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1、智课网托福备考资料托福阅读――地质地貌类（四）Icebergsaremassiveblocksofice,irregularinshape;theyfloatwithonlyabout12percentoftheirmassabovetheseasurface.Theyareformedbyglaciers—largeriversoficethatbegininlandinthesnowsofGreenland,Antarctica,andAlaska—andmoveslowlytowardthesea.Theforwardmovement,theme
2、ltingatthebaseoftheglacierwhereitmeetstheocean,andwavesandtidalactioncauseblocksoficetobreakoffandfloatouttosea.Icebergsareordinarilybluetowhite,althoughtheysometimesappeardarkoropaquebecausetheycarrygravelandbitsofrock.Theymaychangecolorwithchanginglightconditionsandcloudcover,
3、glowingpinkorgoldinthemorningoreveninglight,butthiscolorchangeisgenerallyrelatedtothelowangleoftheSunabovethehorizon.However,travelerstoAntarcticahaverepeatedlyreportedseeinggreenicebergsintheWeddellSeaand,morecommonly,closetotheAmeryIceShelfinEastAntarctica.Oneexplanationforgre
4、enicebergsattributestheircolortoanopticalillusionwhenblueiceisilluminatedbyanear-horizonredSun,butgreenicebergsstandoutamongwhiteandblueicebergsunderagreatvarietyoflightconditions.Anothersuggestionisthatthecolormightberelatedtoicewithhighlevelsofmetalliccompounds,includingcopper
5、andiron.Recentexpeditionshavetakenicesamplesfromgreenicebergsandicecores—vertical,cylindricalicesamplesreachingdowntogreatdepths—fromtheglacialiceshelvesalongtheAntarcticcontinent.Analysesofthesecoresandsamplesprovideadifferentsolutiontotheproblem.Theiceshelfcores,withatotalleng
6、thof215meters(705feet),werelongenoughtopenetratethroughglacialice—whichisformedfromthecompactionofsnowandcontainsairbubbles—andtocontinueintotheclear,bubble-freeiceformedfromseawaterthatfreezesontothebottomoftheglacialice.Thepropertiesofthisclearseaicewereverysimilartotheicefrom
7、thegreeniceberg.Thescientistsconcludedthatgreenicebergsformwhenatwo-layerblockofshelficebreaksawayandcapsizes(turnsupsidedown),exposingthebubble-freeshelficethatwasformedfromseawater.AgreenicebergthatstrandedjustwestoftheAmeryIceShelfshowedtwodistinctlayers:bubblyblue-whiteicean
8、dbubble-freegreeniceseparatedbyaone-meter-longi
