A Short History of Nearly Everything- 第47部分

by　coincidence　just　at　this　time　nasa　decided　to　test　some　new　high…altitude　cameras　bytaking　photographs　of　yellowstone；　copies　of　which　some　thoughtful　official　passed　on　to　thepark　authorities　on　the　assumption　that　they　might　make　a　nice　blow…up　for　one　of　thevisitors’　centers。　as　soon　as　christiansen　saw　the　photos　he　realized　why　he　had　failed　to　spotthe　caldera：　virtually　the　whole　park—2。2　million　acres—was　caldera。　the　explosion　had　lefta　crater　more　than　forty　miles　across—much　too　huge　to　be　perceived　from　anywhere　atground　level。　at　some　time　in　the　past　yellowstone　must　have　blown　up　with　a　violence　farbeyond　the　scale　of　anything　known　to　humans。

yellowstone；　it　turns　out；　is　a　supervolcano。　it　sits　on　top　of　an　enormous　hot　spot；　areservoir　of　molten　rock　that　rises　from　at　least　125　miles　down　in　the　earth。　the　heat　fromthe　hot　spot　is　what　powers　all　of　yellowstone’s　vents；　geysers；　hot　springs；　and　popping　mudpots。　beneath　the　surface　is　a　magma　chamber　that　is　about　forty…five　miles　across—roughlythe　same　dimensions　as　the　park—and　about　eight　miles　thick　at　its　thickest　point。　imagine　apile　of　tnt　about　the　size　of　rhode　island　and　reaching　eight　miles　into　the　sky；　to　about　theheight　of　the　highest　cirrus　clouds；　and　you　have　some　idea　of　what　visitors　to　yellowstoneare　shuffling　around　on　top　of。　the　pressure　that　such　a　pool　of　magma　exerts　on　the　crustabove　has　lifted　yellowstone　and　about　three　hundred　miles　of　surrounding　territory　about1；700　feet　higher　than　they　would　otherwise　be。　if　it　blew；　the　cataclysm　is　pretty　well　beyondimagining。　according　to　professor　bill　mcguire　of　university　college　london；　“youwouldn’t　be　able　to　get　within　a　thousand　kilometers　of　it”　while　it　was　erupting。　theconsequences　that　followed　would　be　even　worse。

superplumes　of　the　type　on　which　yellowstone　sits　are　rather　like　martini　glasses—thin　onthe　way　up；　but　spreading　out　as　they　near　the　surface　to　create　vast　bowls　of　unstable　magma。

some　of　these　bowls　can　be　up　to　1；200　miles　across。　according　to　theories；　they　don’talways　erupt　explosively　but　sometimes　burst　forth　in　a　vast；　continuous　outpouring—aflood—of　molten　rock；　such　as　with　the　deccan　traps　in　india　sixty…five　million　years　ago。

（trap　in　this　context　es　from　a　swedish　word　for　a　type　of　lava；　deccan　is　simply　anarea。）　these　covered　an　area　of　200；000　square　miles　and　probably　contributed　to　the　demiseof　the　dinosaurs—they　certainly　didn’t　help—with　their　noxious　outgassings。　superplumesmay　also　be　responsible　for　the　rifts　that　cause　continents　to　break　up。

such　plumes　are　not　all　that　rare。　there　are　about　thirty　active　ones　on　the　earth　at　themoment；　and　they　are　responsible　for　many　of　the　world’s　best…known　islands　and　islandchains—iceland；　hawaii；　the　azores；　canaries；　and　galápagos　archipelagos；　little　pitcairn　inthe　middle　of　the　south　pacific；　and　many　others—but　apart　from　yellowstone　they　are　alloceanic。　no　one　has　the　faintest　idea　how　or　why　yellowstone’s　ended　up　beneath　acontinental　plate。　only　two　things　are　certain：　that　the　crust　at　yellowstone　is　thin　and　that　theworld　beneath　it　is　hot。　but　whether　the　crust　is　thin　because　of　the　hot　spot　or　whether　the　hotspot　is　there　because　the　crust　is　thin　is　a　matter　of　heated　（as　it　were）　debate。　the　continentalnature　of　the　crust　makes　a　huge　difference　to　its　eruptions。　where　the　other　supervolcanoestend　to　bubble　away　steadily　and　in　a　paratively　benign　fashion；　yellowstone　blowsexplosively。　it　doesn’t　happen　often；　but　when　it　does　you　want　to　stand　well　back。

since　its　first　known　eruption　16。5　million　years　ago；　it　has　blown　up　about　a　hundredtimes；　but　the　most　recent　three　eruptions　are　the　ones　that　get　written　about。　the　last　eruptionwas　a　thousand　times　greater　than　that　of　mount　st。　helens；　the　one　before　that　was　280　timesbigger；　and　the　one　before　was　so　big　that　nobody　knows　exactly　how　big　it　was。　it　was　atleast　twenty…five　hundred　times　greater　than　st。　helens；　but　perhaps　eight　thousand　timesmore　monstrous。

we　have　absolutely　nothing　to　pare　it　to。　the　biggest　blast　in　recent　times　was　that　ofkrakatau　in　indonesia　in　august　1883；　which　made　a　bang　that　reverberated　around　the　worldfor　nine　days；　and　made　water　slosh　as　far　away　as　the　english　channel。　but　if　you　imaginethe　volume　of　ejected　material　from　krakatau　as　being　about　the　size　of　a　golf　ball；　then　thebiggest　of　the　yellowstone　blasts　would　be　the　size　of　a　sphere　you　could　just　about　hidebehind。　on　this　scale；　mount　st。　helens’s　would　be　no　more　than　a　pea。

