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TheSpectacularEruptionofMountSt.HelensATheeruptioninMay1980ofMountSt.Helens,WashingtonState,astoundedtheworldwithitsviolence.Agiganticexplosiontoremuchofthevolcano'ssummittofragments;theenergyreleasedwasequaltothatof500ofthenuclearbombsthatdestroyedHiroshimain1945.BTheeventoccurredalongtheboundaryoftwoofthemovingplatesthatmakeuptheEarth'scrust.TheymeetatthejunctionoftheNorthAmericancontinentandthePacificOcean.OneedgeofthecontinentalNorthAmericanplateover-ridestheoceanicJuandeFucamicro-plate,producingthevolcanicCascaderangethatincludesMountsBaker,RainierandHood,andLassenPeakaswellasMountSt.Helens.CUntilMountSt.Helensbegantostir,onlyMountBakerandLassenPeakhadshownsignsoflifeduringthe20thcentury.AccordingtogeologicalevidencefoundbytheUnitedStatesGeologicalSurvey,therehadbeentwomajoreruptionsofMountSt.Helensintherecent(geologicallyspeaking)past:around1900B.C.,andaboutA.D.1500.SincethearrivalofEuropeansintheregion,ithadexperiencedasingleperiodofspasmodicactivity,between1831and1857.Then,formorethanacentury,MountSt.Helenslaydormant.DBy1979,theGeologicalSurvey,alertedbysignsofrenewedactivity,hadbeenmonitoringthevolcanofor18months.Itwarnedthelocalpopulationagainstbeingdeceivedbythemountain'soutwardcalm,andforecastthataneruptionwouldtakeplacebeforetheendofthecentury.Theinhabitantsoftheareadidnothavetowaitthatlong.OnMarch27,1980,afewcloudsofsmokeformedabovethesummit,andslighttremorswerefelt.Onthe28th,largeranddarkerclouds,.consistingofgasandashes,.emergedandclimbedashighas20,000feet.InAprilaslightlullensued,butthevolcanologistsremainedpessimistic.The,inearlyMay,thenorthernflankofthemountainbulged,andthesummitroseby500feet.EStepsweretakentoevacuatethepopulation.Most-campers,hikers,timbercutterslefttheslopesofthemountain.Eighty-four-year-oldHarryTruman,aholidaylodgeownerwhohadlivedthereformorethan50years,refusedtobeevacuated,inspiteofofficialandpublic,includinganentireclassofschoolchildren,wrotetohim,begginghimtoleave.Heneverdid.FOnMay18,at8.32inthemorning,MountSt.Helensblewitstop.literally.Suddenly,itwas1300feetshorterthanithadbeenbeforeitsgrowthhadbegun.Overhalfacubicmileofrockhaddisintegrated.Atthesamemoment,anearthquakewithanintensityof5ontheRichterscalewasrecorded.Ittriggeredanavalancheofsnowandice.mixedwithhotrock-theentirenorthfaceofthemountainhadfallenaway.Awaveofscorchingvolcanicgasandrockfragmentsshothorizontallyfromthevolcano'srivenflank,ataninescapable200milesperhour.Astheslidingiceandsnowmelted,ittouchedoffdevastatingtorrentsofmudanddebris,whichdestroyedalllifeintheirpath.Pulverised,whichdestroyedalllifeintheirpath.Pulverisedrockclimbedasadustcloudintotheatmosphere.Finally,viscouslava,accompaniedbyburningcloudsofashandgas,welledoutofvolcano'snewcrater,andfromlesserventsandcracksinitsflanks.GAfterwards,scientistswereabletoanalysethesequenceofevents.First,magmamoltenrock-attemperaturesabove2000oF.hadsurgedintothevolcanofromtheEarth'smantle.Thebuild-upwasaccompaniedbyanaccumulationofgas,whichincreasedasthemassofmagmagrew.Itwasthepressureinsidethemountainthatmadeitswell.Next,theriseingaspressurecausedaviolentdecompression.Whichejectedtheshatteredsummitlikeacorkfromashakensodabottle.Withthesummitgone,themoltenrockwithinwasreleasedinajetofgasandfragmentedmagma,andlavawelledfromthecrater.HTheeffectsoftheMountSt.Helenseruptionwerecatastrophic.Almostallthetreesofthesurroundingforest,mainlyDouglasfirs.wereflattened.andtheirbranchesandbarkrippedoffbytheshockwaveoftheexplosion.Ashandmudspreadovernearly200squaremilesofcountry.Allthetownsandsettlementsintheareaweresmotheredinanevencoatingofash.VolcanicashsilteduptheColumbiaRiver35milesaway,reducingthedebristhataccumulatedatthefootofthevolcanoreachedadepth.inplaces,of200feet.ITheeruptionofMountSt.Helenswasoneofthemostcloselyobservedandanalysedinhistory.Becausegeologistshadbeenexpectingtheevent,theywereabletoamassvastamountsoftechnicaldatawhenithappened.Studyofatmosphericparticlesformedasaresultoftheexplosionshowedthatdropletsofsulphuricacid,actingasascreenbetweentheSunandtheEarth'ssurface,causedadistinctdropintemperature.ThereisnodoubtthattheactivityofMountSt.Helensandothervolcanoessince1980hasinfluencedourclimate.Evenso,ithasbeencalculatedthatthequantityofdustejectedbyMountSt.Helens-aquarterofacubicmile-wasnegligibleincomparisonwiththatthrownoutbyearliereruptions,suchasthatofMountKatmaiinAlaskain1912(threecubicmiles).Thevolcanoisstillactive.Lavadomeshaveformedinsidethenewcrater,andhaveperiodicallyburst.ThethreatofMountStHelensliveson..Questions1and2Answerquestions1and2bywritingtheappropriateletterA-Iinboxes1and2onyouranswersheet.ExampleAnswerWhichparagraphcomparestheeruptiontotheenergyAreleasedbynuclearbomb?1.Whichparagraphdescribestheevacuationofthemountain?2.WhichparagraphdescribesthemomentoftheexplosionofMountSt.Helens?Questions3and43.WhatarethedatesoftheTWOmajoreruptionsofMountSt.Helensbefore1980?WriteTWOdatesinbox3onyouranswersheet.4Howdoscientistsknowthatthevolcanoexplodedaroundthetwodatesabove?UsingNOMORETHANTHREEWORDS,writeyouranswerinbox4onyouranswersheetQuestio