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1、11111111TheSpectacularEruptionofMountSt.HelensATheeruptioninMay1980ofMountSt.Helens,WashingtonState,astoundedtheworldwithitsviolence.Agiganticexplosiontoremuchofthevolcano'ssummittofragments;theenergyreleasedwasequaltothatof500ofthenuclearbombsthatdestroyedHiroshimain1945.B
2、TheeventoccurredalongtheboundaryoftwoofthemovingplatesthatmakeuptheEarth'scrust.TheymeetatthejunctionoftheNorthAmericancontinentandthePacificOcean.OneedgeofthecontinentalNorthAmericanplateover-ridestheoceanicJuandeFucamicro-plate,producingthevolcanicCascaderangethatincludes
3、MountsBaker,RainierandHood,andLassenPeakaswellasMountSt.Helens.CUntilMountSt.Helensbegantostir,onlyMountBakerandLassenPeakhadshownsignsoflifeduringthe20thcentury.AccordingtogeologicalevidencefoundbytheUnitedStatesGeologicalSurvey,therehadbeentwomajoreruptionsofMountSt.Helen
4、sintherecent(geologicallyspeaking)past:around1900B.C.,andaboutA.D.1500.SincethearrivalofEuropeansintheregion,ithadexperiencedasingleperiodofspasmodicactivity,between1831and1857.Then,formorethanacentury,MountSt.Helenslaydormant.DBy1979,theGeologicalSurvey,alertedbysignsofren
5、ewedactivity,hadbeenmonitoringthevolcanofor18months.Itwarnedthelocalpopulationagainstbeingdeceivedbythemountain'soutwardcalm,andforecastthataneruptionwouldtakeplacebeforetheendofthecentury.Theinhabitantsoftheareadidnothavetowaitthatlong.OnMarch27,1980,afewcloudsofsmokeforme
6、dabovethesummit,andslighttremorswerefelt.Onthe28th,largeranddarkerclouds,.consistingofgasandashes,.emergedandclimbedashighas20,000feet.InAprilaslightlullensued,butthevolcanologistsremainedpessimistic.The,inearlyMay,thenorthernflankofthemountainbulged,andthesummitroseby500fe
7、et.EStepsweretakentoevacuatethepopulation.Most-campers,hikers,timbercutterslefttheslopesofthemountain.Eighty-four-year-oldHarryTruman,aholidaylodgeownerwhohadlivedthereformorethan50years,refusedtobeevacuated,inspiteofofficialandpublic,includinganentireclassofschoolchildren,
8、wrotetohim,begginghimtoleave.Heneverdid.FOnMay18,at8.32inthemorning,MountSt.Helens
