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Magma spends most of its existence as sludgy mush

Crystals open window into volcanic depths

UNDER THE VOLCANO  Mount Hood looms over Oregon’s White River Valley. A study of magma that poured into this valley around 220 years ago shows that the magma spent most of its long existence under the mountain in a cold, semisolid state.

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When red-hot magma spews from a volcano, it is clearly flowing like a liquid. But before erupting, that same magma may have spent hundreds of thousands of years in a chunky state resembling cold porridge, a new study finds. The discovery suggests that hot, liquid magma pools may be useful as an indicator of impending eruptions.

Volcanic magma, the molten mix of rock and gas below Earth’s surface, leads a sort of double life: It spends some time as a hot liquid and some as a colder, rocky “crystal mush.” Scientists have long known that magma is mostly liquid only above 750° Celsius, and that in its colder state, it is too viscous to flow.  Whether magma spends much of its existence in the hot, largely liquid state or heats up briefly before erupting has been a mystery.

Seeking answers, Kari Cooper of the University of California, Davis and Adam Kent of Oregon State University in Corvallis analyzed crystallized minerals in cooled magma that spewed from the two most recent eruptions of Oregon’s Mount Hood, which occurred around 220 and 1,500 years ago. Some of these crystals contain forms of uranium and thorium that radioactively decay to other elements. By measuring the amounts of uranium and thorium in the minerals, the scientists determined that the crystals began growing inside Earth tens of thousands, and possibly hundreds of thousands, of years ago.

The team then sought to determine how much time the crystals had spent above 750°. Certain elements like strontium diffuse into crystals only at high temperatures, so by measuring the amount of strontium in the crystals the scientists could estimate how long the magma had been hot. The scientists also measured the crystals’ sizes to see how long they had spent growing; the crystals the team studied grow only above 750°. “We're getting kind of a window into what's happening below the surface,” says Cooper.

From both the strontium and the size measurements, Cooper and Kent found that the crystals could have been above 750° for a few thousand years at most; the rest of the time, they were in cold, mushy storage. The scientists conclude February 16 in Nature that the magma was hot and liquid for at most 12 percent of the time it spent under Mount Hood and probably for less than 1 percent.

Cooper and Kent now want to confirm that other volcanoes also have cold, mushy magma beneath them. If scientists can use imaging techniques to find liquid magma beneath a volcano, they may now have a clue the volcano could soon erupt, the authors say. But magma can also heat and cool without the volcano erupting, so just finding a liquid magma pool is not enough to forecast an eruption, says George Bergantz, a geologist at the University of Washington in Seattle.

Still, Bergantz says he was “tremendously excited” when he read the paper. He says he called in his grad students and told them: “Check this out, this is going to change the way we do things.”

Editor's Note: This story was updated on March 4, 2014, to correct the approximate date of Mount Hood's most recent eruption.

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