雅思阅读真题.docx

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雅思阅读真题

TheSpectacularEruptionofMountSt.Helens

ATheeruptioninMay1980ofMountSt.Helens,WashingtonState,astoundedtheworldwith

itsviolence.Agiganticexplosiontoremuchofthevolcano'ssummittofragments;theenergy

releasedwasequaltothatof500ofthenuclearbombsthatdestroyedHiroshimain1945.

BTheeventoccurredalongtheboundaryoftwoofthemovingplatesthatmakeuptheEarth's

crust.TheymeetatthejunctionoftheNorthAmericancontinentandthePacificOcean.One

edgeofthecontinentalNorthAmericanplateover-ridestheoceanicJuandeFucamicro-plate,

producingthevolcanicCascaderangethatincludesMountsBaker,RainierandHood,andLassen

PeakaswellasMountSt.Helens.

CUntilMountSt.Helensbegantostir,onlyMountBakerandLassenPeakhadshownsignsof

lifeduringthe20thcentury.AccordingtogeologicalevidencefoundbytheUnitedStates

GeologicalSurvey,therehadbeentwomajoreruptionsofMountSt.Helensintherecent

（geologicallyspeaking）past:

around1900B.C.,andaboutA.D.1500.Sincethearrivalof

Europeansintheregion,ithadexperiencedasingleperiodofspasmodicactivity,between1831

and1857.Then,formorethanacentury,MountSt.Helenslaydormant.

DBy1979,theGeologicalSurvey,alertedbysignsofrenewedactivity,hadbeenmonitoring

thevolcanofor18months.Itwarnedthelocalpopulationagainstbeingdeceivedbythe

mountain'soutwardcalm,andforecastthataneruptionwouldtakeplacebeforetheendofthe

century.Theinhabitantsoftheareadidnothavetowaitthatlong.OnMarch27,1980,afew

cloudsofsmokeformedabovethesummit,andslighttremorswerefelt.Onthe28th,largerand

darkerclouds,.consistingofgasandashes,.emergedandclimbedashighas20,000feet.InApril

aslightlullensued,butthevolcanologistsremainedpessimistic.The,inearlyMay,thenorthern

flankofthemountainbulged,andthesummitroseby500feet.

EStepsweretakentoevacuatethepopulation.Most-campers,hikers,timbercuttersleftthe

slopesofthemountain.Eighty-four-year-oldHarryTruman,aholidaylodgeownerwhohadlived

thereformorethan50years,refusedtobeevacuated,inspiteofofficialandpublic,includingan

entireclassofschoolchildren,wrotetohim,begginghimtoleave.Heneverdid.

FOnMay18,at8.32inthemorning,MountSt.Helensblewitstop.literally.Suddenly,itwas

1300feetshorterthanithadbeenbeforeitsgrowthhadbegun.Overhalfacubicmileofrockhad

disintegrated.Atthesamemoment,anearthquakewithanintensityof5ontheRichterscalewas

recorded.Ittriggeredanavalancheofsnowandice.mixedwithhotrock-theentirenorthfaceof

themountainhadfallenaway.Awaveofscorchingvolcanicgasandrockfragmentsshot

horizontallyfromthevolcano'srivenflank,ataninescapable200milesperhour.Asthesliding

iceandsnowmelted,ittouchedoffdevastatingtorrentsofmudanddebris,whichdestroyedall

lifeintheirpath.Pulverised,whichdestroyedalllifeintheirpath.Pulverisedrockclimbedasa

dustcloudintotheatmosphere.Finally,viscouslava,accompaniedbyburningcloudsofashand

gas,welledoutofvolcano'snewcrater,andfromlesserventsandcracksinitsflanks.

GAfterwards,scientistswereabletoanalysethesequenceofevents.First,magmamolten

rock-attemperaturesabove2000oF.hadsurgedintothevolcanofromtheEarth'smantle.The

build-upwasaccompaniedbyanaccumulationofgas,whichincreasedasthemassofmagma

grew.Itwasthepressureinsidethemountainthatmadeitswell.Next,theriseingaspressure

causedaviolentdecompression.Whichejectedtheshatteredsummitlikeacorkfromashaken

sodabottle.Withthesummitgone,themoltenrockwithinwasreleasedinajetofgasand

fragmentedmagma,andlavawelledfromthecrater.

HTheeffectsoftheMountSt.Helenseruptionwerecatastrophic.Almostallthetreesofthe

surroundingforest,mainlyDouglasfirs.wereflattened.andtheirbranchesandbarkrippedoffby

theshockwaveoftheexplosion.Ashandmudspreadovernearly200squaremilesofcountry.All

thetownsandsettlementsintheareaweresmotheredinanevencoatingofash.Volcanicashsilted

uptheColumbiaRiver35milesaway,reducingthedebristhataccumulatedatthefootofthe

volcanoreachedadepth.inplaces,of200feet.

ITheeruptionofMountSt.Helenswasoneofthemostcloselyobservedandanalysedin

history.Becausegeologistshadbeenexpectingtheevent,theywereabletoamassvastamountsof

technicaldatawhenithappened.Studyofatmosphericparticlesformedasaresultofthe

explosionshowedthatdropletsofsulphuricacid,actingasascreenbetweentheSunandthe

Earth'ssurface,causedadistinctdropintemperature.ThereisnodoubtthattheactivityofMount

St.Helensandothervolcano