A giant reservoir of magma and hot rock - big enough to fill the Grand Canyon 11 times - has been discovered beneath the Yellowstone supervolcano in the US, scientists say.
University of Utah seismologists discovered the reservoir of hot, partly molten rock 19 to 45 kilometres beneath Yellowstone, 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, said postdoctoral researcher Jamie Farrell, a co-author of the study published in the journal Science.
"For the first time, we have imaged the continuous volcanic plumbing system under Yellowstone," said first author Hsin-Hua Huang.
"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," said Huang.
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 9 per cent molten rock - consistent with earlier estimates of 5 per cent to 15 per cent melt - and the lower magma reservoir is about 2 per cent 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 said.
The researchers emphasise that Yellowstone's plumbing system is no larger - nor closer to erupting - than before, only that they now have used advanced techniques to make a complete image of the system that carries hot and partly molten rock upward from the top of the Yellowstone hotspot plume - about 64 km beneath the surface - to the magma reservoir and the magma chamber above it.
"The magma chamber and reservoir are not getting any bigger than they have been, it's just that we can see them better now using new techniques," Farrell said.
"It gives us a better understanding of the Yellowstone magmatic system. We can now use these new models to better estimate the potential seismic and volcanic hazards," study co-author Fan-Chi Lin, said.
Researchers point out 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 is not changed by discovery of the underlying magma reservoir that supplies the chamber.
Yellowstone is among the world's largest supervolcanoes, with frequent earthquakes and Earth's most vigorous continental geothermal system.
