万物简史英文版_比尔·布莱森-第43章

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ut　apart　from　yellowstone　they　are　alloceanic。　no　one　has　the　faintest　idea　how　or　why　yellowstone鈥檚　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鈥檛　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鈥檚　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鈥檚　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鈥檚　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鈥檚　farmers　long　agodiscovered。　in　another　two　million　years；　geologists　like　to　joke；　yellowstone　will　beproducing　french　fries　for　mcdonald鈥檚；　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）鈥攏early　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鈥檚　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鈥檚　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　鈥渧olcanic　winter鈥潯�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鈥檚　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鈥檚wading　pool。　by　1984；　the　whole　central　region　of　the　park鈥攕everal　dozen　square　miles鈥攚as　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鈥攁　restless　magma　chamber。

yellowstone　wasn鈥檛　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鈥檚eruptions　averaged　one　massive　blow　every　600；000　years。　the　last　one；　interestingly　enough；was　630；000　years　ago。　yellowstone；　it　appears；　is　due。

鈥渋t　may　not　feel　like　it；　but　you鈥檙e　standing　on　the　largest　active　volcano　in　the　world；鈥潯�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。　鈥渁nd　i鈥檝e　got　the　best　place　inthe　world　to　do　it；鈥潯�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　whatever　it　is　a　park　geologist　does。　the　first　assignment　today　is　to　give　anintroductory　talk　to　a　new　crop　of　tour　guides。

yellowstone；　i　hardly　need　point　out；　is　sensationally　beautiful；　with　plump；　statelymountains；　bison…specked　meadows；　tumbling　streams；　a　sky…blue　lake；　wildlife　beyondcounting。　鈥渋t　really　doesn鈥檛　get　any　better　than　this　if　you鈥檙e　a　geologist；鈥潯�doss　says。　鈥測ou鈥檝egot　rocks　up　at　beartooth　gap　that　are　nearly　three　billion　years　old鈥攖hree…quarters　of　theway　back　to　earth鈥檚　beginning鈥攁nd　then　you鈥檝e　got　mineral　springs　here鈥濃�攈e　points　at　thesulfurous　hot　springs　from　which　mammoth　takes　its　title鈥斺�渨here　you　can　see　rocks　as　theyare　being　born。　and　in　between　there鈥檚　everything　you　could　possibly　imagine。　i鈥檝e　neverbeen　any　place　where　geology　is　more　evident鈥攐r　prettier。鈥�

鈥渟o　you　like　it？鈥潯�i　say。

鈥渙h；　no；　i　love　it；鈥潯�he　answers　with　profound　sincerity。　鈥渋　mean　i　really　love　it　here。　thewinters　are　tough　and　the　pay鈥檚　not　too　hot；　but　when　it鈥檚　good；　it鈥檚　just鈥斺��

he　interrupted　himself　to　point　out　a　distant　gap　in　a　range　of　mountains　to　the　west；　whichhad　just　e　into　view　over　a　rise。　the　mountains；　he　told　me；　were　known　as　the　gallatins。

鈥渢hat　gap　is　sixty　or　maybe　seventy　miles　across。　for　a　long　time　nobody　could　understandwhy　that　gap　was　there；　and　then　bob　christiansen　realized　that　it　had　to　be　because　themountains　were　just　blown　away。　when　you鈥檝e　got　sixty　miles　of　mountains　just　obliterated；you　know　you鈥檙e　dealing　with　something　pretty　potent。　it　took　christiansen　six　years　to　figureit　all　out。鈥�

i　asked　him　what　caused　yellowstone　to　blow　when　it　did。

鈥渄on鈥檛　know。　nobody　knows。　volcanoes　are　strange　things。　we　really　don鈥檛　understandthem　at　all。　vesuvius；　in　italy；　was　active　for　three　hundred　years　until　an　eruption　in　1944and　then　it　just　stopped。　it鈥檚　been　silent　ever　since。　some　volcanologists　think　that　it　isrecharging　in　a　big　way；　which　is　a　little　worrying　because　two　million　people　live　on　oraround　it。　but　nobody　knows。鈥�

