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Massive eruptions can arise suddenly: Magma merger let Idaho supervolcano blow, study finds.

Massive supervolcanic eruptions can be triggered much more quickly than previously thought, scientists report July 21 in Geology.

The researchers made the discovery while reconstructing the history of a massive eruption 4.5 million years ago in a field of volcanoes called Heise, in what's now eastern Idaho. The Heise supervolcano spewed enough ash and molten rock to fill Lake Ontario. In most supervolcanoes, magma simmers in underground chambers for hundreds of thousands of years before erupting. But in Heise, the team has found, several small magma reservoirs pooled together in less than 10,000 years to spur the eruption.

Supervolcanoes are capable of outbursts thousands of times more powerful than typical volcanic eruptions, causing regional devastation and dramatic changes in global climate. "We now have a better idea of what the magma reservoirs under these supervolcanoes look like just prior to eruption," says geologist and lead author Jorn-Frederik Wotzlaw of the University of Geneva.

Heise was fueled by an abnormally hot spot deep in Earth's mantle that has sprouted a string of North American volcanoes over the last 16.5 million years, including a supervolcano beneath Yellowstone National Park that erupted 640,000 years ago. The new finding could help researchers predict how quickly a new supervolcano could emerge and erupt in the region, Wotzlaw says.

Wotzlaw and colleagues studied tiny translucent crystals embedded in rocks blasted from the Heise eruption. Scientists know that over thousands of years, the crystals grew inside the volcano's subterranean magma and absorbed oxygen from the surrounding molten rock. The oxygen serves as a fingerprint of where the crystals formed, Wotzlaw says, because all crystals within a magma reservoir should have the same ratio of forms of oxygen called isotopes.

Looking at these ratios, the researchers found that the crystals in the Heise sample didn't all arise in a single magma chamber. The researchers could tell from the oldest part of a crystal, its core, that each was born in one of four magma reservoirs. The younger outer rims of the crystals grew inside one of two distinct magma reservoirs. Because previous studies had demonstrated that the Heise supervolcano contained only one magma reservoir when it erupted, the team realized that the crystals must have grown in different magma batches that merged into two, then one before eruption.

While in these reservoirs, the crystals absorbed not only oxygen but also uranium, which slowly decays into lead. By comparing the number of uranium and lead atoms in each crystal, the researchers determined how long ago it formed. They estimate that the four smaller magma chambers formed, merged and erupted in only 1,000 to 10,000 years.

Wotzlaw proposes that the assembly of magma reservoirs set off the Heise eruption without an external trigger such as an earthquake. In January, a team reported that the buoyancy of magma pushing upward against Earth's denser crust might itself be enough to prompt an eruption. Because the force from a huge magma chamber would be stronger than from each individual reservoir, Wotzlaw says that when the reservoirs teamed up, they could have set off the eruption.

Heise's magma is spent and the volcano will not erupt again. The now-simmering Yellowstone supervolcano does contain several large magma reservoirs, but they are less likely to combine or initiate a major eruption, says geochemist Kenneth Sims of the University of Wyoming in Laramie. He warns against comparing the two volcanoes. The magma under Yellowstone is older and less runny than the molten rock that fueled the Heise eruption, he notes. "The possibility of another eruption certainly exists, but right now there isn't that kind of evidence," he says.

But a new supervolcano probably will one day form northeast of Yellowstone, Wotzlaw says. The North American tectonic plate has been slowly and steadily drifting southwest over the region's hot spot, spawning volcanoes. If the new volcano follows Heise's lead, it could quickly evolve into an eruptible state.

"If the process only takes a few thousand years," Wotzlaw says, "then it is relevant on a human scale; maybe not for you and me. But if a volcano erupts in 2,000 years, that may not look good for the people living at that time."

Caption: A hot spot deep in Earth's mantle helps fuel the geothermal activity at Yellowstone National Park (shown). The hot spot also nourished a supervolcano that erupted 4.5 million years ago in Idaho when magma reservoirs quickly merged together, a new study suggests.


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Title Annotation:EARTH & ENVIRONMENT
Author:Sumner, Thomas
Publication:Science News
Geographic Code:1U8ID
Date:Aug 23, 2014
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