万物简史英文版_比尔·布莱森- 第58节

but　to　return　to　washington。　mount　st。　helens　lost　thirteen　hundred　feet　of　peak；　and　230square　miles　of　forest　were　devastated。　enough　trees　to　build　150；000　homes　（or　300；000　insome　reports）　were　blown　away。　the　damage　was　placed　at　2。7　billion。　a　giant　column　ofsmoke　and　ash　rose　to　a　height　of　sixty　thousand　feet　in　less　than　ten　minutes。　an　airlinersome　thirty　miles　away　reported　being　pelted　with　rocks。

ninety　　minutes　　after　　the　　blast；　ash　　began　to　rain　down　on　yakima；　washington；　amunity　of　fifty　thousand　people　about　eighty　miles　away。　as　you　would　expect；　the　ashturned　day　to　night　and　got　into　everything；　clogging　motors；　generators；　and　electricalswitching　equipment；　choking　pedestrians；　blocking　filtration　systems；　and　generally　bringingthings　to　a　halt。　the　airport　shut　down　and　highways　in　and　out　of　the　city　were　closed。

all　this　was　happening；　you　will　note；　just　downwind　of　a　volcano　that　had　been　rumblingmenacingly　for　two　months。　yet　yakima　had　no　volcano　emergency　procedures。　the　city鈥檚emergency　broadcast　system；　which　was　supposed　to　swing　into　action　during　a　crisis；　did　notgo　on　the　air　because　鈥渢he　sunday…morning　staff　did　not　know　how　to　operate　the　equipment。鈥�

for　three　days；　yakima　was　paralyzed　and　cut　off　from　the　world；　its　airport　closed；　itsapproach　roads　impassable。　altogether　the　city　received　just　five…eighths　of　an　inch　of　ashafter　the　eruption　of　mount　st。　helens。　now　bear　that　in　mind；　please；　as　we　consider　what　ayellowstone　blast　would　do。

锛凤挤w。xi锛�os锛╱otxt。锛ｏ集锛�



15　　　　DANGEROUS　BEAUTY


in　the　1960s；　while　studying　the　volcanic　history　of　yellowstone　national　park；　bobchristiansen　of　the　united　states　geological　survey　became　puzzled　about　something　that；oddly；　had　not　troubled　anyone　before：　he　couldn鈥檛　find　the　park鈥檚　volcano。　it　had　been　knownfor　a　long　time　that　yellowstone　was　volcanic　in　nature鈥攖hat鈥檚　what　accounted　for　all　itsgeysers　and　other　steamy　features鈥攁nd　the　one　thing　about　volcanoes　is　that　they　aregenerally　pretty　conspicuous。　but　christiansen　couldn鈥檛　find　the　yellowstone　volcanoanywhere。　in　particular　what　he　couldn鈥檛　find　was　a　structure　known　as　a　caldera。

most　of　us；　when　we　think　of　volcanoes；　think　of　the　classic　cone　shapes　of　a　fuji　orkilimanjaro；　which　are　created　when　erupting　magma　accumulates　in　a　symmetrical　mound。

these　can　form　remarkably　quickly。　in　1943；　at　par铆cutin　in　mexico；　a　farmer　was　startled　tosee　smoke　rising　from　a　patch　on　his　land。　in　one　week　he　was　the　bemused　owner　of　a　conefive　hundred　feet　high。　within　two　years　it　had　topped　out　at　almost　fourteen　hundred　feet　andwas　more　than　half　a　mile　across。　altogether　there　are　some　ten　thousand　of　these　intrusivelyvisible　volcanoes　on　earth；　all　but　a　few　hundred　of　them　extinct。　but　there　is　a　second；　lesscelebrated　type　of　volcano　that　doesn鈥檛　involve　mountain　building。　these　are　volcanoes　soexplosive　that　they　burst　open　in　a　single　mighty　rupture；　leaving　behind　a　vast　subsided　pit；the　caldera　（from　a　latin　word　for　cauldron）。　yellowstone　obviously　was　of　this　second　type；but　christiansen　couldn鈥檛　find　the　caldera　anywhere。

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鈥櫋�centers。　as　soon　as　christiansen　saw　the　photos　he　realized　why　he　had　failed　to　spotthe　caldera：　virtually　the　whole　park鈥�2。2　million　acres鈥攚as　caldera。　the　explosion　had　lefta　crater　more　than　forty　miles　across鈥攎uch　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鈥檚　vents；　geysers；　hot　springs；　and　popping　mudpots。　beneath　the　surface　is　a　magma　chamber　that　is　about　forty…five　miles　across鈥攔oughlythe　same　dimensions　as　the　park鈥攁nd　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；　鈥測ouwouldn鈥檛　be　able　to　get　within　a　thousand　kilometers　of　it鈥潯�while　it　was　erupting。　theconsequences　that　followed　would　be　even　worse。

superplumes　of　the　type　on　which　yellowstone　sits　are　rather　like　martini　glasses鈥攖hin　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鈥檛always　erupt　explosively　but　sometimes　burst　forth　in　a　vast；　continuous　outpouring鈥攁flood鈥攐f　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鈥攖hey　certainly　didn鈥檛　help鈥攚ith　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鈥檚　best…known　islands　and　islandchains鈥攊celand；　hawaii；　the　azores；　canaries；　and　gal谩pagos　archipelagos；　little　pitcairn　inthe　middle　of　the　south　pacific；　and　many　others鈥攂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