A previously unknown reservoir of magma has been discovered under the Yellowstone supervolcano — even larger than the one that caused the caldera to erupt some 640,000 years ago.
At 11,000 cubic miles of material, the reservoir contains enough magma and heated rock to fill the Grand Canyon almost 14 times over. It is about 28 miles deep and more than four times the size of the long-known, shallower magma chamber that produced the ancient eruption and still powers the region's geysers, steam vents and hot springs.
"Its existence has been suspected for a while," said the study's lead author, geophysicist Hsin-Hua Huang from the University of Utah.
That's because much more carbon dioxide has been detected emanating from the ground around Yellowstone than what could be produced by the smaller magma reservoir that is about 12 miles underground.
There has been no growth in the "plumbing" system of the region — and therefore no greater chance of an eruption. The new findings have, however, provided a better understanding of how hot and partly molten rock rises from a hotspot plume 40 miles beneath the Earth's surface up to the magma reservoirs underlying the supervolcano.
"For the first time, we have imaged the continuous volcanic plumbing system under Yellowstone," Huang said. "That includes the upper crustal magma chamber we have seen previously plus a lower crustal magma reservoir that has never been imaged before and which connects the upper chamber to the Yellowstone hotspot plume below."
Contrary to popular assumption, the magma reservoirs and chambers are not full of molten rock — they contain a form of spongy but solid hot rock with pockets of melted rock, the researchers explained.
That makes it "hard to infer anything about a future eruption," said study co-author Fan-Chi Lin — although experts consider the chances of the Yellowstone supervolcano erupting to be only 1 in 700,000.
For the study, the researchers combined seismographic data from the university's seismographic stations throughout the region and from the EarthScope array of seismometers, funded by the National Science Foundation. The foundation has been mapping the underground structures of all 48 of the lower U.S. states.
"Every additional thing we learn about the Yellowstone volcanic system is one more piece in the puzzle, and that gets us closer to really understanding how the volcanic system works," said Lin. "If we could better understand the transport properties of magmatic fluids, we could get a better understanding of the timing and, therefore, where we are in the volcanic cycle."
This study was published in the journal ScienceExpress.