Mount Hood eruption 'style' studied

CORVALLIS, Ore., Feb. 14 (UPI) -- U.S. scientists say they've discovered why Mount Hood in Oregon doesn't have a history of explosive eruptions like so many of its Cascade Mountains cousins.

Researchers at Oregon State University say the way in which magma mixes under the volcano, which has towered over the landscape for half a million years, appears to have kept it from erupting violently over the millennia.

Studies show the volcano has never experienced an explosive eruption exhibited by other volcanoes in Oregon, Washington and across the Pacific "ring of fire" despite having similar chemical magma composition and gas content, a university release said Tuesday.

The reason, researcher Alison Koleszar said, is eruptions at Mount Hood appear to be preceded by episodes of intense mixing between magmas of different temperatures, with hotter magma rising from deep below and mixing with cooler magma underlying the volcano.

Heat from the deeper, hotter magma lowers the viscosity of the magma that eventually erupts, so instead of exploding like a Mount St. Helens, magma at Mount Hood slowly oozes out the top of the volcano.

"If you take a straw and blow bubbles into a glass of milk, it will bubble up and allow the pressure to escape," Koleszar said. "But if you blow bubbles into a thick milkshake you need more pressure and it essentially 'erupts' with more force as bits of milkshake get thrown into the air. Add a little heat to the milkshake, though, and it thins out and bubbles gently when you blow into it, more like the glass of milk.

"That what Mount Hood has been doing -- heating things up enough to avoid a major explosion."

Few other volcanoes around the world act quite like Mount Hood, Koleszar said, calling it a poster child for low-explosivity eruptions.