Recent plume surprises scientists

VANCOUVER, Wash. – Scientists at the U.S. Geological Survey’s volcano observatory were getting ready to head 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 a magnitude of 2.0.

Outside the building, the mountain located some 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. This time the mountain’s bark was worse than its bite.

Compared to the eruption that killed 57 people on May 18, 1980, the plume “is really small potatoes,” Major said. Tuesday’s emission lasted for roughly 10 minutes, compared to the eruption 25 years ago that went on for nine hours.

The outpouring began with practically no warning at around 5:25 p.m., about an hour after the 2.0 magnitude 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 several 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 noted that in the hours preceding the incident, the seismograph readings had changed. While the peaks, indicating the strength of each seismic burst, were no higher than normal, the line separating them had become “noisier,” said Major.

“Usually that line is nice and flat. Instead it was creeping up and getting noisier and noisier,” he said, drawing on his pad a vertical squiggle that became darker as he drew it out horizontally.

But volcanologists had seen that pattern before when it did not lead to such a large belch.

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,” said Major. “If so, the plume would have risen more lazily.”

Nevertheless, scientists described the event as “just another beat,” in the volcano’s escalating drumroll, said Peter Frenzen, monument scientist for the U.S. Forest Service at Mount St. Helens.

Volcano enthusiasts rushed to get to the mountain Wednesday even though the massive plume had been replaced by a wispy cloud.

“It’s been twice in a row now that the mountain has blown before I could get up here,” said 31-year-old Josh White, who drove up from Astoria, Ore. “I do think if it goes big this is going to be it – but what a way to go.”

Brian Atchison, 31, of Pittsburgh, Pa., agreed: “The potential for it to go off at any time is amazing.”

Nathan Levans, also 31, of Olympia, shared that he had recently had a dream about nearby Mount Rainier erupting.

“It’s exciting, but it’s slow moving,” he said of the real-life plume. “It’s not like ‘80.”

Scientists will spend the next few days combing through the hours of data just before the plume to see if they missed any markers. They also intend 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.

Geologists have said there is little chance of anything like the massive explosion that removed the top 1,243 feet of the mountain in 1980. But the Forest Service has closed an area in a 5-mile radius around the cone to foot traffic. The area includes the Johnston Ridge Observatory, which officials had hoped to reopen in May for the 25th anniversary of the 1980 eruption. Officials said their plans are on hold.

“The volcano’s really in charge at this point,” said Todd Cullings, assistant director of Mount St. Helens visitor centers.