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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’ centers。 as soon as christiansen saw the photos he realized why he had failed to spotthe caldera： virtually the whole park—2。2 million acres—was caldera。 the explosion had lefta crater more than forty miles across—much 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’s vents； geysers； hot springs； and popping mudpots。 beneath the surface is a magma chamber that is about forty…five miles across—roughlythe same dimensions as the park—and 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； “youwouldn’t be able to get within a thousand kilometers of it” while it was erupting。 theconsequences that followed would be even worse。
superplumes of the type on which yellowstone sits are rather like martini glasses—thin 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’talways erupt explosively but sometimes 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； sev