers thick.Because of their tremendous size and weight, theseglaciers transform the surface of the land. They change thecourses of rivers, create lakes great and small, and moveboulders the size of houses hundreds of kilometers.

Like alpine glaciers, continental glaciers mark theirboundaries by leaving behind moraines. The map showsthe location of major moraines in the United States. Thesemoraines are left from the last ice age, when much of theNorthern Hemisphere was covered by continental glaciers.The ice age started more than a million years ago andended about 10,000 years ago.

The moraines show the shape of continental glaciersthat once covered part of the United States.


b. Ice block getspartly buriedin sediment.

As the glaciers melt, huge blocks of ice are left behind. Theice may be partly covered with sediment. When the icemelts, it leaves a hole in the ground. The hole fills withwater and becomes a lake. Many of the lakes of Wisconsinand Minnesota formed this way.

Some large lakesformed ascontinental glaciersmoved throughlarge valleys. Theglaciers carved thevalleys into wide,deep basins. Themelting glacierhelped fill thebasins with water.Moraines dammedparts of the lakes.This processcreated the GreatLakes, the FingerLakes in New York,and Lake Winnipegin Canada.

a. Ice block breaksoff glacier.

c. Ice block meltsto form lake.

Self-Check1. Describe the two kinds of glaciers.

2. How does a glacier erode the land?

3. What is a moraine?


Objectives

After readingthis lesson, youshould be able to*• describe how

wind erodesland.

> explain howsand dunesform.

^ identifyexamples oferosion bygravity.

Did You Know?Some sand dunesin the Sahara, adesert in Africa,grow to behundreds ofmeters tall.

How Do Wind and GravityShape the Land?

Wind ErosionWind is another cause of erosion. Like water, wind picksup and carries materials from one place to another. Windalso erodes by blowing sand against rock. This action issimilar to a sandblaster used to clean buildings discoloredby pollution. If you have ever been stung in the face bywindblown sand, you know wind can be an effective agentof erosion. Much rock in desert areas is pitted with tinyholes from windblown sand.

Wind DepositionYou are probably familiar with the wind deposit calledsand dunes. These are mounds formed as the wind blowssand from one place to another. Sand dunes are mostcommon in deserts, but they also occur around beaches.

Wind may bounce sand along the ground until it hits anobstacle, such as a small rock. A small pile forms behindthe rock. The pile blocks other sand grains, and a largermound forms. The mound continues to grow, forming asand dune. The dune moves as wind blows sand up thegentle slope and deposits it on the steeper back slope, asshown below.

Wind direction[rectipji)

Gentleslope

A sand dune moves as sand isblown from one side to the other.

Steepslope

Movcmont of e><ind dune

Weathering and Erosion C.haptcr 12 255

Gravity Moves the LandGravity plays a part in all erosion. For example, rivers andalpine glaciers flow because of gravity. Gravity can moveonly material that has been loosened in some way, such as,by freezing and thawing or by lubrication. Sometimesgravity makes erosion happen rapidly. For example, thephoto shows a common form of erosion on hillsides inCalifornia. Heavy winter rains lubricate the soil. The soilflows downhill as a mudflow. If youVe ever seen a signthat read CAUTION: FALLING ROCK, you are aware ofanother way gravity makes erosion happen rapidly.

- •

Gravity works slowly, too. You may have noticed oldtelephone poles or tombstones that tilt downhill. Loosesoil and rock is moving slowly downhill, tilting objectsalong the way.

Self-Check1. What are two ways that wind erodes the land?2. How does a sand dune form?

3 What are two causes of a mudflow?


Chapte SUMMARY

All rock exposed at the surfacebegins to break apart.

Mechanical weathering is theprocess of breaking up rockswithout changing their minerals.

Chemical weathering is theprocess of breaking up rocks bychanging the minerals in them.

Soil is a mixture of weatheredrock and the remains of plantsand animals.

Fully developed soil includes atopsoil, a subsoil, and a layer ofpartially weathered rock.

The process by which weatheredrock bits and soil are moved iscalled erosion.

Erosion is caused by water,glaciers, wind, and gravity.

As a river erodes the land, itcarves out a valley. The size ofthe river and valley changes withtime.

A river deposits sediment whereit flows into a lake or an ocean,forming a delta.

Waves wear away the shorelinein some places and build it up inothers.

Glaciers are moving sheets ofice.

Glaciers form U-shaped valleysand scrape the land.

Glaciers leave ridges of sedimentcalled moraines. Glaciers formedmany lakes.

Wind erodes by carryingsediment and by blowing itagainst rock.

Sand dunes form as sand collectsinto a huge mound.

Gravity moves rock and soildownhill. This process can occurquickly or slowly.

Science Words alluvial fan, 245chemical weathering, 237cirque, 251delta, 245deposition, 245erosion, 242flood plain, 244glacier, 250horn, 251

meander, 244mechanical weathering, 236moraine, 252oxbow lake, 244oxidation, 237soil, 238subsoil, 239topsoil, 238weathering, 236


R E V I E W

Soil layers

Vocabulary ReviewNumber your paper from 1 to 8. Then, from each pair ofterms in parentheses, choose the one that best completeseach sentence.

1. (Erosion, Weathering) is the wearing away and movingof rock.

2. An example of chemical weathering is (deposition,oxidation).

3. The part of the soil that includes most remains of plantsand animals is the (subsoil, topsoil).

4. Large, looping curves in a river are (meanders, oxbows).

5. Sediment settles out where a river empties into anocean, forming a (delta, horn).

6. Moving bodies of ice are (alluvial fans, glaciers).

7. Glaciers that form in mountain valleys are(alpine, continental) glaciers.

8. Sediment that drops from a glacier formsridges called (cirques, moraines).

Concept ReviewNumber your paper from 1 to 4. Then nameeach layer shown in the diagram.


Number your paper from 5 to 9. Then choose the word orwords that best complete each sentence. Write the letter ofthe answer on your paper.

5. Water freezing in the cracks of rocks is an example of

a. deposition c. mechanical weatheringb. chemical weathering

6. During oxidation, oxygen combines with iron to formiron oxide, or .a. acid b. rust c. feldspar

7. both cause erosion.a. Gravity and wind c. Oxygen and limestoneb. Topsoil and subsoil

8. Two characteristics of a young river are a .a. U-shaped valley and slow-movingb. V-shaped valley and fast-movingc. wide valley and many meanders

9. The main process that forms a beach is .a. weathering b. erosion c. deposition

Critical ThinkingWrite the answer to each of the following questions.

1. Why would a farmer plow across a hillside instead ofplowing straight down the slope?

2. Once rock breaks into pieces, weathering occurs faster.Explain how this is true.

3. Suppose sand dunes around a beach are being blowntoward a neighborhood. Residents want to keep the dunesbut they don't want them blowing into the neighborhood.What can the residents do to solve this problem?

Try to answer all questions as completely as possible. Whenasked to explain your answer, do so in complete sentences.


the earth's water -...

Documents