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d 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； 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nd 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auldoss； 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e says as w