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EncyclopedicEntry ForTeachers

landslide

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Alandslideisthemovementofrock,earth,ordebrisdownaslopedsectionofland.Landslidesarecausedbyrain,earthquakes,volcanoes,orotherfactorsthatmaketheslopeunstable.Geologists,scientistswhostudythephysicalformationsoftheEarth,sometimesdescribelandslidesasonetypeofmasswasting.AmasswastingisanydownwardmovementinwhichtheEarth'ssurfaceiswornaway.Othertypesofmasswastingincluderockfallsandtheflowofshoredepositscalledalluvium.Nearpopulatedareas,landslidespresentmajorhazardstopeopleandproperty.Landslidescauseanestimated25to50deathsand$3.5billionindamageeachyearintheUnitedStates.WhatCausesLandslides?Landslideshavethreemajorcauses:geology,morphology,andhumanactivity.Geologyreferstocharacteristicsofthematerialitself.Theearthorrockmightbeweakorfractured,ordifferentlayersmayhavedifferentstrengthsandstiffness.Morphologyreferstothestructureoftheland.Forexample,slopesthatlosetheirvegetationtofireordroughtaremorevulnerabletolandslides.Vegetationholdssoilinplace,andwithouttherootsystemsoftrees,bushes,andotherplants,thelandismorelikelytoslideaway.Aclassicmorphologicalcauseoflandslidesiserosion,orweakeningofearthduetowater.InApril1983,thetownofThistle,Utah,experiencedadevastatinglandslidebroughtonbyheavyrainsandrapidlymeltingsnow.Amassofeartheventuallytotaling305meterswide,61metersthick,and1.6kilometerslong(1,000feetwide,200feetthick,andonemilelong)slidacrossthenearbySpanishForkRiver,dammingitandseveringrailroadandhighwaylines.ThelandslidewasthecostliestinU.S.history,causingover$400millionindamageanddestroyingThistle,whichremainsanevacuatedghosttowntoday.Humanactivity,suchasagricultureandconstruction,canincreasetheriskofalandslide.Irrigation,deforestation,excavation,andwaterleakagearesomeofthecommonactivitiesthatcanhelpdestabilize,orweaken,aslope.TypesofLandslidesTherearemanywaystodescribealandslide.Thenatureofalandslide'smovementandthetypeofmaterialinvolvedaretwoofthemostcommon.Thereareseveralwaysofdescribinghowalandslidemoves.Theseincludefalls,topples,translationalslides,lateralspreads,andflows.Infallsandtopples,heavyblocksofmaterialfallafterseparatingfromaverysteepslopeorcliff.Boulderstumblingdownaslopewouldbeafallortopple.Intranslationalslides,surfacematerialisseparatedfromthemorestableunderlyinglayerofaslope.Anearthquakemayshaketheloosetoplayerofsoilfreefromtheharderearthbeneathinthistypeoflandslide.Alateralspreadorflowisthemovementofmaterialsideways,orlaterally.Thishappenswhenapowerfulforce,suchasanearthquake,makesthegroundmovequickly,likealiquid.Alandslidecaninvolverock,soil,vegetation,water,orsomecombinationofallthese.Alandslidecausedbyavolcanocan

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VOCABULARY

agriculture noun theartandscienceofcultivatingthelandforgrowingcrops(farming)orraisinglivestock(ranching).

alluvium noun gravel,sand,andsmallermaterialsdepositedbyflowingwater.

boulder noun largerock.

bush noun low-lyingplantwithmanybranches.

characteristic noun physical,cultural,orpsychologicalfeatureofanorganism,place,orobject.

complex adjective complicated.

construction noun arrangementofdifferentparts.

costly adjective expensiveorhavingalotofvalue.

creep verb tomoveslowlyandclosetotheground.

dam verb toblockaflowofwater.

debris noun remainsofsomethingbrokenordestroyed,waste,orgarbage.

deforestation noun destructionorremovalofforestsandtheirundergrowth.

destroy verb toruinormakeuseless.

devastate verb todestroy.

drought noun periodofgreatlyreducedprecipitation.

earth noun soilordirt.

earthquake noun thesuddenshakingofEarth'scrustcausedbythereleaseofenergyalongfaultlinesorfromvolcanicactivity.

erosion noun actinwhichearthiswornaway,oftenbywater,wind,orice.

eruption noun releaseofmaterialfromanopeningintheEarth'scrust.

