Washington, April 24 (IANS): A huge reservoir of hot, partly molten rock has been found 20-45 kms beneath the Yellowstone supervolcano in the US, and it is 4.4 times larger than the shallower, long-known magma chamber, US researchers said on Thursday.

The hot rock in the newly-discovered, deeper magma reservoir would fill the 4,170-cubic-km Grand Canyon 11.2 times, while the previously known magma chamber would fill the Grand Canyon 2.5 times.

The findings, published in the US journal Science, were based on seismic imaging of Yellowstone's volcanic plumbing system, which is somewhat like a medical CT scan but uses earthquake waves instead of X-rays to distinguish rock of various densities, Xinhua reported.

"For the first time, we have imaged the continuous volcanic plumbing system under Yellowstone," said first author Hsin-Hua Huang, a postdoctoral researcher of the University of Utah.

"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 said the new study indicates the upper magma chamber averages about nine percent molten rock, consistent with earlier estimates of five percent to 15 percent melt, and the lower magma reservoir is about two percent melt.

The researchers noted that the previously known upper magma chamber was the immediate source of three cataclysmic eruptions of the Yellowstone caldera 2 million, 1.2 million and 640,000 years ago, and that isn't changed by discovery of the underlying magma reservoir that supplies the magma chamber.

Study co-author Fan-Chi Lin, assistant professor at the University of Utah, said the findings have no relation to the possibility of a supervolcano eruption in the US national park, which is among the world's largest supervolcanoes, with frequent earthquakes and Earth's most vigorous continental geothermal system.

The three supervolcano eruptions at Yellowstone covered much of North America in volcanic ash. A supervolcano eruption today would be cataclysmic, but the researchers said the annual chance is 1 in 700,000.

The discovery solves a longstanding mystery as to why Yellowstone's soil and geothermal features emit more carbon dioxide than can be explained by gases from the upper magma chamber, the researchers said.