Long before any recorded human history in Yellowstone, a
massive volcanic eruption spewed an immense volume of ash that covered all of
the western U.S., much of the Midwest, northern Mexico and some areas of the
eastern Pacific Coast. The eruption may have been as much as one thousand times
more powerful than the 1980 eruption of Mt. St. Helens, and it left a caldera
approximately 34 by 45 miles (55 by 72 km). The Yellowstone super volcano is
believed to erupt every 600,000 to 900,000 years with the last event occurring
640,000 years ago. Its eruptions are among the largest known to have ever
