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The Ever€hangingSurfaceof the Earth- Part 1: Erosion
lf you'veever hadthe privilegeof travelingto foreigncountriesor evenacrossthe United
States,you've certainly observed the many different landforms
that make up the fascinatingsurface of our PlanetEarth! But
there's somethingabout these landformsthat is impossibleto
noticewith just a casualglance.Fromthe vast plainsand
plateausto the rolling hills,towering mountains,and deep
valleys;the Earth'ssurface is very gradually,but very
constantly,being moldedand worn down by a processcalfed
erosion.
2 Erosionhappenswhen runningwater, sea waves,wihd, or
glacierspick up materialsfrom the Earth'ssurfaceand carry
them to other locations.The actions of erosionand deposition
work togetherto alter existinglandformsand create new ones.
Depositionoccurs when silt, sand and other materialsare
carried by flowing water and depositedat other placeson the
Earth'ssurface.
t Movingwaters,such as rivers and streams,are the mostimportantnaturalagentsof
erosion.Wheneverit rains,water washesover the land,picks up loosesoil, and carries it
away.Wateralsowashesweatheredrocks (rocksalteredthrougha processcalled
weathering)into rivers and streams.We'lllearnmoreabouthow weatheringimpactsthe
erosionprocessin Part 2 of "The Ever-changingSurfaceof the Earth."
a Gravitationalpull playsa majorrole in the processof erosionand can causehuge
amountsof soil or rock debristo be carried downa slope,suchas in the caseof a mudslide.
Thisoccurrenceis referredto as a massmovement.Anothertype of maasmovementis when
certainareasof the Earth'ssurfaceare pulledby gravityand sink belowgroundlevel.
5 GravitSr
also naturallypulls rivers and streamsdownhill.Mostof thesestreamscarry silt,
clay,andsandthatare suspendedin the water.Pebblesandgranulesare rolled,tumbled,
and bouncedalonga stream'sbed- the part of the landcoveredby a river or stream.All of
the rocks and mineralsthat a streamcarries are calledits load.Little streamspassthelr
loadsalongto biggerstreams,and biggerstreamspasstheir loadsalongto rivers.
6 Whenthe sedimentcontainedin theseloadsis depositedat the mouthof a river, a delta
(or low-lyingplain)is formed.Sometimesthe sand,clay,and silt sedimentsfrom thesefanshapeddeltasare pickedup and droppedonto surroundinglandareasby floodwaters,
creatingvery fertile farmland.Forthis reason,deltashavebeerrimportantto major
civilizationssince prehistorictimes! ln recentyears,geologistshavediscoveredthat much
of the world's petroleumresourcesare foundin ancientdeltaicrocks. Somerivers haveno
deltasbecausepowerfulsea currentsand waveswhiskthe sedimentawaybeforeit can
buildup.
t Seawavesare very activeagentsof coastalerosion.As wavespoundagainstthe cliffs at
the water'sedge,they breakoff piecesof rock, draggingthem back into the ocean.Other
wavescatch the rocks and hurl them back at the basesof thesecliffs. LitUeby litUe,the cliffs
losetheir supportand collapseinto the ocean.As thesewavesare carvingawaythe
shoreline,othersare depositingsandand pebblesthat were broughtto the oceanby rivers.
Thisconstant,powerfulactionof erosionand depositioncausessubstantial,noticeable
changeto our rocky shoresand sandybeaches,
I In contrastto rapidlyflowing riversand waves,glaciersare agentsof erosionthat move
very slowlyover the Earth'ssurface.Glaciersform in polar regions,such as Antarcticaand
Greenland,and in high mountainswhere low temperafurescauselarge amountsof snowto
buildup andturn into ice.A glacierflowsdownslopebecauseof the pullof gravity,usually
movinglessthan a foot per day. Thesehugemasses,rangingin thicknessfrom about300to
10,000feet, passover the land and shapeits featuresbyplucking,transporting,and
depositingrock debrisfrom the sizeof blocksor bouldersto sandand clay.As the debris
pilesup, it formsunevenridgescalledmoraines.Severalkindsof morainesare recognized,
includinglateralmorainesalongthe sidesof the glacier,andterminalmorainesat its leading
edge.Manyother landformsare associatedwith glaciers,the mostfamousof which are
foundin Europe.
