University of Utah seismologists discovered and made images of a reservoir of hot, partly molten rock 12 to 28 miles beneath the Yellowstone supervolcano, and it is 4.4 times larger than the shallower, long-known magma chamber. The hot rock in the newly discovered, deeper magma reservoir would fill the 1,000-cubic-mile Grand Canyon 11.2 times, while the previously known magma chamber would fill the Grand Canyon 2.5 times, says postdoctoral researcher Jamie Farrell, a co-author of the study published online today in the journal Science.

"For the first time, we have imaged the continuous volcanic plumbing system under Yellowstone," says first author Hsin-Hua Huang, also a postdoctoral researcher in geology and geophysics. "That includes the upper crustal magma chamber we have seen previously plus a lower crustal magma reservoir that has never been imaged before and that connects the upper chamber to the Yellowstone hotspot plume below."

Contrary to popular perception, the magma chamber and magma reservoir are not full of molten rock. Instead, the rock is hot, mostly solid and spongelike, with pockets of molten rock within it. Huang says the new study indicates the upper magma chamber averages about 9 percent molten rock -- consistent with earlier estimates of 5 percent to 15 percent melt -- and the lower magma reservoir is about 2 percent melt.

So there is about one-quarter of a Grand Canyon worth of molten rock within the much larger volumes of either the magma chamber or the magma reservoir, Farrell says.