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1、智课网托福备考资料托福阅读――指代题（六）GeologyandLandscapeMostpeopleconsiderthelandscapetobeunchanging,butEarthisadynamicbody,anditssurfaceiscontinuallyaltering-slowlyonthehumantimescale,butrelativelyrapidlywhencomparedtothegreatageofEarth(about4,500billionyears).Therearetwop
2、rincipalinfluencesthatshapetheterrain:constructiveprocessessuchasuplift,whichcreatenewlandscapefeatures,anddestructiveforcessuchaserosion,whichgraduallywearawayexposedlandforms.Hillsandmountainsareoftenregardedastheepitomeofpermanence,successfullyresistingth
3、edestructiveforcesofnature,butinfacttheytendtoberelativelyshort-livedingeologicalterms.Asageneralrule,thehigheramountainis,themorerecentlyitwasformed;forexample,thehighmountainsoftheHimalayasareonlyabout50millionyearsold.Lowermountainstendtobeolder,andareoft
4、entheerodedrelicsofmuchhighermountainchains.About400millionyearsago,whenthepresent-daycontinentsofNorthAmericaandEuropewerejoined,theCaledonianmountainchainwasthesamesizeasthemodernHimalayas.Today,however,therelicsoftheCaledonianorogeny(mountain-buildingperi
5、od)existasthecomparativelylowmountainsofGreenland,thenorthernAppalachiansintheUnitedStates,theScottishHighlands,andtheNorwegiancoastalplateau.TheEarth'scrustisthoughttobedividedintohuge,movablesegments,calledplates,whichfloatonasoftplasticlayerofrock.Somemou
6、ntainswereformedasaresultoftheseplatescrashingintoeachotherandforcinguptherockattheplatemargins.Inthisprocess,sedimentaryrocksthatoriginallyformedontheseabedmaybefoldedupwardstoaltitudesofmorethan26,000feet.Othermountainsmayberaisedbyearthquakes,whichfractur
7、etheEarth'scrustandcandisplaceenoughrocktoproduceblockmountains.Athirdtypeofmountainmaybeformedasaresultofvolcanicactivitywhichoccursinregionsofactivefoldmountainbelts,suchasintheCascadeRangeofwesternNorthAmerica.TheCascadesaremadeupoflavasandvolcanicmateria
8、ls.Manyofthepeaksareextinctvolcanoes.Whateverthereasonformountainformation,assoonaslandrisesabovesealevelitissubjectedtodestructiveforces.Theexposedrocksareattackedbythevariousweatherprocessesan
