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ba； 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 range of mountains to the west； whichhad just e into view over a rise。 the mountains； he told me； were known as the gallatins。
“that gap is sixty or maybe seventy miles across。 for a long time nobody could understandwhy that gap was there； and then bob christiansen realized that it had to be because themountains were just blown away。 when you’ve got sixty miles of mountains just obliterated；you know you’re dealing with something pretty potent。 it took christiansen six years to figureit all out。”
i asked him what caused yellowstone to blow when it did。
“don’t know。 nobody knows。 volcanoes are strange things。 we really don’t understandthem at all。 vesuvius； in italy； was active for three hundred years until an eruption in 1944and then it just stopped。 it’s been silent ever since。 some volcanologists think that it isrecharging in a big way； which is a little worrying because two million people live on oraround it。 but nobody knows。”
“and how much warning would you get if yellowstone was going to go？”
he shrugged。 “nobody was around the last time it blew； so nobody knows what thewarning signs are。 probably you would have swarms of earthquakes and some surface upliftand possibly some changes in the patterns of behavior of the geysers and steam vents； butnobody really knows。”
“so it could just blow without warning？”
he nodded thoughtfully。 the trouble； he explained； is that nearly all the things that wouldconstitute warning signs already exist in some measure at yellowstone。 “earthquakes aregenerally a precursor of volcanic eruptions； but the park already has lots of earthquakes—1；260 of them last year。 most of them are too small to be felt； but they are earthquakesnonetheless。”
a change in the pattern of geyser eruptions might also be taken as a clue； he said； but thesetoo vary unpredictably。 once the most famous geyser in the park was excelsior geyser。 itused to erupt regularly and spectacularly to heights of three hundred feet； but in 1888 it juststopped。 then in 1985 it erup