Volcanic ‘wagging’ could be key to predicting eruptions

Volcanic ‘wagging’ could be key to predicting eruptions

Volcanoes such as Mount Baker shake and vibrate in distinct and predictable ways when they are going to blow because giant columns of magma “wag” back and forth inside them, researchers say
Postmedia News files
Volcanoes such as Mount Baker shake and vibrate in distinct and predictable ways when they are going to blow because giant columns of magma “wag” back and forth inside them, researchers say

Margaret Munro, Postmedia News · Wednesday, Feb. 23, 2011

VANCOUVER — Cloaked in snow and visible across the U.S. border from Vancouver, Mount Baker looks like a gentle giant.
But UBC volcanologist Mark Jellinek says Mount Baker, located in Washington state, is probably overdue for an eruption — an explosion he and his colleagues hope to predict well in advance based on the how much “wagging” goes on inside the volcano as magma rises up from the deep.
According to their research, to be published Thursday in the journal Nature, volcanoes shake and vibrate in distinct and predictable ways when they are going to blow because giant columns of magma “wag” back and forth inside them.
“It’s basically like a dog wagging its tail,” says Mr. Jellinek, except that the magma columns are up to a kilometre high.
They are so powerful they shake mountains and when they blow they can hurl hot ash up to 40 kilometres into the atmosphere, with sometimes devastating impact as the ash spreads across surrounding areas. (Vancouver is far enough from Mount Baker it will be spared the worst effect, though Mr. Jellinek says he expects the city could be covered in a thick layer of fine ash. “It would make a huge mess,” he says.)
It has long been known that volcanoes vibrate at pretty much the same frequency before they explode, whether there are in B.C., Alaska, the Caribbean or the Philippines. But until now no one has been able to explain why volcanoes that are so different in size and character behave in the same way.
“Magma wagging” is the most plausible explanation yet, and may help forecast deadly eruptions, say Mr. Jellinek and David Bercovici from Yale University and co-author of the new study.
Their model of the “magma wagging” explains why tremors in nearly all explosive volcanoes stay in a narrow band of frequencies that can be felt but are so low humans can’t hear them. Just before and during eruptions, the frequency climbs to a higher pitch, and the range spreads out.
It provides “a fundamental mechanism for tremor that is generic to nearly all volcanically explosive systems,” the researchers report.
As Mr. Bercovici put it, the shaking is both a warning “and a vital clue about what is going on in the belly of the beast.”
Mr. Jellinek said the magma columns have the consistency of toffee, can be up to a kilometre high and 50 metres across, and sway or wag several metres back and forth inside the volcano.
“It’s like a Greek column, surrounded by a layer of gas bubbles that act like springs,” he said. “Every time one of these ginormous columns wags in one direction, the bubbles push it back.” Eventually, so much pressure builds up the volcano blows its top.
Mr. Jellinek is keen to test the predictive power of magma wagging in the real world and does not expect to have to wait long, given the volcanic activity in the Caribbean, Indonesia and Alaska. There are also plenty of volcanoes along the Pacific coast including Mount Meager in B.C. and Mount St. Helens, Mount Rainier and Mount Baker in Washington state.
There can be weeks to months of warning before volcanoes erupt but some come to life quickly.
“The most recent eruption in the Aleutians in Alaska had five hours notice,” says Mr. Jellinek. “But in general we do better than that.”
Postmedia News
(http://www.nationalpost.com)

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