“One way of looking at the power of volcanoes is what scientists call the Volcano Explosivity Index (VEI) — sort of a Richter scale for eruptions. And like the Richter scale used to measure earthquakes, the power of an eruption increases exponentially from number to number in the VEI index.”
http://dsc.discovery.com/convergence/supervolcano/under/under_03.html
Mega Caldera
“The crater left by a super volcanic eruption is called a mega caldera. In Yellowstone National Park there is a mega caldera measuring about 34 miles across. Experts know of at least three occasions in which the Yellowstone super volcano has erupted: 2.2 million years ago, 1.3 million years ago, and 640,000 years ago. If Yellowstone keeps to this schedule, we still have about half a million years til the next eruption. Geologist Chuck Wicks of the U.S. Geological Survey says explosions of this magnitude “happen about every 600,000 years.” Yellowstone, if it sticks to its history, isn’t due just yet,….” http://hubpages.com/hub/Yellowstone-State-Park-Super-Volcano
SwittersB & Exploring 