The geysers of Yellowstone are a reminder of the potential danger that lies below---a supervolcano that last erupted some 70,000 years ago. The Yellowstone region sits on a volcanic hotspot, similar to the one that creates the Hawaiian islands. That hotspot first pushed through the Earth's surface near the current border of Oregon, Idaho and Nevada 17 million years ago. The North American plate has been drifting slowly over the hotspot; the continent reached its present location relative to the hotspot about 2 million years ago. Since that time, there have been three huge, caldera-forming eruptions, 2 million, 1.3 million and 642,000 years ago. (Some people have suggested the volcano might be overdue for another of these large eruptions, but that's probably not true.)
In 2009, scientists used seismic images to map the volcanic plume beneath Yellowstone and found that it extended about 150 miles west-northwest to a point at least 410 miles beneath the border of Montana and Idaho. Those scientists recently applied a different imaging technique and found that the plume is even bigger. "It's like comparing ultrasound and MRI in the human body," said University of Utah geophysicist Michael Zhdanov, lead author on the paper that will soon be published in Geophysical Research Letters.
Zhdanov and his colleagues used electromagnetic field data collected by 115 EarthScope stations in Wyoming, Montana and Idaho to produce their new image of the plume and found that it extends about 400 miles from East to West and at least 200 miles deep, as far as this imaging technique can penetrate.