按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
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s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enters。 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hile 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