Supervolcanos produce eruptions with ejecta greater than 1,000 cubic kilometers (240 cubic miles) – thousands of times larger than most historic volcanic eruptions. Supervolcanoes occur when magma rises into the crust from a hotspot but is unable to break through that crust until pressure builds to a phenomenally high level. Yellowstone National Park is a supervolcano that last erupted 640,000 years ago.
http://dsc.discovery.com/convergence/supervolcano/supervolcano.html
Some scientists are worried that we’re overdue for another eruption. The floor of the Yellowstone supervolcano has risen 3 inches a year for the past 3 years, the fastest rate since records began in 1923.
http://www.earthmountainview.com/yellowstone/yellowstone.htm
Here’s what National Geographic predicts will happen when Yellowstone explodes:
http://ngm.nationalgeographic.com/2009/08/yellowstone/achenbach-text
It's Interesting 