WhatIsAVolcano?Theword"volcano"comesfromthelittleislandofVulcanointheMediterraneanSeaoffSicily.Centuriesago,thepeoplelivinginthisareabelievedthatVulcanowasthechimneyoftheforgeofVulcan--theblacksmithoftheRomangods.TheythoughtthatthehotlavafragmentsandcloudsofdusteruptingformVulcanocamefromVulcan'sforgeashebeatoutthunderboltsforJupiter,kingofthegods,andweaponsforMars,thegodofwar.InPolynesiathepeopleattributederuptiveactivitytothebeautifulbutwrathfulPele,GoddessofVolcanoes,whenevershewasangryorspiteful.Todayweknowthatvolcaniceruptionsarenotsuper-naturalbutcanbestudiedandinterpretedbyscientists.TheNatureofVolcanoesVolcanoesaremountains,buttheyareverydifferentfromothermountains;theyarenotformedbyfoldingandcrumplingorbyupliftanderosion.Instead,volcanoesarebuiltbytheaccumulationoftheirowneruptiveproducts--lava,bombs(crustedoverlavablobs),ashflows,andtephra(airborneashanddust).AvolcanoismostcommonlyaconicalhillormountainbuiltaroundaventthatconnectswithreservoirsofmoltenrockbelowthesurfaceoftheEarth.Thetermvolcanoalsoreferstotheopeningorventthroughwhichthemoltenrockandassociatedgasesareexpelled.Drivenbybuoyancyandgaspressurethemoltenrock,whichislighterthanthesurroundingsolidrock,forcesitswayupwardandmyultimatelybreakthroughzonesofweaknessesintheEarth'scrust.Ifso,aneruptionbegins,andthemoltenrockmaypourfromtheventasnonexplosivelavaflows,oritmayshootviolentlyintotheairasdensecloudsoflavafragments.Largerfragmentsfallbackaroundthevent,andaccumulations用心爱心专心offallbackfragmentsmaymovedownslopeasashflowsundertheforceofgravity.Someofthefinerejectedmaterialsmaybecarriedbythewindonlytofalltothegroundmanymilesaway.Thefinestashparticlesmaybeinjectedmilesintotheatmosphereandcarriedmanytimesaroundtheworldbystratosphericwindsbeforesettlingout.用心爱心专心
