Lava source runs deep, USGS says
Saturday, March 12, 2005 8:23 AM PST
By Courtney Sherwood
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E-mail this articleA new stream of molten rock has been feeding the earthquakes and eruptions shaking Mount St. Helens since September, U.S. Geological Survey scientists said Friday.
For months researchers have wondered if the volcano's recent activity might be linked to the series of nonviolent eruptions that built an older lava dome from 1980 to 1986.
Ash analyzed Friday gave scientific sleuths definitive evidence that the new lava dome is not related to its older counterpart, however, said Carolyn Driedger, USGS hydrologist and outreach coordinator.
The gray ash, which landed on the volcano's flanks after Tuesday's eruption, is pure dacite, Driedger said.
Dacite is a form of lava rich in silica. Its presence is a sign that the molten rock feeding the recent eruptions, called magma, is fresh and comes from a deep source.
If the magma had been stewing since the 1980s, it would contain less silica, Driedger said.
Scientists believe that a straw-like channel is sucking magma to the crater surface from a well that's 3.5 miles to 5.5 miles deep, Driedger said.
The "straw" is 200 to 300 feet across, she said.
Since October, magma has been oozing up the straw and spewing out to form the new lava dome, nicknamed "whaleback" by scientists because of its smooth, arcing surface.
By Feb. 1, the whaleback rose 1,400 feet above the 1980 crater floor and 500 feet above the top of the older lava dome. It's even bigger now, Driedger said, but scientists don't expect to have new measurements of the whaleback until next week.
The volume of the new lava dome is more than 50 million cubic yards, but it still has a way to go before catching up with its 97 million-cubic yard twin.
Scientists have no idea how deep the well of underground magma could be, so they can't predict how big the new lava dome may eventually become, Driedger said.
Tuesday's eruption and other explosions of ash and rock also came out of the straw-like channel, Driedger said.
"When you look at the ballistics and the ash that came out, it all goes back to the same location," she said.
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