Sunday, May 20, 2007

Is Mount St.Helens entering a new phase?

Mount St. Helens' latest eruption, which began more than two and half years ago, continues to slowly wind down, and it's unlikely now that the mountain's new lava dome will peek above the crater rim anytime soon.By last summer, the hissing, steaming dome of hot rock had risen to within less than 10 feet of the height of "Shoestring Notch," a low point on the crater rim on the Southeast side of the peak.

But the dome has settled and partially collapsed, and its top now is a good 60 to 70 feet below the notch, said Tom Pierson, a U.S. Geological Survey hydrologist who will give a free public presentation about the volcano today at the Johnston Ridge Observatory."It's anyone's guess" when the dome may again approach the volcano's rim, Pierson said earlier this week. "It's expanding sideways now. It wants to build sideways as much as it wants to build up."

Friday marks the 27th anniversary of the eruption on May 18, 1980, which flattened 230 square miles of forest, killed 57 people, destroyed 200 homes, sent a cubic mile of ash around the Earth.In contrast to that blast, the dome-building eruption that began in October 2004 has been a placid affair, with new lava piling up in the crater atop and adjoining a previous dome built in sporadic eruptions from 1980 to 1986.

Between them, the two lava domes have replaced about 11 percent of the mass of the volcano that shot skyward or toppled into the Toutle Valley in 1980. At its current rate of dome growth -- now less than a cubic meter a day -- the volcano could take 180 years to replace the old summit, the USGS estimates. Scientists still don't know when the current dome-building eruption will end, though some volcanoes have had dome-building eruptions that have lasted decades.

For example, Guatemala's Santa Maria volcano has a dome that has grown more or less continuously since 1922.In the Russian Far East, Bezymianny volcano erupted in 1956 in so similar a fashion to Mount St. Helens that it's been called a "twin" to the Southwest Washington peak. However, the Russian volcano has been dome-building near continuously since the late 1950s."That dome has filled up a huge percentage of the crater. It's way above the crater rim," Pierson said of Bezymianny, which means "no name" in Russian.

For an eruption to continue for decades, scientists believe volcanoes must periodically be "refueled " with fresh batches of gas-rich molten rock from deep in the earth's mantle. Whether that's happening at Mount St. Helens is under debate, but indicators point against it, Pierson said."We're not seeing fresher, hotter magma. There is hardly any gas coming off it now. When you bring in fresh magma, the gas content goes up in what is being released."Also, he said, there are no seismic signals to indicate a deep movement of magma.When it began in October 2004, the current eruption was pumping about 10 cubic yards of lava onto the surface every second.

That rate has fallen 90 percent, Pierson said."We don't really know how much (the dome) will keep growing. It can't go on ... too many more years," Pierson said.Even if it stopped today, the latest eruption has made some major changes in the crater. For one thing, it has divided the crater glacier in two. The glacier, which formed against the crater's shaded southern wall, had been a horseshoe-shaped mass of ice and rock. The two "arms" of the glacier have flowed northward, circling the dome, currently at a rate of about a meter a day in some places.

They've reached the breach at the open end of crater. They've also moved toward one another, now separated at their northern ends by less than the length of two football fields.They may not grow much more, Pierson said. The crater glacier is among the lowest-elevation glaciers in the Cascade Range --- roughly 6,000 feet above sea level -- and the more it creeps downward the faster it will melt.

Also, the new dome has filled up the glacier's "accumulation zone" -- the niche in the back of the crater where snow and rock get compressed together to feed the glacier."At some point the glacier will run out of 'oomph' " and may not grow much farther," Pierson said.

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