ver 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 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。 鈥渋t really doesn鈥檛 get any better than this if you鈥檙e a geologist；鈥潯oss says。 鈥測ou鈥檝egot rocks up at beartooth gap that are nearly three billion years old鈥攖hree…quarters of theway back to earth鈥檚 beginning鈥攁nd then you鈥檝e got mineral springs here鈥濃�攈e points at thesulfurous hot springs from which mammoth takes its title鈥斺�渨here you can see rocks as theyare being born。 and in between there鈥檚 everything you could possibly imagine。 i鈥檝e neverbeen any place where geology is more evident鈥攐r prettier。鈥�
鈥渟o you like it？鈥潯 say。
鈥渙h； no； i love it；鈥潯e answers with profound sincerity。 鈥渋 mean i really love it here。 thewinters are tough and the pay鈥檚 not too hot； but when it鈥檚 good； it鈥檚 just鈥斺��
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。
鈥渢hat 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鈥檝e got sixty miles of mountains just obliterated；you know you鈥檙e dealing with something pretty potent。 it took christiansen six years to figureit all out。鈥�
i asked him what caused yellowstone to blow when it did。
鈥渄on鈥檛 know。 nobody knows。 volcanoes are strange things。 we really don鈥檛 understandthem at all。 vesuvius； in italy； was active for three hundred years until an eruption in 1944and then it just stopped。 it鈥檚 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。鈥�
鈥渁nd how much warning would you get if yellowstone was going to go？鈥�
he shrugged。 鈥渘obody 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。鈥�
鈥渟o it could just blow without warning？鈥�
he nodded thoughtfully。 the trouble； he explained； is that nearly all the things that wouldconstitute warning signs already exist in some measure at yellowstone。 鈥渆arthquakes aregenerally a precursor of volcanic eruptions； but the park already has lots of earthquakes鈥�1；260 of them last year。 most of them are too small to be felt； but they are earthquakesnonetheless。鈥�
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 erupted again； though only to a height of eighty feet。 steamboatgeyser is the biggest geyser in the world when it blows； shooting water four hundred feet intothe air； but the intervals between its eruptions have ranged from as little as four days to almost fifty years。 鈥渋f it blew today and again next week； that wouldn鈥檛 tell us anything at all aboutwhat it might do the following week or the week after or twenty years from now；鈥潯oss says。
鈥渢he whole park is so volatile that it鈥檚 essentially impossible to draw conclusions from almostanything that happens。鈥�
evacuating yellowstone would never be easy。 the park gets some three million visitors ayear； mostly in the three peak months of summer。 the park鈥檚 roads are parati