托福考试真题练习.docx

托福考试真题练习

托福考试真题练习2017 加紧学习，抓住中心，宁精勿杂，宁专勿多。以下是小编为大家搜索整理的托福考试真题练习2017，希望对大家有所帮助!更多精彩内容请及时关注我们应届毕业生考试网! Mostpeopleconsiderthelandscapetobeunchanging,butEarthisadynamicbody,anditssurfaceiscontinuallyaltering-slowlyonthehumantimescale,butrelativelyrapidlywhencomparedtothegreatageofEarth(about4,500billionyears).Therearetwoprincipalinfluencesthatshapetheterrain:constructiveprocessessuchasuplift,whichcreatenewlandscapefeatures,anddestructiveforcessuchaserosion,whichgraduallywearawayexposedlandforms. Hillsandmountainsareoftenregardedastheepitomeofpermanence,successfullyresistingthedestructiveforcesofnature,butinfacttheytendtoberelativelyshort-livedingeologicalterms.Asageneralrule,thehigheramountainis,themorerecentlyitwasformed;forexample,thehighmountainsoftheHimalayasareonlyabout50millionyearsold.Lowermountainstendtobeolder,andareoftentheerodedrelicsofmuchhighermountainchains.About400millionyearsago,whenthepresent-daycontinentsofNorthAmericaandEuropewerejoined,theCaledonianmountainchainwasthesamesizeasthemodernHimalayas.Today,however,therelicsoftheCaledonianorogeny(mountain-buildingperiod)existasthecomparativelylowmountainsofGreenland,thenorthernAppalachiansintheUnitedStates,theScottishHighlands,andtheNorwegiancoastalplateau. TheEarth'scrustisthoughttobepidedintohuge,movablesegments,calledplates,whichfloatonasoftplasticlayerofrock.Somemountainswereformedasaresultoftheseplatescrashingintoeachotherandforcinguptherockattheplatemargins.Inthisprocess,sedimentaryrocksthatoriginallyformedontheseabedmaybefoldedupwardstoaltitudesofmorethan26,000feet.Othermountainsmayberaisedbyearthquakes,whichfracturetheEarth'scrustandcandisplaceenoughrocktoproduceblockmountains.Athirdtypeofmountainmaybeformedasaresultofvolcanicactivitywhichoccursinregionsofactivefoldmountainbelts,suchasintheCascadeRangeofwesternNorthAmerica.TheCascadesaremadeupoflavasandvolcanicmaterials.Manyofthepeaksareextinctvolcanoes. Whateverthereasonformountainformation,assoonaslandrisesabovesealevelitissubjectedtodestructive