Sunday, April 10, 2005

Dome formed by last volcano eruption crashed

Mount St. Helens' once-smooth lava dome has fractured, sending cascades of rock falling into ashy piles on the crater floor, scientists said Thursday.Even as it breaks apart, however, the dome continues to grow. At its current rate, the gradually erupting dome could fill in Mount St. Helens' crater within 80 years, U.S. Geological Survey geologist Tom Pierson confirmed.The new lava dome, made of cool volcanic rock on the outside with oven-hot rock inside, has been growing since October. Last time scientists measured it, in February, the new dome was roughly half the size of an older dome that formed during a series of eruptions from 1980 to 1986.Before clouds and rain obscured the view of Mount St. Helens in March, the growing dome had a smooth gray surface, nicknamed "whaleback" because of its appearance.

Since Sunday, however, larger-than-typical earthquakes, several with a magnitude of above 3.0, had scientists guessing that the dome might be fracturing.Clear skies Wednesday proved them right. The overall shape of the whaleback dome is the same, but now its surface is rougher, Pierson said. "There are big chunks and slabs of rock sliding off the dome and causing small ash flows and hot avalanches of rock and dust," he said.The force of the constant eruption may have contributed to the fractures, with newly erupting rocks squeezing the lava dome against Mount St. Helens' crater wall.Other fractures are probably caused by cold weather, Pierson said. The hard rock on the outside of the dome is chilled by snow and cool air, but rock deep inside the dome is hot, possibly even molten. Just as an ice cube fractures when dropped into warm water, the collision of hot and cold is creating cracks."It's also cracking because it is getting taller," Pierson said. "The taller it gets, the steeper the sides of the dome and the more that falls off."Clouds and rain were back Thursday, once again obscuring scientists' view of the mountain. During the brief break in the weather, however, scientists were able to lower four new instruments into the volcano's crater."We're collecting a lot of data," Pierson said. Now scientists are waiting for their next clear day, so they can corroborate their instruments' latest measurements with the scene inside the crater.

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