University of Utah seismologists have discovered a reservoir of hot, partly molten rock 12 to 28 miles beneath the Yellowstone supervolcano that is 4.4 times larger than the shallower, long-known magma chamber.
Yellowstone is among the world’s largest supervolcanoes, with frequent earthquakes and Earth’s most vigorous continental geothermal system.
The hot rock in the newly-discovered, deeper magma reservoir would fill the 1,000-cubic-mile Grand Canyon 11.2 times, while the previously known magma chamber would fill the Grand Canyon 2.5 times, says postdoctoral researcher Jamie Farrell, a co-author of the study published online April 23, 2015 in the journal Science.
The researchers emphasize 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 40 miles beneath the surface – to the magma reservoir and the magma chamber above it. Farrell said:
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.
The 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 isn’t changed by discovery of the underlying magma reservoir that supplies the magma chamber.
Study co-author Robert B. Smith is a research and emeritus professor of geology and geophysics at the University of Utah. Smith said:
The actual hazard is the same, but now we have a much better understanding of the complete crustal magma system.
The three supervolcano eruptions at Yellowstone – on the Wyoming-Idaho-Montana border – covered much of North America in volcanic ash. A supervolcano eruption today would be cataclysmic, but Smith says the annual chance is 1 in 700,000.
The three ancient Yellowstone supervolcano eruptions were only the latest in a series of more than 140 that happened as the North American plate of Earth’s crust and upper mantle moved southwest over the Yellowstone hotspot, starting 17 million years ago at the Oregon-Idaho-Nevada border. The hotspot eruptions progressed northeast before reaching Yellowstone 2 million years ago.
Hsin-Hua Huang, a postdoctoral researcher in geology and geophysics, is the paper’s first author. Huang said:
For the first time, we have imaged the continuous volcanic plumbing system under Yellowstone. 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 says the new study indicates the upper magma chamber averages about 9 percent molten rock – consistent with earlier estimates of 5 percent to 15 percent melt – and the lower magma reservoir is about 2 percent melt.
As with past studies that made images of Yellowstone’s volcanic plumbing, the new study used seismic imaging, which is somewhat like a medical CT scan but uses earthquake waves instead of X-rays to distinguish rock of various densities. Quake waves go faster through cold rock, and slower through hot and molten rock.
For the new study, Huang developed a technique to combine two kinds of seismic information: Data from local quakes detected in Utah, Idaho, the Teton Range and Yellowstone by the University of Utah Seismograph Stations and data from more distant quakes detected by the National Science Foundation-funded EarthScope array of seismometers, which was used to map the underground structure of the lower 48 states.
Bottom line: University of Utah seismologists have discovered a reservoir of hot, partly molten rock 12 to 28 miles beneath the Yellowstone supervolcano that is 4.4 times larger than the shallower, long-known magma chamber. The hot rock in the newly discovered magma reservoir would fill the 1,000-cubic-mile Grand Canyon 11.2 times, say the researchers. The study was published online April 23, 2015 in the journal Science.
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