St. Helens Quiet After Big Burp Tuesday
"It just kept on going and going and going," said research hydrologist Jon Major, referring to the seismic line which registered Tuesday's earthquake measuring a magnitude of 2.0.
Outside the building, the mountain - 50 miles northeast - was belching steam and ash seven 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 blast which killed 57 people on May 18, 1980, the plume "is really small potatoes," said Major. Tuesday's emission lasted for roughly 10 minutes, compared to the eruption 25 years ago which went on for nine hours.
The outpouring began with practically no warning at around 5:25 p.m., about an hour after the 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. It also may have created a new vent.
Scientists did not know what had caused the larger-than-normal plume, but noted that in the hours preceding the incident the seismic 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 tourists rushed to the mountain to snap pictures, hoping to capture the drama up close.
Brian Atchison, 31, was visiting Washington from Pittsburgh, Penn., and took the opportunity to drive up to the last outlook open to visitors at Mount St. Helens. "It's cool to watch," he said. "It's remarkable. The potential for it to go off at any time is amazing."
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.
As a precaution, officials with the U.S. Forest Service said they had 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 the U.S. Forest Service's visitor center at Mount St. Helens.