the　yellowstone　eruption　of　two　million　years　ago　put　out　enough　ash　to　bury　new　yorkstate　to　a　depth　of　sixty…seven　feet　or　california　to　a　depth　of　twenty。　this　was　the　ash　thatmade　mike　voorhies’s　fossil　beds　in　eastern　nebraska。　that　blast　occurred　in　what　is　nowidaho；　but　over　millions　of　years；　at　a　rate　of　about　one　inch　a　year；　the　earth’s　crust　hastraveled　over　it；　so　that　today　it　is　directly　under　northwest　wyoming。　（the　hot　spot　itselfstays　in　one　place；　like　an　acetylene　torch　aimed　at　a　ceiling。）　in　its　wake　it　leaves　the　sort　ofrich　volcanic　plains　that　are　ideal　for　growing　potatoes；　as　idaho’s　farmers　long　agodiscovered。　in　another　two　million　years；　geologists　like　to　joke；　yellowstone　will　beproducing　french　fries　for　mcdonald’s；　and　the　people　of　billings；　montana；　will　be　steppingaround　geysers。

the　ash　fall　from　the　last　yellowstone　eruption　covered　all　or　parts　of　nineteen　westernstates　（plus　parts　of　canada　and　mexico）—nearly　the　whole　of　the　united　states　west　of　themississippi。　this；　bear　in　mind；　is　the　breadbasket　of　america；　an　area　that　produces　roughlyhalf　the　world’s　cereals。　and　ash；　it　is　worth　remembering；　is　not　like　a　big　snowfall　that　willmelt　in　the　spring。　if　you　wanted　to　grow　crops　again；　you　would　have　to　find　some　place　toput　all　the　ash。　it　took　thousands　of　workers　eight　months　to　clear　1。8　billion　tons　of　debrisfrom　the　sixteen　acres　of　the　world　trade　center　site　in　new　york。　imagine　what　it　wouldtake　to　clear　kansas。

and　that’s　not　even　to　consider　the　climatic　consequences。　the　last　supervolcano　eruptionon　earth　was　at　toba；　in　northern　sumatra；　seventy…four　thousand　years　ago。　no　one　knowsquite　how　big　it　was　other　than　that　it　was　a　whopper。　greenland　ice　cores　show　that　the　tobablast　was　followed　by　at　least　six　years　of　“volcanic　winter”　and　goodness　knows　how　manypoor　growing　seasons　after　that。　the　event；　it　is　thought；　may　have　carried　humans　right　to　thebrink　of　extinction；　reducing　the　global　population　to　no　more　than　a　few　thousandindividuals。　that　means　that　all　modern　humans　arose　from　a　very　small　population　base；which　would　explain　our　lack　of　genetic　diversity。　at　all　events；　there　is　some　evidence　tosuggest　that　for　the　next　twenty　thousand　years　the　total　number　of　people　on　earth　was　nevermore　than　a　few　thousand　at　any　time。　that　is；　needless　to　say；　a　long　time　to　recover　from　asingle　volcanic　blast。

all　this　was　hypothetically　interesting　until　1973；　when　an　odd　occurrence　made　itsuddenly　momentous：　water　in　yellowstone　lake；　in　the　heart　of　the　park；　began　to　run　overthe　banks　at　the　lake’s　southern　end；　flooding　a　meadow；　while　at　the　opposite　end　of　the　lakethe　water　mysteriously　flowed　away。　geologists　did　a　hasty　survey　and　discovered　that　a　largearea　of　the　park　had　developed　an　ominous　bulge。　this　was　lifting　up　one　end　of　the　lake　andcausing　the　water　to　run　out　at　the　other；　as　would　happen　if　you　lifted　one　side　of　a　child’swading　pool。　by　1984；　the　whole　central　region　of　the　park—several　dozen　square　miles—was　more　than　three　feet　higher　than　it　had　been　in　1924；　when　the　park　was　last　formallysurveyed。　then　in　1985；　the　whole　of　the　central　part　of　the　park　subsided　by　eight　inches。　itnow　seems　to　be　swelling　again。

the　geologists　realized　that　only　one　thing　could　cause　this—a　restless　magma　chamber。

yellowstone　wasn’t　the　site　of　an　ancient　supervolcano；　it　was　the　site　of　an　active　one。　it　wasalso　at　about　this　time　that　they　were　able　to　work　out　that　the　cycle　of　yellowstone’seruptions　averaged　one　massive　blow　every　600；000　years。　the　last　one；　interestingly　enough；was　630；000　years　ago。　yellowstone；　it　appears；　is　due。

“it　may　not　feel　like　it；　but　you’re　standing　on　the　largest　active　volcano　in　the　world；”　pauldoss；　yellowstone　national　park　geologist；　told　me　soon　after　climbing　off　an　enormousharley…davidson　motorcycle　and　shaking　hands　when　we　met　at　the　park　headquarters　atmammoth　hot　springs　early　on　a　lovely　morning　in　june。　a　native　of　indiana；　doss　is　anamiable；　soft…spoken；　extremely　thoughtful　man　who　looks　nothing　like　a　national　parkservice　employee。　he　has　a　graying　beard　and　hair　tied　back　in　a　long　ponytail。　a　smallsapphire　stud　graces　one　ear。　a　slight　paunch　strains　against　his　crisp　park　service　uniform。

he　looks　more　like　a　blues　musician　than　a　government　employee。　in　fact；　he　is　a　bluesmusician　（harmonica）。　but　he　sure　knows　and　loves　geology。　“and　i’ve　got　the　best　place　inthe　world　to　do　it；”　he　says　as　we　set　off　in　a　bouncy；　battered　four…wheel…drive　vehicle　in　thegeneral　direction　of　old　faithful。　he　has　agreed　to　let　me　acpany　him　for　a　day　as　he　goesabout　doing