A short history of nearly everything - Bill Bryson [110]
Ninety minutes after the blast, ash began to rain down on Yakima, Washington, a community of fifty thousand people about eighty miles away. As you would expect, the ash turned day to night and got into everything, clogging motors, generators, and electrical switching equipment, choking pedestrians, blocking filtration systems, and generally bringing things to a halt. The airport shut down and highways in and out of the city were closed.
All this was happening, you will note, just downwind of a volcano that had been rumbling menacingly for two months. Yet Yakima had no volcano emergency procedures. The city's emergency broadcast system, which was supposed to swing into action during a crisis, did not go on the air because “the Sunday-morning staff did not know how to operate the equipment.” For three days, Yakima was paralyzed and cut off from the world, its airport closed, its approach roads impassable. Altogether the city received just five-eighths of an inch of ash after the eruption of Mount St. Helens. Now bear that in mind, please, as we consider what a Yellowstone blast would do.
15 DANGEROUS BEAUTY
IN THE 1960s, while studying the volcanic history of Yellowstone National Park, Bob Christiansen of the United States Geological Survey became puzzled about something that, oddly, had not troubled anyone before: he couldn't find the park's volcano. It had been known for a long time that Yellowstone was volcanic in nature—that's what accounted for all its geysers and other steamy features—and the one thing about volcanoes is that they are generally pretty conspicuous. But Christiansen couldn't find the Yellowstone volcano anywhere. In particular what he couldn't find was a structure known as a caldera.
Most of us, when we think of volcanoes, think of the classic cone shapes of a Fuji or Kilimanjaro, which are created when erupting magma accumulates in a symmetrical mound. These can form remarkably quickly. In 1943, at Parícutin in Mexico, a farmer was startled to see smoke rising from a patch on his land. In one week he was the bemused owner of a cone five hundred feet high. Within two years it had topped out at almost fourteen hundred feet and was more than half a mile across. Altogether there are some ten thousand of these intrusively visible volcanoes on Earth, all but a few hundred of them extinct. But there is a second, less celebrated type of volcano that doesn't involve mountain building. These are volcanoes so explosive 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 second type, but Christiansen couldn't find the caldera anywhere.
By coincidence just at this time NASA decided to test some new high-altitude cameras by taking photographs of Yellowstone, copies of which some thoughtful official passed on to the park authorities on the assumption that they might make a nice blow-up for one of the visitors' centers. As soon as Christiansen saw the photos he realized why he had failed to spot the caldera: virtually the whole park—2.2 million acres—was caldera. The explosion had left a crater more than forty miles across—much too huge to be perceived from anywhere at ground level. At some time in the past Yellowstone must have blown up with a violence far beyond the scale of anything known to humans.
Yellowstone, it turns out, is a supervolcano. It sits on top of an enormous hot spot, a reservoir of molten rock that rises from at least 125 miles down in the Earth. The heat from the hot spot is what powers all of Yellowstone's vents, geysers, hot springs, and popping mud pots. Beneath the surface is a magma chamber that is about forty-five miles across—roughly the same dimensions as