Lurking beneath Old Faithful, scientists have long known there was a supervolcano in Yellowstone National Park with the potential to make the Mt. St. Helens' eruption look like child's play.
Now, they are coming to grips with the entire volcanic system. They have discovered and mapped a blob-like reservoir of hot, partly molten rock reaching 12 to 28 miles beneath the surface. It has a volume of 46,000 cubic kilometers, 4.5 times larger than the shallower, long-known upper crustal magma chamber.
This deep reservoir, according the findings published Thursday in Science, could fill the 1,000-cubic-mile Grand Canyon as much as 11.2 times over.
"For the first time, we have imaged the continuous volcanic plumbing system under Yellowstone," University of Utah's Hsin-Hua Huang, a lead author on the study and 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."
The researchers said the discovery may lead to better estimates of region's future seismic and volcanic hazards as well as helping explain one of the mysteries of Yellowstone's geology. Its soil and geothermal features emit more carbon dioxide than can be explained by gases from the upper chamber.
"The new study is the missing link between the shallow magma system that we imaged last year and the mantle plume deep in the Earth," Robert B. Smith, a research and emeritus professor of geology and geophysics at the University of Utah and a co-author on the study, told CBS News. "By putting in this new body we just discovered, it accounts pretty well for the total of the CO2 that comes out of the system."
And by mapping the magma reservoir, the researchers also have learned it is not full of molten rock. Rather, the rock is hot and sponge-like, with pockets of molten rock within it.
However, the experts say this discovery isn't anything to worry about. "The actual hazard is the same, but now we have a much better understanding of the complete crustal magma system," said Smith, who has worked in Yellowstone for 57 years.
There have only been three giant eruptions in the past 2.1 million years. If the next eruption is anything like the last, which happened 640,000 years ago, it will spew large amounts of volcanic ash and material into the atmosphere -- enough to encircle the Earth.
But the scientists say chances of that happening again are just 1 in 700,000.
The trio of ancient eruptions were only the latest in a series of more than 140 as the North American plate of Earth's crust and upper mantle moved southwest over the Yellowstone hotspot, starting 17 million years ago at what's now the Oregon-Idaho-Nevada border. The hotspot eruptions progressed northeast before reaching Yellowstone 2 million years ago.
More recently, the volcano has been active but on a much smaller scale.
Since the last major eruption, Smith said the volcano has experienced 50 to 60 smaller eruptions - hundreds of times smaller than the catastrophic ones - most recently about 170,000 years ago. The last one created what is now the West Thumb of Yellowstone Lake. The last lava flow was about 70,000 years ago.