雅思(IELTS)閱讀段落練習(xí)試題

雅思(IELTS)閱讀段落練習(xí)試題Volcanoes-earth-shatteringnewsWhenMountPinatubosuddenlyeruptedon9June1991,thepowerofvolcanoespastandpresentagainhittheheadlinesAVolcanoesaretheultimateearth-movingmachinery.Aviolenteruptioncanblowthetopfewkilometresoffamountain,scatterfineashpracticallyallovertheglobeandhurlrockfragmentsintothestratospheretodarkentheskiesacontinentaway.Buttheclassiceruption—cone-shapedmountain,bigbang,mushroomcloudandsurgesofmoltenlava—isonlyatinypartofaglobalstory.Vulcanism,thenamegiventovolcanicprocesses,reallyhasshapedtheworld.Eruptionshaveriftedcontinents,raisedmountainchains,constructedislandsandshapedthetopographyoftheearth.Theentireoceanfloorhasabasementofvolcanicbasalt.Volcanoeshavenotonlymadethecontinents,theyarealsothoughttohavemadetheworld'sfirststableatmosphereandprovidedallthewaterfortheoceans,riversandice-caps.Therearenowabout600activevolcanoes.Everyyeartheyaddtwoorthreecubickilometresofrocktothecontinents.Imagineasimilarnumberofvolcanoessmokingawayforthelast3,500millionyears.Thatisenoughrocktoexplainthecontinentalcrust.Whatcomesoutofvolcaniccratersismostlygas.Morethan90%ofthisgasiswatervapourfromthedeepearth:enoughtoexplain,over3,500millionyears,thewaterintheoceans.Therestofthegasisnitrogen,carbondioxide,sulphurdioxide,methane,ammoniaandhydrogen.Thequantityofthesegases,againmultipliedover3,500millionyears,isenoughtoexplainthemassoftheworld'satmosphere.Wearealivebecausevolcanoesprovidedthesoil,airandwaterweneed.BGeologistsconsidertheearthashavingamoltencore,surroundedbyasemi-moltenmantleandabrittle,outerskin.Ithelpstothinkofasoft-boiledeggwitharunnyyolk,afirmbutsquishywhiteandahardshell.Iftheshellisevenslightlycrackedduringboiling,thewhitematerialbubblesoutandsetslikeatinymountainchainoverthecrack—likeanarchipelagoofvolcanicislandssuchastheHawaiianIslands.Buttheearthissomuchbiggerandthemantlebelowissomuchhotter.Eventhoughthemantlerocksarekeptsolidbyoverlyingpressure,theycanstillslowly'flow'likethicktreacle.Theflow,thoughttobeintheformofconvectioncurrents,ispowerfulenoughtofracturethe'eggshell'ofthecrustintoplates,andkeepthembumpingandgrindingagainsteachother,orevenoverlapping,attherateofafewcentimetresayear.Thesefracturezones,wherethecollisionsoccur,arewhereearthquakeshappen.And,veryoften,volcanoes.CThesezonesarelinesofweakness,orhotspots.Everyeruptionisdifferent,butputatitssimplest,wherethereareweaknesses,rocksdeepinthemantle,heatedto1,350℃,willstarttoexpandandrise.Astheydoso,thepressuredrops,andtheyexpandandbecomeliquidandrisemoreswiftly.Sometimesitisslow:vastbubblesofmagma—moltenrockfromthemantle—inchtowardsthesurface,coolingslowly,toshowthroughasgraniteextrusions(asonSkye,ortheGreatWhinSill,thelavadykesqueezedoutliketoothpastethatcarriespartofHadrian'sWallinnorthernEngland).Sometimes—asinNorthernIreland,WalesandtheKarooinSouthAfrica—themagmarosefaster,andthenflowedouthorizontallyontothesurfaceinvastthicksheets.IntheDeccanplateauinwesternIndia,therearemorethantwomillioncubickilometresoflava,someofit2,400metresthick,formedover500,000yearsofslurpingeruption.Sometimesthemagmamovesveryswiftlyindeed.Itdoesnothavetimetocoolasitsurgesupwards.Thegasestrappedinsidetheboilingrockexpandsuddenly,thelavaglowswithheat,itbeginstofroth,anditexplodeswithtremendousforce.Thentheslightlycoolerlavafollowingitbeginstoflowoverthelipofthecrater.IthappensonMars,ithappenedonthemoon,itevenhappensonsomeofthemoonsofJupiterandUranus.Bystudyingtheevidence,vulcanologistscanreadtheforceofthegreatblastsofthepast.Isthepumicelightandfullofholes?Theexplosionwastremendous.Aretherocksheavy,withhugecrystallinebasaltshapes,liketheGiant'sCausewayinNorthernIreland?Itwasaslow,gentleeruption.Thebiggesteruptionsaredeeponthemid-oceanfloor,wherenewlavaisforcingthecontinentsapartandwideningtheAtlanticbyperhapsfivecentimetresayear.Lookatmapsofvolcanoes,earthquakesandislandchainslikethePhilippinesandJapan,andyoucanseetheroughoutlinesofwhatarecalledtectonicplates—theplateswhichmakeuptheearth'scrustandmantle.ThemostdramaticoftheseisthePacific'ringoffire'wheretherehavebeenthemostviolentexplosions—MountPinatubonearManila,MountStHelen'sintheRockiesandElChichóninMexicoaboutadecadeago,nottomentionworld-shakingblastslikeKrakatoaintheSundaStraitsin1883.DButvolcanoesarenotverypredictable.Thatisbecausegeologicaltimeisnotlikehumantime.Duringquietperiods,volcanoescapthemselveswiththeirownlavabyformingapowerfulconefromthemoltenrockssloppingovertherimofthecrater;laterthelavacoolsslowlyintoahuge,hard,stableplugwhichblocksanyfurthereruptionuntilthepressurebelowbecomesirresistible.Inthecase