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1、智课网TOEFL备考资料文勇ibt黄金阅读:冰川的形成摘要：刘文勇新托福黄金阅读最新版文本及答案，高清PDF格式支持免费下载，一共71篇，包括原刘文勇黄金阅读29篇，下面是文勇ibt黄金阅读:冰川的形成，希望对大家备考托福阅读考试有所帮助。文勇ibt黄金阅读:GlacierFormation冰川的形成Glaciersareslowlymovingmassesoficethathaveaccumulatedonlandinareaswheremoresnowfallsduringayearthanmelts.Snowfa
2、llsashexagonalcrystals,butonceontheground,snowissoontransformedintoacompactedmassofsmaller,roundedgrains.Astheairspacearoundthemislessenedbycompactionandmelting,thegrainsbecomedenser.Withfurthermelting,refreezing,andincreasedweightfromnewersnowfallabove,thesnowr
3、eachesagranularrecrystallizedstageintermediatebetweenflakesandiceknownasfirn.Withadditionaltime,pressure,andrefrozenmeltwaterfromabove,thesmallfirngranulesbecomelarger,interlockedcrystalsofblueglacialice.Whentheiceisthickenough,usuallyover30meters,theweightofthe
4、snowandfirnwillcausetheicecrystalstowardthebottomtobecomeplasticandtoflowoutwardordownwardfromtheareaofsnowaccumulation.Glaciersareopensystems,withsnowasthesystem'sinputandmeltwaterasthesystem’smainoutput.Theglacialsystemisgovernedbytwobasicclimaticvariables:pre
5、cipitationandtemperature.Foraglaciertogrowormaintainitsmass,theremustbesufficientsnowfalltomatchorexceedtheannuallossthroughmelting,evaporation,andcalving,whichoccurswhentheglacierlosessolidchunksasicebergstotheseaortolargelakes.Ifsummertemperaturesarehighfortoo
6、long,thenallthesnowfallfromthepreviouswinterwillmelt.Surplussnowfallisessentialforaglaciertodevelop.Asurplusallowssnowtoaccumulateandforthepressureofsnowaccumulatedovertheyearstotransformburiedsnowintoglacialicewithadepthgreatenoughfortheicetoflow.Glaciersaresom
7、etimesclassifiedbytemperatureasfaster-flowingtemperateglaciersorasslower-flowingpolarglaciers.GlaciersarepartofEarth'shydrologiccycleandaresecondonlytotheoceansinthetotalamountofwatercontained.About2percentofEarth'swateriscurrentlyfrozenasice.Twopercentmaybeadec
8、eivingfigure,however,sinceover80percentoftheworld'sfreshwaterislockedupasiceinglaciers,withthemajorityofitinAntarctica.Thetotalamountoficeisevenmoreawesomeifweestimat