n Windplaysa key role in the processof erosionin arid regionslike deserts,as blowing
sandbreaksdown rock and dislodgessurfacesandfrom unprotectedsanddunes.As sand
and silt are lifted, larger materials,such as pebblesand cobblesare left behind.Overtime,
thesepebblesand cobblespack closelytogetherforminga desert pavement.The removalof
largeamountsof sandand silt is calleddeflation,which lowersthe landscapeslowly.The
DustBowlof the '1930'sis an exampleof deflaUon,which occurredover an area of 50 million
acres in the GreatPlainsregionof the UnitedStates.This seriesof dust stormsdestroyed
the protectivecover ofvegetationthat had alreadybeenweakenedby poorfarming
practicesand the grazingof too manyanimals.
'o Somehistoriansbelievethatsoilerosionhasimpactedvariouspopulationshaftsandhas
causedthe fall of certain civilizations.Ruinsof towns and cities havebeenfound in arid
regions,such as the desertsof Mesopotamia,indicatingthat agriculturewas once
widespreadin the surroundingterritory.
" The naturalprocessof erosionis slow and sure. lts in amazingoccurrencethat, over
the courseof hundredsof thousandsof years,can wear awaya mountainuntil it is levelwith
the plain,or cut a steep-sidedgorge out of a cliffto form a wonderof naturesuch as Niagara
Falls.Todayand in the yearsto come,the surfaceof the Earthwill continueto be alteredby
the manyactiveagentsof erosion.
'|-. List three of the major agentsof erosion.
2. Erosionand
ones.
work togetherto alter existinglandformsand create new
3. Whengravity causeshugeamountsof rock or soil to be carried downa slope,this
form of erosionis referredto as a
(E) Glacier
(D MassMovement
(D Landform
(D Rockpile
4. Whena river or streamtransportssilt, clay, pebbles,sand,and minerals,the material
it carries is called its _.
@ Riverbed
(D Load
@ Deposit
(D 661ge
5. Sedimentsdepositedatthe mouthof a river ereatethis fan-shapedlandform.
@ Oelta
(E) Oceanfloor
@ Bay
(D Basin
6. Whencomparingthe movementof sea waveswittr the movementof glaciers,whichof
the followingstatementsis true?
G Seawavesmovemuchfaster than glaciers.
(E) Seawavesbreakdownand buildup landforms,but glaciersjust makethe land
flatter.
G Seawavescarry pebbles,andglaciersjust carry ice chunks.
(D Seawavesmoveconstantly,but glaciersonlymoveoncein a while.
7. As glaciersdepositrock debris,unevenridgescalled
G
@
@
@
are formed.
Peaks
Moraines
Ledges
Crevasses
8. TheDustBowl,whichoccurredduringthe 1930'sin the GieatPlainsregionof the
UnitedStates,is an exampleof
(D lnflation
(D Deflation
G Wind erosion
(D Both (b) and (c) are correct
20
IN SCIENCE
5 TAKS
THEGRADE
MASTERING
flowerson a plant
7 Jameshasdecidedto investigatewhetherthe numberof
has four
will increaseif water suppliedto that plant is increased.James
pots of geraniumsto useduring his experiment.
plantsin order
What factor in the experimentshouldbe variedfor the four
to answerthe question?
of the water
A Temperature
B Numberof hoursin sunlight
C Age of the seeds
D Amountof water
what haPPens.
Alice placesa magnet next to a metal fork. She records
She
of
Next, she placesthe same magnet next to a fork made Plastic.
recordswhat haPPensnext.
which questionis Alice most likery exploringwith this experiment?
F
G
H
J
Does the size of the magnetaffect the magnet'spower?
What causesmagnetsto be attractedto metal objects?
How do different objectsreact to magnets?
Do objects other than magnetshave magneticpower?
has on the
Kara did an experiment to find out the effect that temPerature
exPeriment?
activity of yeast.which step would come last in Karats
A Move one bowl to a waffner location'
B Observewhat haPPensin eachbowl'
c Put 120 milliliters of water in eachbowl.
D Add 1 gram of Yeastto eachbowl'
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