鈥渁nd　how　much　warning　would　you　get　if　yellowstone　was　going　to　go？鈥�

he　shrugged。　鈥渘obody　was　around　the　last　time　it　blew；　so　nobody　knows　what　thewarning　signs　are。　probably　you　would　have　swarms　of　earthquakes　and　some　surface　upliftand　possibly　some　changes　in　the　patterns　of　behavior　of　the　geysers　and　steam　vents；　butnobody　really　knows。鈥�

鈥渟o　it　could　just　blow　without　warning？鈥�

he　nodded　thoughtfully。　the　trouble；　he　explained；　is　that　nearly　all　the　things　that　wouldconstitute　warning　signs　already　exist　in　some　measure　at　yellowstone。　鈥渆arthquakes　aregenerally　a　precursor　of　volcanic　eruptions；　but　the　park　already　has　lots　of　earthquakes鈥�1；260　of　them　last　year。　most　of　them　are　too　small　to　be　felt；　but　they　are　earthquakesnonetheless。鈥�

a　change　in　the　pattern　of　geyser　eruptions　might　also　be　taken　as　a　clue；　he　said；　but　thesetoo　vary　unpredictably。　once　the　most　famous　geyser　in　the　park　was　excelsior　geyser。　itused　to　erupt　regularly　and　spectacularly　to　heights　of　three　hundred　feet；　but　in　1888　it　juststopped。　then　in　1985　it　erupted　again；　though　only　to　a　height　of　eighty　feet。　steamboatgeyser　is　the　biggest　geyser　in　the　world　when　it　blows；　shooting　water　four　hundred　feet　intothe　air；　but　the　intervals　between　its　eruptions　have　ranged　from　as　little　as　four　days　to　almostfifty　years。　鈥渋f　it　blew　today　and　again　next　week；　that　wouldn鈥檛　tell　us　anything　at　all　aboutwhat　it　might　do　the　following　week　or　the　week　after　or　twenty　years　from　now；鈥潯�doss　says。

鈥渢he　whole　park　is　so　volatile　that　it鈥檚　essentially　impossible　to　draw　conclusions　from　almostanything　that　happens。鈥�

evacuating　yellowstone　would　never　be　easy。　the　park　gets　some　three　million　visitors　ayear；　mostly　in　the　three　peak　months　of　summer。　the　park鈥檚　roads　are　paratively　few　andthey　are　kept　intentionally　narrow；　partly　to　slow　traffic；　partly　to　preserve　an　air　ofpicturesqueness；　and　partly　because　of　topographical　constraints。　at　the　height　of　summer；　itcan　easily　take　half　a　day　to　cross　the　park　and　hours　to　get　anywhere　within　it。　鈥渨heneverpeople　see　animals；　they　just　stop；　wherever　they　are；鈥潯�doss　says。　鈥渨e　get　bear　jams。　we　getbison　jams。　we　get　wolf　jams。鈥�

in　the　autumn　of　2000；　representatives　from　the　u。s。　geological　survey　and　national　parkservice；　along　with　some　academics；　met　and　formed　something　called　the　yellowstonevolcanic　observatory。　four　such　bodies　were　in　existence　already鈥攊n　hawaii；　california；alaska；　and　washington鈥攂ut　oddly　none　in　the　largest　volcanic　zone　in　the　world。　the　yvois　not　actually　a　thing；　but　more　an　idea鈥攁n　agreement　to　coordinate　efforts　at　studying　andanalyzing　the　park鈥檚　diverse　geology。　one　of　their　first　tasks；　doss　told　me；　was　to　draw　up　an鈥渆arthquake　and　volcano　hazards　plan鈥濃�攁　plan　of　action　in　t