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Molten magma survives in upper crust for hundreds of millennia

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Reservoirs of silica-rich magma - the kind that causes the most explosive volcanic eruptions - can persist in Earth's upper crust for hundreds of thousands of years without triggering an eruption, according to a new study.

The findings by University of researchers mean that an area known to have experienced a massive volcanic eruption in the past, such as Yellowstone National Park, could have a large pool of magma festering beneath it and still not be close to going off as it did 600,000 years ago.

"You might expect to see a stewing magma chamber for a long period of time and it doesn't necessarily mean an eruption is imminent," said Sarah Gelman, a UW doctoral student in Earth and space sciences and lead author of the study.

Recent research models have suggested that reservoirs of silica-rich magma, or molten rock, form on and survive for geologically short time scales - in the tens of thousands of years - in the Earth's cold upper crust before they solidify.

They also suggested that the magma had to be injected into the Earth's crust at a high rate to reach a large enough volume and pressure to cause an eruption.

However, Gelman and her collaborators took the models further, incorporating changes in the crystallisation behaviour of silica-rich magma in the upper crust and temperature-dependent heat conductivity.

They found that the magma could accumulate more slowly and remain molten for a much longer period than the models previously suggested.

The findings are significant for volcanic 'arcs', found near subduction zones where one of Earth's tectonic plates is diving beneath another.

Arcs are found in various parts of the world, including the Andes Mountains of South America and the Cascades Range of the Pacific Northwest.

Scientists have developed techniques to detect magma pools beneath these arcs, but they cannot determine how long the reservoirs have been there. Because volcanic magma becomes more silica-rich with time, its explosive potential increases.

"If you see melt in an area, it's important to know how long that melt has been around to determine whether there is eruptive potential or not," Gelman said.

"If you image it today, does that mean it could not have been there 300,000 years ago? Previous models have said it couldn't have been. Our model says it could. That doesn't mean it was there, but it could have been there," she said.

The study was published in the journal Geology.

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