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ometimes burst forth in a vast； continuous outpouring—aflood—of 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—they certainly didn’t help—with 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’s best…known islands and islandchains—iceland； hawaii； the azores； canaries； and galápagos archipelagos； little pitcairn inthe middle of the south pacific； and many others—but apart from yellowstone they are alloceanic。 no one has the faintest idea how or why yellowstone’s 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’t 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’s 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’s 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’s 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’s farmers long agodiscovered。 in another two million years； geologists like to joke； yellowstone will beproducing french fries for mcdonald’s； 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）—nearly 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’s 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’s 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 “volcanic winter” and 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’s 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’swading pool。 by 1984； the whole central region of the park—several dozen square miles—was 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—a restless magma chamber。
yellowstone wasn’t 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’seruptions averaged one massive blow every 600；000 years。 the last one； interestingly enough；was 630；000 years ago。 yellowstone； it appears； is due。
“it may not feel like it； but you’re standing on the largest active volcano in the world；” pauldoss； 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。 “and i’ve got the best place inthe world to do it；” he says as we set off in a bouncy； battered four…wheel…drive vehicle in thegeneral direction of old faithful。 he has agreed to let me acpany him for a day as he goesabout doing whatever it is a park geologist does。 the first assignment today is to give anintroductory talk to a new crop of tour guides。
yellowstone； i hardly need point out； is sensationally beautiful； with plump； statelymountains； bison…specked meadows； tumbling streams； a sky…blue lake； wildlife beyondcounting。 “it really doesn’t get any better than this if you’re a geologist；” doss says。 “you’vegot rocks up at beartooth gap that are nearly three billion years old—three…quarters of theway back to earth’s beginning—and then you’ve got mineral springs here”—he points at thesulfurous hot springs from which mammoth takes its title—“where you can see rocks as theyare being born。 and in between there’s everything you could possibly imagine。 i’ve neverbeen any place where geology is more evident—or prettier。”
“so you like it？” i say。
“oh； no； i love it；” he answers with profound sincerity。 “i mean i really love it here。 thewinters are tough and the pay’s not too hot； but when it’s good； it’s just—”
he interrupted himself to point out a distant gap in a r