tes to refrigerators and keeping ourpasses pointing the right way； plays a vital role in keeping us alive。 space is full ofdangerous cosmic rays that in the absence of magnetic protection would tear through ourbodies； leaving much of our dna in useless tatters。 when the magnetic field is working；these rays are safely herded away from the earth鈥檚 surface and into two zones in near spacecalled the van allen belts。 they also interact with particles in the upper atmosphere to createthe bewitching veils of light known as the auroras。
a big part of the reason for our ignorance； interestingly enough； is that traditionally therehas been little effort to coordinate what鈥檚 happening on top of the earth with what鈥檚 going oninside。 according to shawna vogel： 鈥済eologists and geophysicists rarely go to the samemeetings or collaborate on the same problems。鈥�
perhaps nothing better demonstrates our inadequate grasp of the dynamics of the earth鈥檚interior than how badly we are caught out when it acts up； and it would be hard to e upwith a more salutary reminder of the limitations of our understanding than the eruption ofmount st。 helens in washington in 1980。
at that time； the lower forty…eight united states had not seen a volcanic eruption for oversixty…five years。 therefore the government volcanologists called in to monitor and forecast st。
helens鈥檚 behavior primarily had seen only hawaiian volcanoes in action； and they； it turnedout； were not the same thing at all。
st。 helens started its ominous rumblings on march 20。 within a week it was eruptingmagma； albeit in modest amounts； up to a hundred times a day； and being constantly shakenwith earthquakes。 people were evacuated to what was assumed to be a safe distance of eightmiles。 as the mountain鈥檚 rumblings grew st。 helens became a tourist attraction for the world。
newspapers gave daily reports on the best places to get a view。 television crews repeatedlyflew in helicopters to the summit； and people were even seen climbing over the mountain。 onone day； more than seventy copters and light aircraft circled the summit。 but as the dayspassed and the rumblings failed to develop into anything dramatic； people grew restless； andthe view became general that the volcano wasn鈥檛 going to blow after all。
on april 19 the northern flank of the mountain began to bulge conspicuously。 remarkably；no one in a position of responsibility saw that this strongly signaled a lateral blast。 theseismologists resolutely based their conclusions on the behavior of hawaiian volcanoes；which don鈥檛 blow out sideways。 almost the only person who believed that something reallybad might happen was jack hyde； a geology professor at a munity college in taa。 hepointed out that st。 helens didn鈥檛 have an open vent； as hawaiian volcanoes have； so anypressure building up inside was bound to be released dramatically and probablycatastrophically。 however； hyde was not part of the official team and his observationsattracted little notice。
we all know what happened next。 at 8：32 a。m。 on a sunday morning； may 18； the northside of the volcano collapsed； sending an enormous avalanche of dirt and rock rushing downthe mountain slope at 150 miles an hour。 it was the biggest landslide in human history andcarried enough material to bury the whole of manhattan to a depth of four hundred feet。 aminute later； its flank severely weakened； st。 helens exploded with the force of five hundredhiroshima…sized atomic bombs； shooting out a murderous hot cloud at up to 650 miles anhour鈥攎uch too fast； clearly； for anyone nearby to outrace。 many people who were thought tobe in safe areas； often far out of sight of the volcano； were overtaken。 fifty…seven people werekilled。 twenty…three of the bodies were never found。 the toll would have been much higherexcept that it was a sunday。 had it been a weekday many lumber workers would have beenworking within the death zone。 as it was； people were killed eighteen miles away。
the luckiest person on that day was a graduate student named harry glicken。 he had beenmanning an observation post 5。7 miles from the mountain； but he had a college placementinterview on may 18 in california； and so had left the site the day before the eruption。 hisplace was taken by david johnston。 johnston was the first to report the volcano exploding；moments later he was dead。 his body was never found。 glicken鈥檚 luck； alas； was temporary。
eleven years later he was one of forty…three scientists and journalists fatally caught up in alethal outpouring of superheated ash； gases； and molten rock鈥攚hat is known as a pyroclasticflow鈥攁t mount unzen in japan when yet another volcano was catastrophically misread。
volcanologists may or may not be the worst scientists in the world at making predictions；but they are without question the worst in the world at realizing how bad their predictions are。
less than two years after the unzen catastrophe another group of volcano watchers； led bystanley williams of the university of arizona； descended into the rim of an active volcanocalled galeras in colombia。 despite the deaths of recent years； only two of the sixteenmembers of williams鈥檚 party wore safety helmets or other protective gear。 the volcano erupted； killing six of the scientists； along with three tourists who had followed them； andseriously injuring several others； including williams himself。
in an extraordinarily unself…critical book called surviving galeras； williams said he could鈥渙nly shake my head in wonder鈥潯hen he learned afterward that his colleagues in the worldof volcanology had suggested that he had overlooked or disregarded important seismic signalsand behaved recklessly。 鈥渉ow easy it is to snipe after the fact； to apply the knowledge wehave now to the events of 1993；鈥潯e wrote。 he was guilty of nothing worse； he believed； thanunlucky timing when galeras 鈥渂ehaved capriciously； as natural forces are wont to do。 i wasfooled； and for that i will take responsibility。 but i do not feel guilty about the deaths of mycolleagues。 there is no guilt。 there was only an eruption。鈥�
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