托福阅读――地质地貌类(一)-豆柴文库

您所在位置：网站首页 / 托福阅读――地质地貌类(一).pdf / 文档详情

免费试读已结束，剩余 8 页请下载文档后查看

16 金币

下载文档

/ 13

下载提示

如果您无法下载资料，请参考说明：

1、部分资料下载需要金币，请确保您的账户上有足够的金币

2、已购买过的文档，再次下载不重复扣费

3、资料包下载后请先用软件解压，在使用对应软件打开

文本预览

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