Supervolcano

Here are a few more excerpts from Bill Bryson's book that will make your day. These are about Yellowstone Park.
In the 1960's, while studying the volcanic history of Yellowstone Park, Bob Christianson became puzzled. He couldn't find the caldera.
By coincidence just at this time, NASA decided to test some new high-altitude cameras by taking photographs of Yellowstone, copies of which were passed on to park authorities. As soon as Christianson saw the photos he realized why he had failed to find the caldera: virtually the whole park - 2.2 million acres - was caldera. At some time in the past Yellowstone must have blown up with a violence far beyond the scale of anything known to humans.
Yellowstone is a supervolcano. It sits on top of an enormous hot spot that rises from 125 miles down in the Earth. Beneath the surface is a magma chamber about 45 miles across and about 8 miles thick. The pressure that such a pool of magma exerts on the crust above has lifted Yellowstone and about 300 miles of surrounding territory about 1,700 feet higher than it would be. If it blew, the cataclysm is pretty well beyond imagining.
Since its first known eruption 16.5 million years ago, Yellowstone has blown up about a hundred times, but the most recent three eruptions are the ones that get written about. The last eruption was a thousand times greater than that of Mt. St. Helens; the one before that was 280 times bigger, and the one before was so big that nobody knows exactly how big it was.
The Yellowstone eruption of 2 million years ago put out enough ash to bury California to a depth of 20 feet. Ash, it is worth remembering, is not like a big snowfall that will melt in the spring. It took thousands of workers eight months to clear 1.8 billion tons of debris from the 16 acres of the World Trade Center site in New York. Imagine what it would take to clear Kansas.
In 1973, water in Yellowstone Lake began to run over the banks at the lake's southern end, while at the other end of the lake the water mysteriously flowed away. Geologists did a hasty survey and discovered that a large area of the park had developed an ominous bulge. This was lifting up one end of the lake and causing water to run out at the other. By 1984, the whole central region region of the park was more than 3 feet higher than it had been in 1924, when the park was last formally surveyed. Then in 1985, the whole central part of the park subsided by 8 inches. It now seems to be swelling again.
The geologists realized that only one thing could cause this -a restless magma chamber. Yellowstone wasn't the site of an ancient supervolcano; it was the site of an active one. It was also at this time that they were able to work out that the cycle of Yellowstone's eruptions averaged one massive blow every 600,000 years. The last one, interestingly enough, was 630,000 years ago. Yellowstone, it appears, is due.