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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).Therearetwoprincipalinfl
2、uencesthatshapetheterrain:constructiveprocessessuchasuplift,whichcreatenewlandscapefeatures,anddestructiveforcessuchaserosion,whichgraduallywearawayexposedlandforms.Hillsandmountainsareoftenregardedastheepitomeofpermanence,successfullyresistingthedestructiveforcesofnatu
3、re,butinfacttheytendtoberelativelyshort-livedingeologicalterms.Asageneralrule,thehigheramountainis,themorerecentlyitwasformed;forexample,thehighmountainsoftheHimalayasareonlyabout50millionyearsold.Lowermountainstendtobeolder,andareoftentheerodedrelicsofmuchhighermountai
4、nchains.About400millionyearsago,whenthepresent-daycontinentsofNorthAmericaandEuropewerejoined,theCaledonianmountainchainwasthesamesizeasthemodernHimalayas.Today,however,therelicsoftheCaledonianorogeny(mountain-buildingperiod)existasthecomparativelylowmountainsofGreenlan
5、d,thenorthernAppalachiansintheUnitedStates,theScottishHighlands,andtheNorwegiancoastalplateau.TheEarth'scrustisthoughttobedividedintohuge,movablesegments,calledplates,whichfloatonasoftplasticlayerofrock.Somemountainswereformedasaresultoftheseplatescrashingintoeachothera
6、ndforcinguptherockattheplatemargins.Inthisprocess,sedimentaryrocksthatoriginallyformedontheseabedmaybefoldedupwardstoaltitudesofmorethan26,000feet.Othermountainsmayberaisedbyearthquakes,whichfracturetheEarth'scrustandcandisplaceenoughrocktoproduceblockmountains.Athirdty
7、peofmountainmaybeformedasaresultofvolcanicactivitywhichoccursinregionsofactivefoldmountainbelts,suchasintheCascadeRangeofwesternNorthAmerica.TheCascadesaremadeupoflavasandvolcanicmaterials.Manyofthepeaksareextinctvolcanoes.Whateverthereasonformountainformation,assoonasl
8、andrisesabovesealevelitissubjectedtodestructiveforces.Theexposedrocksareattackedbythevariousweatherprocessesan
