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US warns of big Mount St. Helens blast
Government scientists raised the alert level Saturday for Mount St. Helens after its second steam eruption in two days was followed by a powerful tremor. They said the next eruption was imminent or in progress, and could threaten life and property in the remote area near the volcano.
Hundreds of visitors at the building closest to the volcano — Johnston Ridge Observatory five miles away — were asked to leave. They went quickly to their cars and drove away, with some relocating several miles north to Coldwater Ridge Visitors Center, which officials said was safe.
Pierson said the volcano has released more seismic energy since quake activity began Sept. 23 than it has at any point since its devastating May 18, 1980, eruption, which killed 57 people and coated much of the Northwest with ash. But scientists expect the impending eruption to be much smaller than the 1980 blast.
A day after the volcano spewed a plume of steam and ash thousands of feet into the air, there was a very brief steam release Saturday — a puff of white cloud, followed by a dust-raising landslide in the crater. A volcanic tremor signal that came next was what prompted the heightened alert level.
The signal "was far stronger after today's steam eruption" than the tremor that followed Friday's blast, Steele said. "We were picking it up throughout western Washington and into central Oregon. Yesterday we had a very weak tremor signal."
A tremor — a steady vibration — "indicates movement of gases or fluid within the volcano," Steele said, while individual earthquakes indicate "a pounding and breaking of rock."
Saturday's tremor lasted about an hour before it was drowned out by a series of earthquakes — one or two a minute, with a maximum magnitude of "well over 2," said Tom Yelin, a USGS seismologist at the UW lab in Seattle.
Interior Secretary Gale Norton, who flew over the mountain Saturday, said the seismic activity has weakened the 1,000-foot lava dome that began forming in the volcano's crater after the 1980 eruption.
Norton said the chances of an eruption or lava flow have increased, and that the volcano most likely will see moderate ash eruptions.
"The greatest concern at this point is an ash plume and the spread of ash itself that might come from an explosion," Norton said. "This is a concern for aircraft travel."
The growing consensus among scientists is that new magma is probably entering the volcano's upper levels, possibly bringing with it volatile gases that could lead to eruptions, said Bill Steele at the University of Washington's seismic laboratory in Seattle.
Explosions from the crater could occur without warning, possibly throwing rock onto the flanks of the volcano, the USGS said in a news release. Still, scientists said the evacuation of the observatory was primarily a precaution in case of heavy ash discharge, which could make it difficult to drive.
"We still feel the risk is confined to this area," Pierson said.
No communities are near Mount St. Helens; the closest, Toutle, is 30 miles west.
Near the town, an impromptu tailgate party was under way along the shore of Silver Lake, which offers a straight-on view of the mountain from a safe distance.
Dozens of cars lined both sides of State Route 504, their festive occupants spilled out along the roadside. Folks set up lawn chairs and cameras on tripods. Radios were tuned to news reports, and a nearby pizza joint was doing a thriving business.
"I missed it the first time back in the '80s eruption and I certainly wasn't going to this time," said welder Dean Strahm, 48, of Gold Bar, who brought camping gear and a cooler and planned to stay all night. "It's a once-in-a-lifetime experience so I thought I'd come on down." The 1980 blast obliterated the top 1,300 feet of the volcano, devastated miles of forest and buried the North Fork of the Toutle River in debris and ash as much as 600 feet deep. The latest seismic activity "probably just reflects the fact that more rock needs to be broken for magma to reach the surface," said geologist Dan Dzurisin at the U.S. Geological Survey's Cascades Volcanic Observatory in Vancouver, Wash., about 50 miles south. The 1980 eruption reamed open the route to the surface, and for six years smaller eruptions piled lava into the massive dome that marks the main conduit for magma. Friday's relatively small eruption, which generated a plume of ash and smoke 16,000 feet high, was the first since a 1986 dome-building event at the volcano. Scientists had believed the recent flurry of shallow earthquakes may reflect movement of magma that came up the volcano's pipe during a 1998 swarm of quakes, but Pierson said Saturday's activity suggested at least some new magma was involved, making a larger explosion more likely. Air sampling had detected only tiny amounts of the volcanic gases that new magma produces, but scientists said the gases could be sealed inside the system or have been dissolved by water on the mountain. The volcano holds a 600-foot-deep glacier and has received several inches of rain recently. Melting of the glacier could trigger debris flows down onto the barren pumice plain at the foot of the mountain, the USGS said, noting a "very low probability" that downstream communities would be affected. Few people live near the mountain, the centerpiece of the Mount St. Helens National Volcanic Monument in the Gifford Pinchot National Forest about 100 miles south of Seattle. |
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