Sunday, October 30, 2005
Mount St.Helen seems to calm down
David Sherrod, a geologist with the U.S. Geological Survey who operates out of the Cascade Volcano Observatory at Vancouver, Wash., spoke of the past, current and future conditions of the volcano in Portland State’s Department of Geology Wednesday.
Geologists say the grumbling mountain is going through a quarter-century eruption cycle, but its explosive energy seems to be petering out. It did go through two relatively big explosions this year, Jan. 16 and March 8.
Lately, the mountain has occasionally given off plumes from eruptive activity, mostly white steam and black dust. When they become substantial enough, the black dust can become hazardous to airplanes because it is hot enough to melt cockpit windshields.
Looking on the brighter side, Sherrod said, “How many times do you get to see an eruption in your lifetime?” He counted only four St. Helens eruptions in the last 200 years. Besides the big one in 1980, there was another major explosion March 18, 1983, so many Oregon and Washington residents could have seen two.
Since 1980, St. Helens has been intermittently active, mainly building lava domes and experiencing less disruptive events. From the big one in 1980 to 1986 geologists counted 20 magmatic, or lava, eruptions. These contributed to a growing new lava dome in the crater left by the 1980 disaster. A new glacier appeared surrounding the emerging dome.
From 1989 to 1991 the mountain experienced a large number of shallow earthquakes, leading mainly to gas and steam explosions.
Last year the mountain seemed to awaken with new threats. From Sept. 27 to Oct. 8 it became active enough to restrict access under a red alert. Lava began pushing to the surface on Oct. 11 and by Oct. 14, the crater seemed to be forming a new rim. After the first week, activity quieted down somewhat.
A huge lava dome, dubbed the whaleback, pushed up beginning Nov. 20, 2004. It rose higher than the Empire State building or the Wells Fargo bank building, tallest building in Portland. The whaleback was estimated to cover 450,000 square yards in the crater and be composed of 81 million cubic yards of magma.
After that, the mountain began forming three new domes, side by side, pushing the whaleback against the crater rim and cutting the glacier in half. Sherrod likened the upward pushing of the new domes to watermelons pushing against each other.
This year’s explosion March 8 sent a shower of big rocks up into the air.
“They were the size of Volkswagens,” Sherrod said. “But they didn’t get out of the crater.” They fell back down within the crater.
No one is hanging out in the crater these days although geologists are able with helicopters to place a variety of remote recording instruments in and near the crater
“If you’re a remote instrument in the crater, your life may be short,” Sherrod said.
As the months and years go by since the 1980 big one, geologists see the seismic energy fading. Gas emissions have been remaining low, suggesting the energy of the underlying magma is fading away.
“We don’t always know if an eruption is going on,” Sherrod explained. “We’re in a 25-year eruption cycle and we’re just getting to the end of it.”