ne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 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