The term supervolcano was first used in a TV documentary to describe eruptions of more than 1,000 cubic kilometers of magma. Such volcanoes are devastating, but very rare. The last one happened at Toba in Sumatra, Indonesia, 74,000 years ago.

Lurking deep below the surface in California and Wyoming are two hibernating volcanoes of almost unimaginable fury. Were they to go critical, they would blanket the western U.S. with many centimeters of ash in a matter of hours. Between them, they have done so at least four times in the past two million years. Similar supervolcanoes smolder underneath Indonesia and New Zealand.

Most have heard of the Battle of Waterloo, but who has heard of the volcano called Tambora? No school textbook I’ve seen mentions that only two months before Napoleon’s final defeat in Belgium on June 18, 1815, the faraway Indonesian island of Sumbawa was the site of the most devastating volcanic eruption on Earth in thousands of years. The death toll claimed around 100,000 people, from the thick pyroclastic flows of lava, from the tsunami that struck nearby coasts, and from the thick ash that blanketed South-East Asia’s farmlands, destroyed crops and plunged it into darkness for a week. Both events – Napoleon’s defeat and the eruption – had monumental impacts on human history. But while a library of scholarship has been devoted to Napoleon’s undoing at Waterloo, the scattered writings on Tambora would scarcely fill your in-tray.

Just a few years after Boyd's paper appeared, the U.S. Geological Survey mounted an extensive investigation of Yellowstone's geology, assigning some of its brightest young scientists to the task. Among them was Bob Christiansen, who studied the young ash flow tuffs in great detail. This article is based on his research and that of his co-workers, including geologists, chemists, and geophysicists, some of whom continue their studies of Yellowstone today.

Calderas are some of the most spectacular features on Earth. They are large volcanic craters that form by two different methods: 1) an explosive volcanic eruption; or, 2) collapse of surface rock into an empty magma chamber. The image at right is a satellite view of one of the most famous calderas - Crater Lake in Oregon. Crater Lake was formed about 7700 years ago when an enormous volcanic eruption of Mount Mazama emptied a large magma chamber below the mountain. The fractured rock above the magma chamber collapsed to produce a massive crater over six miles across. Centuries of rain and snow filled the caldera creating Crater Lake. With a depth of 1932 feet (589 meters), Crater Lake is the deepest lake in the United States and the seventh deepest lake in the world.