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

http://news.nationalgeographic.com/news/2011/01/110119-yellowstone-park-supervolcano-eruption-magma-science/

Here’s what National Geographic predicts will happen when Yellowstone explodes:

http://ngm.nationalgeographic.com/2009/08/yellowstone/achenbach-text