Yellowstone National Park's Grand Prismatic Spring is one of many geothermal features created by the region's supervolcano.| "Windows into the Earth," Robert B. Smith and Lee J. Siegel
Geologists finally have a complete picture of the active volcanic system underneath Yellowstone National Park in the United States, after discovering a large reservoir of magma in the planet's lower crust.
This pool of partly molten rock, which had been predicted but never glimpsed before, connects two well-known features of the park's volcanic plumbing: a smaller chamber of magma in the upper crust and a particularly hot spot of the Earth's mantle below.
The findings do not suggest any increased risk of eruption, but the discovery of the magma pool represents the last piece of the puzzle regarding Yellowstone's supervolcano.
Hsin-Hua Huang from the University of Utah in Salt Lake City and a team of U.S. colleagues combined data from several seismic arrays in North America to identify this magma reservoir in the lower crust — a region that has been difficult to study.
Their study, which appears in the 24 April issue of the journal Science , should help geologists understand how these major volcano systems work and, perhaps, provide a way to assess hazards in the future.
"Our study provides firsthand estimates of the lower-crustal reservoir's general size and geometry, which we knew little about before," said Huang. "Further modeling studies could use these estimates to model how magmatic fluid was transported to the crust, which in turn may provide insight into the dynamics of the system and its possible impacts on the planet's surface."
This cross-section illustration, cutting southwest to northeast under Yellowstone, shows the newly identified magma reservoir (lower crust). | Hsin-Hua Huang/ University of Utah
Previous studies had identified a small, upper magma chamber in the crust, along with a heat-supplying mantle plume underneath it. But the two features alone could never account for the very high levels of carbon dioxide gas venting from the molten rock into the atmosphere.
This newly identified reservoir of magma in the lower crust — about 4.5 times larger than the upper chamber — likely bridges the mantle's hotspot and the upper-crust magma chamber, and its presence helps to account for all of the escaping gas.
Huang and his colleagues used data from both local and distant earthquakes to create an image of the Yellowstone crust and uppermost mantle. "Local earthquake data provides resolution for shallow depths, and distant earthquakes, on the contrary, provide information on deeper depths," explained Huang. "This is because the farther a seismic wave travels, the deeper it dives."
Similarly large magma reservoirs situated below smaller chambers in the upper crust, where rock deforms into sponge-like material and occasionally melts, might be common features of supervolcanoes. The new findings beneath Yellowstone are bound to help researchers find out.
"On geologic time scales, rocks could flow in a constant state of flux," said Huang. "Think of the colorful, stained glass windows in Europe's oldest churches. If you look close, you find that they are usually thicker at the bottom and thinner near the top. This is an example of a flowing — or more correctly, 'deforming' — solid…and this kind of rock deformation may accommodate magma transport, given the high temperatures and pressure at deep depths."
Yellowstone is considered a supervolcano because some of its past eruptions have explosively evacuated more than 300 cubic kilometers of magma from its subsurface magma chambers. One such "super-eruption" in Yellowstone, about 2.1 million years ago, spewed more than eight times that amount of magma from its chambers. But other eruptions have been much smaller.
Researchers estimate that the most recent lava flow in Yellowstone occurred about 70,000 years ago, and the park hasn't blown its top in about 640,000 years. The annual chance of a super-eruption in Yellowstone today remains about 1 in 700,000, they say.