A giant reservoir of magma and hot rock beneath the Yellowstone supervolcano has been found and imaged. The newly found reservoir lies 12-28 miles below the surface, and is four-and-a-half times larger than the shallower, hot melted rock zone that powers current Yellowstone geysers and caused the caldera's last eruption some 70,000 years ago.
The volume of the newly imaged, deeper reservoir is a whopping 11,000 cubic-miles (46,000 cubic kilometers), which is about the volume of Long Island with 9 miles of hot rock piled on it, or 300 Lake Tahoes. The discovery begins to fill in a gray area about how Yellowstone connects to a far deeper plume of heat rising up from the Earth's mantle.
"It's existence has been suspected for a while," said University of Utah geophysicist Hsin-Hua Huang of the newly imaged hot reservoir. Huang is the lead author of a paper announcing the discovery in the Thursday issue of the journal ScienceExpress.
Far more carbon dioxide was being released from the ground at Yellowstone, previous research suggested, than could be explained by the smaller, shallower magma reservoir. That smaller reservoir was found using data from a local seismic array. The array used local earthquakes to seismically illuminate structures in the crust.
To see deeper, however, scientists needed a wider array to record how seismic waves from more distant earthquakes behaved as they passed through the unexplored zone under Yellowstone, Huang said. Seismic waves reveal a lot about the crust because they travel slower through hot rocks than through colder rocks. That wider seismic network came in the form of the USArray, a portable, temporary seismic network that's been crossing the continental United States since 2004.
"Until now we hadn't combined this data," Huang said. It's the blending of that data that allowed Huang and his colleagues to see the giant hot, partially melted zone. "It's not a new technique, but no one had ever applied it to Yellowstone."
The data also reveal that the shallower magma zone is about 9 percent melted rock, and the newly found lower zone is 2 percent melted. Neither is a giant chamber of magma and neither is in any danger of erupting -- contrary to popular misconceptions -- say the researchers.
The actual image Huang and his colleagues produced was created by what's called seismic tomography. The method is a lot like computerized tomography, or CT-scans used in medicine -- except that instead of x-rays passing through living tissues, geophysicists use seismic waves from earthquakes that pass through the Earth's interior.
"This is a snapshot," said geoscientist Shaul Hurwitz of the U.S. Geological Survey. It's a higher-resolution view than was available before, he said, somewhat like improving a CT-scan from just seeing bones to seeing the finer structures of tissues. "I think they did a very good job looking at this gray area."