高中英语Unit5 The power of nature文章 What is a volcano人教版选修六VIP专享VIP免费

Whatisavolcano?Avolcanoisaventor'chimney'thatconnectsmoltenrock(magma)totheEarth'ssurface.Itincludesthesurroundingconeofbuilt-upmaterial.Magmaeruptingfromavolcanoiscalledlava.Gasesandpiecesofrockeruptfromvolcanoestoo.Avolcanoisactiveifiteruptslava,releasesgasorshowsseismicactivity.Itisdormantifithasn'teruptedforalongtimebutcouldagainoneday.Anextinctvolcanowillnevereruptagain.Theexplosivenessofavolcaniceruptiondependsonhoweasilymagmacanflowandtheamountofgastrappedinit.Largeamountsofwaterandcarbondioxidearedissolvedinmagma.Theybehavelikegasinfizzydrinks.Afteropeningthebottlethegasexpands,formingbubblesthatescape.Thisalsohappenswhenmagmarisesquicklythroughthecrust-gasbubblesformandexpandupto1000timestheiroriginalsize.TypesofvolcanoesTheshapeofavolcanoisrelatedtohowiterupts.用心爱心专心Compositevolcano-alsoknownasastrato-volcano.Ifmagmafromaneruptingvolcanoisthickandsticky(viscous)itisveryhardforgasbubblestoexpand.Theycanforcethemagmaapartsuddenly,blastingitintotinypiecescalledvolcanicash.Theseeruptionsareveryexplosiveandcreatesteepconesofashandrocks.Theysometimescreatelayersoflava,hotashcloudscalledpyroclasticflowsanddangerousmudflows.AvolcanoinPapuaNewGuinea,isanexampleofaCompositevolcano.Shieldvolcanoes-Whenmagmaisrunny(notviscous)gasescanescapeeasily.Theseeruptionsarenotveryexplosiveandlotsofmagmareachesthesurfacetoformlavaflows.Thistypeofvolcanomakesaconebuiltupfromlayersoflava.Becausethelavaflowseasily,theconeisbroadandhasaflatteneddome-likeshape.KilaueavolcanoinHawaiiisanexampleofashieldvolcano.Calderavolcano-eruptssoexplosivelythatlittlematerialbuildsupnearthevent.Eruptionsemptypartoralloftheunderlyingmagmachamber,leavingtheregionaroundtheventunsupportedandcausingtheregiontosinkunderitsownweight.Thisresultsinabasin-shapeddepressionthatisroughlycircularandisusuallyseveralkilometresormoreindiameter.Thelavaeruptedisthemostviscousandgenerallythecoolest.用心爱心专心