estimate verb toguessbasedonknowledgeofthesituationorobject.

evacuate verb toleaveorremovefromadangerousplace.

excavation noun areathathasbeenduguporexposedforstudy.

expel verb toejectorforceout.

alsocontainhotvolcanicashandlavafromtheeruption.Alandslidehighinthemountainsmayhavesnowandsnowmelt.Volcaniclandslides,alsocalledlahars,areamongthemostdevastatingtypeoflandslides.Thelargestlandslideinrecordedhistorytookplaceafterthe1980eruptionofMountSt.HelensintheU.S.stateofWashington.Theresultingflowofash,rock,soil,vegetationandwater,withavolumeofabout2.9cubickilometers(0.7cubicmiles),coveredanareaof62squarekilometers(24squaremiles).Anotherfactorthatmightbeimportantfordescribinglandslidesisthespeedofthemovement.Somelandslidesmoveatmanyfeetpersecond,whileotherscreepalongataninchortwoayear.Theamountofwater,iceorairintheearthshouldalsobeconsidered.SomelandslidesincludetoxicgasesfromdeepintheEarthexpelledbyvolcanoes.Somelandslides,calledmudslides,containahighamountofwaterandmoveveryquickly.Complexlandslidesconsistofacombinationofdifferentmaterialormovementtypes.

Term PartofSpeech Definition

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fall noun movementofpiecesofrockorsoildownwardinalandslide.

fire noun achemicalprocessthatreleasesheatandlightduetoburning.

flow noun quickmovementofmaterialinalandslide,asifitwereliquid.

fracture verb tobreak.

gas noun stateofmatterwithnofixedshapethatwillfillanycontaineruniformly.Gasmoleculesareinconstant,randommotion.

geologist noun personwhostudiesthephysicalformationsoftheEarth.

geology noun studyofthephysicalhistoryoftheEarth,itscomposition,itsstructure,andtheprocessesthatformandchangeit.

ghosttown noun urbanareathathasbeenabandonedbyallresidents.

hazard noun dangerorrisk.

highway noun largepublicroad.

ice noun waterinitssolidform.

irrigation noun wateringland,usuallyforagriculture,byartificialmeans.

lahar noun flowofmudandotherwetmaterialfromavolcano.

landslide noun thefallofrocks,soil,andothermaterialsfromamountain,hill,orslope.

lateralspread noun movementofmaterialsidewaysduringalandslide.

lava noun moltenrock,ormagma,thateruptsfromvolcanoesorfissuresintheEarth'ssurface.

masswasting noun downwardmovementofrock,soil,andothermaterial.

morphology noun studyoftheformandstructureoforganismsormaterials.

mudslide noun rapid,downhillflowofsoilandwater.Alsocalledamudflow.

plant noun organismthatproducesitsownfoodthroughphotosynthesisandwhosecellshavewalls.

railroad noun roadconstructedwithmetaltracksonwhichtrainstravel.

rain noun liquidprecipitation.

rock noun naturalsubstancecomposedofsolidmineralmatter.

rockfall noun suddendescentoflargerocks.

rootsystem noun allofaplant'sroots.

scheme noun structureordiagramofthewayinformationisstudied,documented,andunderstood.

sever verb toseparateorcutaway.

slope noun slant,eitherupwardordownward,fromastraightorflatpath.

snow noun precipitationmadeoficecrystals.

snowmelt noun watersuppliedbysnow.

topple noun movementofsmallerpiecesofrockorsoildownwardinalandslide.

toxic adjective poisonous.

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translationalslide

noun movementofallsurfacematerial(includingrocks,soil,andvegetation)downwardduringalandslide.

tree noun typeoflargeplantwithathicktrunkandbranches.

unstable adjective unsteadyorlikelytofallapart.

vegetation noun alltheplantlifeofaspecificplace.

volcanicash noun fragmentsoflavalessthan2millimetersacross.

volcano noun anopeningintheEarth'scrust,throughwhichlava,ash,andgaseserupt,andalsotheconebuiltbyeruptions.

vulnerable adjective capableofbeinghurt.

FORFURTHEREXPLORATION

Articles&ProfilesNationalGeographicNews:BiggestKnownLandslideFoundonMars?

Audio&VideoNationalGeographicVideo:LandslideDevoursHomeNationalGeographicChannel:NakedScience—Landslides

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