Much of Yellowstone National Park lies in the crater of a massive volcano, formed in a landscape-altering eruption 640,000 years ago. The crater, or caldera, measures some 28 miles wide by 47 miles long.
Subsequent lava flows—most recently 70,000 years ago—filled in much of the blasted-out crater, disguising the area's volcanic identity.
Now a study by scientists with the U. S. Geological Survey (USGS) and the Yellowstone Volcano Observatory attributes changes in both surface terrain and geyser behavior to flows of magma, or molten rock, 9 miles below the Earth's surface.
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