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Oregon State scientists examine Mount St. Helens formation

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A plume of condensation rises off Mount St. Helens, as viewed from the roof of the Cascades Volcano Observatory December 19, 2006 in Vancouver, Washington. The plume results from condensation of warm moist air rising off the growing lava dome. (Photo by Steve Schillling./The U.S. Geological Survey via Getty Images)

PORTLAND, Ore. — Oregon State University scientists may have the answer to why Mount St. Helens is out of line with other major volcanoes on the Cascade Range.

University geophysicist Adam Schultz and his team think a giant subsurface rock formation diverted magma, causing Mount St. Helens to form west of the Cascadia Arc, Oregon Public Broadcasting reported Monday.

“It seems like what we call the Spirit Lake batholith is probably the reason why Mount St. Helens actually pops up far to the west of where you would anticipate it to be,” Schultz said.

The team of researchers layered together different imaging studies to gain better insight into what’s below the volcano’s surface. They paired older studies — showing the structure, density and temperature — with newer ones that used magnetotelluric measurements, which show subsurface electrical conductivity.

Their work is being published this week in the peer-reviewed journal “Nature Geoscience.”

“Either method by itself can lead to a level of uncertainty, but when you layer them together as we have done in this project you get a much clearer picture of what lies below,” Schultz said.

Mount St. Helens formed because of the Juan de Fuca plate off the coast of Washington and Oregon went under the North America plate. The plate went past crustal blocks with marine sediments that “slammed into the continent, where they accreted,” Schultz said.

“This material is more permeable than surrounding rock and allows the magma to move through it,” Schultz said. “The big batholith acts kind of like a plug in the crust and diverted magma that normally would have erupted in line with the other major Cascade volcanoes, resulting in St. Helens forming to the west of the Cascadia Arc, and Mount Adams slightly to the east.”

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