"So what exactly is a supervolcano? Just picture a volcano with 10,000 times the explosive force of Mount St. Helens. And unlike Mount Fuji, supervolcanoes aren’t available in nice cone shapes. Rather, these extreme volcanoes form in depressions called calderas, where the magma gets so thick that gas can’t escape. The pressure keeps building and building until all hell literally breaks loose.
We have our very own supervolcano under Yellowstone National Park. The entire park. In fact, the caldera under Yellowstone [wiki] is so big – 4,000 square kilometers – no one knew it was there until satellite images told us so. By all estimates, it erupts about every 600,000 years, and the last eruption was 640,000 years ago. We’re due.
So what happens if it blows? The last eruption of a supervolcano was at Lake Toba in Sumatra 75,000 years ago. So much ash was released into the atmosphere that the sun was blocked out, the global temperature dropped 21 degrees, and three-quarters of all plant life in the Northern Hemisphere died. Ice age, anyone? Hopeful geologists content that we may be saved by the venting that occurs at Yellowstone through geysers like Old Faithful, relieving a bit of pressure from the caldera. Let’s hope they’re right."
http://www.neatorama.com/2007/02/13/...ral-disasters/