托福阅读――地质地貌类(一)

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1、智课网托福备考资料托福阅读――地质地貌类(一）ThesurfaceofMarsThesurfaceofMarsshowsawiderangeofgeologicfeatures,includinghugevolcanoes-thelargestknowninthesolarsystem-andextensiveimpactcratering.ThreeverylargevolcanoesarefoundontheTharsisbulge,anenormousgeologicareanearMars’sequator.NorthwestofTharsisisthelargestvolc

2、anoofall:OlympusMons,withaheightof25kilometersandmeasuringsome700kilometersindiameteratitsbase.ThethreelargevolcanoesontheTharsisbulgearealittlesmaller-a“mere”18kilometershigh.NoneofthesevolcanoeswasformedasaresultofcollisionsbetweenplatesoftheMartiancrust-thereisnoplatemotiononMars.Instead,the

3、yareshieldvolcanoes—volcanoeswithbroad,slopingslidesformedbymoltenrock.AllfourshowdistinctivelavachannelsandotherflowfeaturessimilartothosefoundonshieldvolcanoesonEarth.ImagesoftheMartiansurfacerevealmanyhundredsofvolcanoes.MostofthelargestvolcanoesareassociatedwiththeTharsisbulge,butmanysmalle

4、ronesarefoundinthenorthernplains.ThegreatheightofMartianvolcanoesisadirectconsequenceoftheplanet’slowsurfacegravity.Aslavaflowsandspreadstoformashieldvolcano,thevolcano’seventualheightdependsonthenewmountain’sabilitytosupportitsownweight.Thelowerthegravity,thelessertheweightandthegreatertheheig

5、htofthemountain.ItisnoaccidentthatMaxwellMonsonVenusandtheHawaiianshieldvolcanoesonEarthrisetoaboutthesameheight(about10kilometers)abovetheirrespectivebases-EarthandVenushavesimilarsurfacegravity.Mars’ssurfacegravityisonly40percentthatofEarth,sovolcanoesriseroughly2.5timesashigh.AretheMartiansh

6、ieldvolcanoesstillactive?Scientistshavenodirectevidenceforrecentorongoingeruptions,butifthesevolcanoeswereactiveasrecentlyas100millionyearsago(anestimateofthetimeoflasteruptionbasedontheextentofimpactcrateringontheirslopes),someofthemmaystillbeatleastintermittentlyactive.Millionsofyears,though,

7、maypassbetweeneruptions.AnotherprominentfeatureofMars’ssurfaceiscratering.TheMarinerspacecraftfoundthatthesurfaceofMars,aswellasthatofitstwomoons,ispittedwithimpactcratersformedbymeteoroidsfallinginfromspace.AsonourMoon,thesmaller