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智课网托福备考资料 托福阅读――地质地貌类(一) ThesurfaceofMars ThesurfaceofMarsshowsawiderangeofgeologicfeatures, includinghugevolcanoes-thelargestknowninthesolar system-andextensiveimpactcratering.Threeverylarge volcanoesarefoundontheTharsisbulge,anenormousgeologic areanearMars’sequator.NorthwestofTharsisisthelargest volcanoofall:OlympusMons,withaheightof25kilometersand measuringsome700kilometersindiameteratitsbase.Thethree largevolcanoesontheTharsisbulgearealittlesmaller-a “mere”18kilometershigh. Noneofthesevolcanoeswasformedasaresultofcollisions betweenplatesoftheMartiancrust-thereisnoplatemotionon Mars.Instead,theyareshieldvolcanoes—volcanoeswithbroad, slopingslidesformedbymoltenrock.Allfourshowdistinctive lavachannelsandotherflowfeaturessimilartothosefoundon shieldvolcanoesonEarth.ImagesoftheMartiansurfacereveal manyhundredsofvolcanoes.Mostofthelargestvolcanoesare associatedwiththeTharsisbulge,butmanysmalleronesare foundinthenorthernplains. ThegreatheightofMartianvolcanoesisadirect consequenceoftheplanet’slowsurfacegravity.Aslavaflows andspreadstoformashieldvolcano,thevolcano’seventual heightdependsonthenewmountain’sabilitytosupportits ownweight.Thelowerthegravity,thelessertheweightandthe greatertheheightofthemountain.ItisnoaccidentthatMaxwell MonsonVenusandtheHawaiianshieldvolcanoesonEarthrise toaboutthesameheight(about10kilometers)abovetheir respectivebases-EarthandVenushavesimilarsurfacegravity. Mars’ssurfacegravityisonly40percentthatofEarth,so volcanoesriseroughly2.5timesashigh.AretheMartianshield volcanoesstillactive?Scientistshavenodirectevidencefor recentorongoingeruptions,butifthesevolcanoeswereactive asrecentlyas100millionyearsago(anestimateofthetimeof lasteruptionbasedontheextentofimpactcrateringontheir slopes),someofthemmaystillbeatleastintermittentlyactive. Millionsofyears,though,maypassbetweeneruptions. AnotherprominentfeatureofMars’ssurfaceiscratering. TheMarinerspacecraftfoundthatthesurfaceofMars,aswellas thatofitstwomoons,ispittedwithimpactcratersformedby meteoro