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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。
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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