Eruption surprised volcano observers

12 March 2005 | 16:04 Code : 4794 Geoscience events
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Scientists at the U.S. Geological Survey's volcano observatory were getting ready to go home when the squiggly line on the computer that tracks seismic activity at Mount St. Helens suddenly turned a solid black.
Scientists at the U.S. Geological Survey's volcano observatory were getting ready to go home when the squiggly line on the computer that tracks seismic activity at Mount St. Helens suddenly turned a solid black.

"It just kept on going and going and going," said research hydrologist Jon Major, describing the seismic line that registered Tuesday's earthquake measuring magnitude 2.0.Outside the building, the mountain, 50 miles away, was belching steam and ash 7 miles high, signaling the most powerful blast since Mount St. Helens reawakened last fall.

Volcanologists said they were surprised but not too worried.Compared with the eruption that killed 57 people May 18, 1980, the plume "is really small potatoes," Major said. Tuesday's emission lasted about 10 minutes; the eruption 25 years ago lasted nine hours. The eruption began with practically no warning at 5:25 p.m., about an hour after the magnitude-2.0 quake registered on the east side of the 8,364-foot volcano, said Bill Steele, coordinator of the Pacific Northwest Seismograph Network in Seattle.The blast of ash and gas was not powerful enough to visibly scar the surface of the crater, but it was strong enough to destroy monitoring stations used by scientists to keep tabs on the mountain, officials said.

Scientists did not know what caused the larger-than-normal plume, but said that in the hours preceding the incident, the seismograph readings had changed. Although the peaks, indicating the strength of each seismic burst, were not higher than normal, the line separating them had become "noisier," Major said.What scientists do know is that the plume rose very rapidly and much higher than in previous months. That indicates that there was an explosive element inside, rather than just a collapse of the crater's roof.

"The fact that it rose so fast and so high means it's not just a simple collapse of the lava dome," Major said. "If so, the plume would have risen more lazily."Scientists will spend the next few days combing through the hours of data before the plume to see if they missed any markers. They also plan to gather ash samples near the crater to study its rock chemistry and to determine if the composition of the magma has become richer in explosive gases.


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