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1、NotesonvolcanoesDefinitionofavolcano:anopeninginthesurfaceoftheEarththroughwhichmagmaerupts(aslavaorpyroclasticfragments).Avolcanoisunderlainbyamagmachamber,areservoirthatstoresmagmabeforeitrisesasaneruption.Notallvolcanoeseruptlava;someeruptrockandmagmafragments(pyroclasts);someeruptg
2、ases.Acommonmisconceptionisthatmagmaexistseverywherebeneaththesurface,andseepsoutwhereverthesurfaceiscracked,producingavolcano.Thisisnottrue.Experimentshaveshownthatifasignificant(2%)ofmagmaexistsinthemantle,itwillrisesduetobuoyancyandwilleventuallyreachthesurfaceasavolcano.Allvolcanoe
3、sareunderlainbyasourceofmagma,andallsignificantsourcesofmagmaproducevolcanoes.Thereforethepresenceofavolcanoimpliesmelting,andtheABSENCEofvolcanoesimpliesNOMELTING.TodisprovethemisconceptionyouneedonlyexaminethedistributionofvolcanoesonEarth:1.Mostvolcanoesoccuralongtheedgesofcontinent
4、s,butnotallcontinentaledgeshavevolcanoes.Forexample,theeasternmarginofNorthAmericadoesnothavevolcanoes.2.Somevolcanoesformislandchainsinthemiddleofthesea,forexampletheAleutianchainofAlaska;3.Therearesomeisolatedvolcanoesinthemiddleofcontinentsoroceans,suchastheHawaiianislands(inthemidd
5、leoftheocean),Yellowstonevolcano(inthemiddleofacontinent),orvolcanoesineasternAfrica(i.e.,Mt.Kilimanjaro).4.Thereisacontinuousstringofsubmarinevolcanoesinthemiddleoftheoceans.Thisdistributioncanbeinterpretedintermsofplatetectonics.Ifyoulookatthedistributionoftheworld’svolcanoesonaplate
6、-tectonicmapyoucansee:1.Thevolcanoesattheedgesofcontinentsoccurabovesubductionzones.Continentalmarginswithoutsubductionzonesdonothavevolcanoes;2.Volcanicislandchainsinthemiddleoftheseaoccurabovesubductionzonesaswell.3.TheunderseavolcanicchainscorrespondtotheMORs.4.Someofthevolcanoesint
7、hemiddleofcontinentsareduetocontinentalrifts(again,aplateboundary)whereacontinentisbeingtornapart;forexample,theeastAfricanvolcanoes.5.Thevolcanoesthatoccurinthemiddleofplates,suchasHawaii,arecalledhotspotvolcanoes,andarecausedbyanomalouslywarmmantlebeneaththem.Itisimportanttonoticet
