Don't put your vacation plans on hold just yet; the big bang probably won't happen for millennia. The last major eruption was 640,000 years ago. That event created a crater 28 miles wide and 47 miles long. (By comparison, the volcano at Mt. St. Helens measures 2 miles across.) The Yellowstone caldera is so huge, in fact, that scientists needed satellite imagery to confirm it.
Unlike the mountain-building volcanoes of the Cascades Range, the Yellowstone supervolcano is an enormous subterranean chamber of magma, or molten rock, sealed off from the surface 5 miles down. Its only heat release comes from steam vents, boiling springs and geysers. In other words, Old Faithful works like a valve on a pressure cooker. Geologists keep daily track of temperature changes and seismic shifts across the park. In fact, hundreds of earthquakes occur at Yellowstone each month, though most are too small to be felt.
In the last great eruption, the prehistoric volcano blasted more than 200 cubic miles of molten debris into the atmosphere, blanketing most of North America in ash. The underground magma chamber collapsed, leaving behind a sunken crater. The Yellowstone caldera ranks among the largest on earth, along with Long Valley in California and Lake Toba in Sumatra, Indonesia. A similar eruption in modern times would destroy most of the Western United States and plunge the planet into volcanic winter.
Though this doomsday scenario is good for Hollywood, most experts believe that a volcano as large as Yellowstone will give decades—if not centuries—of warning before it erupts. There is great debate over whether another catastrophic eruption will happen at all. You can learn more about supervolcanoes at the Canyon Visitor Education Center , which maps out hot spots around the globe and describes the geothermal features at Mammoth Hot Springs , Norris Geyser Basin , Lower and Midway Geyser Basins and Upper Geyser Basin .
After your vacation to Yellowstone, you can say you've walked across one of the largest volcanoes in the world.