Yellowstone sits above a melting anomaly within the Earth, called a “hotspot.” This hotspot is powered by a plume of hot (but not molten) material that may extend as deep as the boundary between the planet's mantle and core.
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The Yellowstone supervolcano last erupted about 640,000 years ago. A sleeping giant is nestled in the western part of the United States. Though it stirs occasionally, it has not risen from slumber in nearly 70,000 years.
As pressure is released, gases dissolved in the magma come out of solution, turning the magma into a boiling froth. The total energy released would be equivalent to an 875,000 megaton explosion. The shockwave would kill 90,000 people. Most of the lava would fall back into the crater.
The lava flows themselves would be contained within a relatively small radius within the park — say, 40 miles or so. In fact, only about one-third of the material would actually make it up into the atmosphere.
Subsequent activity has been focused within the area of the National Park, and another huge eruption 631,000 years ago formed the Yellowstone caldera as we now see it. The three caldera-forming eruptions, respectively, were about 6,000, 700, and 2,500 times larger than the May 18, 1980 eruption of Mt. St.