The annual probability of a Yellowstone supereruption is a much smaller 0.00014 percent, Smith said.
Yellowstone National Park is cradled inside a gentle depression created by a giant volcanic eruption 640,000 years ago. The ground collapsed, leaving a bowl-shaped caldera. It was the third in a series of massive eruptions, the first of which exploded 2.1 million years ago.
A mantle plume (also called a hotspot) feeds Yellowstone's supereruptions. Hotspots are massive rising blobs of hot rock from Earth's mantle, the layer beneath the crust. As the planet's tectonic plates trundle over hotspots, the plumes punch through the crust, forming volcanic chains like Hawaii or the Idaho's Snake River Plain and Yellowstone.
In the millennia since the last massive volcanic blowout, magma has again built up beneath Yellowstone. The park trembles constantly with tiny earthquakes as gas and hot fluids course through underground fractures, escaping from the molten rock below.
Led by graduate student Jamie Farrell, the University of Utah group used these tremors like a CT scan, building a precise image of the underground magma reservoir.