IN THE 1960s, while studying the volic history of Yellowstoional Park, BobChristiansen of the Uates Geological Survey became puzzled about something that,oddly, had not troubled anyone before: he couldn』t find the park』s volo. It had been knownfor a long time that Yellowstone was voli nature—that』s what ated for all itsgeysers and other steamy features—and the ohing about voloes is that they aregenerally pretty spicuous. But Christiansen couldn』t find the Yellowstone voloanywhere. In particular what he couldn』t find was a structure known as a caldera.
Most of us, whehink of voloes, think of the classie shapes of a Fuji orKilimanjaro, which are created wheing magma accumulates in a symmetrical mound.
These form remarkably quickly. In 1943, at Parí in Mexico, a farmer was startled tosee smoke rising from a pat his land. In one week he was the bemused owner of a efive hundred feet high. Within two years it had topped out at almost fourteen hundred feet andwas more than half a mile across. Altogether there are some ten thousand of these intrusivelyvisible voloes oh, all but a few hundred of them extinct. But there is a sed, lesscelebrated type of volo that doesn』t involve mountain building. These are voloes soexplosive that they burst open in a single mighty rupture, leaving behind a vast subsided pit,the caldera (from a Latin word for cauldron). Yellowstone obviously was of this sed type,but Christiansen couldn』t find the caldera anywhere.
By ce just at this time NASA decided to test some new high-altitude cameras bytaking photographs of Yellowstone, copies of whie thoughtful official passed on to thepark authorities on the assumption that they might make a nice blow-up for one of thevisitors』 ters. 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 auch 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.
Yellowsto turns out, is a supervolo. 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 owers all of Yellowstone』s vents, geysers, hot springs, and popping mudpots. Beh the surface is a magma chamber that is about forty-five miles acrhlythe 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 reag 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 surroundiory about1,700 feet higher than they would otherwise be. If it blew, the cataclysm is pretty well beyondimagining. Acc to Professor Bill McGuire of Uy College London, 「youwouldn』t be able to get within a thousand kilometers of it」 while it was erupting. Thesequehat followed would be even worse.
Superplumes of the type on which Yellowstos are rather like martini glasses—thin onthe , but spreading out as they he surface to create vast bowls of unstable magma.
Some of these bowls be up to 1,200 miles across. Acc to theories, they don』talways erupt explosively but sometimes burst f