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1、20130131GeologyandLandscapeMostpeopleconsiderthelandscapetobeunchanging,butEarthisadynamicbody,anditssurfaceiscontinuallyaltering-slowlyonthehumantimescale,butrelativelyrapidlywhencomparedtothegreatageofEarth(about4,500billionyears).Therearetwoprincipalinfluencesthatshapetheterrain:constructivepro
2、cessessuchasuplift,whichcreatenewlandscapefeatures,anddestructiveforcessuchaserosion,whichgraduallywearawayexposedlandforms.Hillsandmountainsareoftenregardedastheepitomeofpermanence,successfullyresistingthedestructiveforcesofnature,butinfacttheytendtoberelativelyshort-livedingeologicalterms.Asagen
3、eralrule,thehigheramountainis,themorerecentlyitwasformed;forexample,thehighmountainsoftheHimalayasareonlyabout50millionyearsold.Lowermountainstendtobeolder,andareoftentheerodedrelicsofmuchhighermountainchains.About400millionyearsago,whenthepresent-daycontinentsofNorthAmericaandEuropewerejoined,the
4、CaledonianmountainchainwasthesamesizeasthemodernHimalayas.Today,however,therelicsoftheCaledonianorogeny(mountain-buildingperiod)existasthecomparativelylowmountainsofGreenland,thenorthernAppalachiansintheUnitedStates,theScottishHighlands,andtheNorwegiancoastalplateau.TheEarth'scrustisthoughttobediv
5、idedintohuge,movablesegments,calledplates,whichfloatonasoftplasticlayerofrock.Somemountainswereformedasaresultoftheseplatescrashingintoeachotherandforcinguptherockattheplatemargins.Inthisprocess,sedimentaryrocksthatoriginallyformedontheseabedmaybefoldedupwardstoaltitudesofmorethan26,000feet.Otherm
6、ountainsmayberaisedbyearthquakes,whichfracturetheEarth'scrustandcandisplaceenoughrocktoproduceblockmountains.Athirdtypeofmountainmaybeformedasaresultofvolcanicactivitywhichoccursinregionsofactivefoldmountainbelts,suchasintheCascadeRangeofwesternNorthAmerica.TheCascadesaremadeupoflavasandvolcanicma
7、terials.Manyofthepeaksareextinctvolcanoes.Whateverthereasonformountainformation,assoonaslandrisesabovesealevelitissubjectedtodestructiveforces.Theexposedrocksareattackedbythevariousweatherprocessesandgraduallybro
