Supervolcanoes evolve superquickly

Huge underground magma chambers appear and erupt within just several hundred years

5:31pm, May 30, 2012

California’s Long Valley is the remains of a supervolcano that exploded 760,000 years ago. Quartz grains in rocks from the eruption  suggest the magma chamber beneath it was there only briefly.

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The biggest eruptions on Earth may happen faster than volcanologists had thought. Giant blobs of magma appear underground and then pour onto the surface within centuries, suggests a new study of a California supereruption.

If the work holds true for other volcanoes, it means the most powerful eruptions don’t have magma chambers beneath them for very long. So if big changes start happening, like the ground rising or new geysers spouting, volcanologists might expect an eruption sooner rather than later. Yellowstone, for one, experienced a supereruption about 2.1 million years ago.

“The fact that at Yellowstone there’s no giant magma body right now doesn’t mean that in hundreds to thousands of years we couldn’t have one,” says Guilherme Gualda, a geologist at Vanderbilt University in Nashville. “By understanding these time scales better, we know better what to expect.”

Gualda and his colleagues report the discovery May 30 in PLoS ONE.

The researchers studied 760,000-year-old rocks from Long Valley in eastern California. These rocks, known as the Bishop Tuff, formed during one of the biggest eruptions known, which spread some 600 cubic kilometers of ash and other debris across the landscape. Earlier studies, which looked at crystals such as zircon, suggested that the Bishop magma had sat underground, chemically evolving and slowly crystallizing for more than 100,000 years before erupting.

Gualda’s team looked instead at crystals of quartz. Certain properties of the quartz — such as how much the element titanium migrated within it and how glass pieces within it changed shape over time — can indicate how long ago the quartz solidified within the liquid magma. That process would begin almost as soon as the magma body appears, the researchers’ calculations showed. Once quartz forms, gas pressure builds up and eventually forces the magma to erupt.

Quartz would have crystallized less than 10,000 years before the eruption — and usually more like 500 to 3,000 years, the team reports. The other studies that used zircon, and found a much older age, may represent a longer-term history of volcanism across the entire region, Gualda says.

Research at other supervolcanoes has shown how quickly an eruption can happen once magma starts moving toward the surface (SN: 3/10/12, p. 12). “What we’re finding is that it wasn’t there for very long to begin with,” says Gualda.

The discovery supports the idea that magma can accumulate rapidly before supereruptions, says Erik Klemetti, a geologist at Denison University in Granville, Ohio, who has found similar fast changes in zircon crystals from New Zealand eruptions. Still, he says, “this study really doesn’t address a key question — just how do these large magma bodies initially form?”

For now, Gualda is leaving that challenge to others. He is working to apply the new finding to other supervolcanoes to see whether they play by the same rules as Long Valley.

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