MOUNT ST. HELENS — Part of the lava dome in Mount St. Helens' crater has risen 50 to 100 feet since Tuesday but earthquake activity remained low, indicating molten rock is moving upward with little resistance, scientists said.
"You can think of it as if the skids are greased," U.S. Geological Survey volcanologist Jake Lowenstern said. "In some ways, it lessens the immediate likelihood of large explosions."
The dome's south side has been rising for the past week, about 250 feet so far, and is now nearly as tall as the dome's 1,000-foot summit, USGS geologist Tom Pierson said Thursday.
Despite the swelling, scientists said there was no reason to raise the alert level around the 8,364-foot volcano, although they said there is a "small but real" chance an eruption could send ash 15 miles into the atmosphere.
"This is probably going to be a prolonged period of activity that involves both steam and ash eruptions and lava growth," said Larry Mastin, a USGS expert in the physics of volcano eruptions.
The new period of volcanic unrest could last for months or longer, he said, adding there is no indication that any eruption is imminent, or that one would be powerful enough to threaten lives or property.
There's no way to tell when magma — molten rock — might reach the surface, Lowenstern told a news conference at the Cascades Volcano Observatory in Vancouver, Wash., Thursday.
Earthquake activity remained relatively low early today.
An inch or more of rain forecast for today could trigger small debris flows in the crater that could flow out onto the pumice plain north of the volcano, scientists predicted this morning.
Water seeping into cracks is also creating steam as it hits hot rocks.
Since Sept. 23, thousands of small earthquakes have shaken the peak in the Cascade Range. Each day from Oct. 1 through Tuesday, Mount St. Helens spewed clouds of steam mixed with small amounts of old volcanic ash.
An hour-long blast Tuesday morning was the largest, sending a thick gray cloud thousands of feet in the air and dusting some areas in Lewis County northeast of the volcano with gritty ash.
Seismic activity began diminishing afterward, and geologists said the most likely scenario now is for weeks or months of occasional steam blasts, and possibly some eruptions of fresh volcanic rock.
"It's not stopped," Lowenstern said Thursday. "We can see that now. Magma is continuing to push, rocks are continuing to deform."
The volcano could be entering a phase of lava dome building, Mastin said.
On Wednesday scientists downgraded the ”volcano alert” to a ”volcano advisory,” indicating the probability of a life-threatening eruption had decreased significantly since Saturday, when thousands of people were evacuated from the visitor center closest to the mountain.
The risk has decreased, but not ended, scientists cautioned.
”Escalation of unrest could occur suddenly and perhaps lead to an eruption with very little warning,” officials at the Mount St. Helens Joint Information Center warned.
Closures around the volcano — including the Johnston Ridge Observatory five miles north of the crater — remained in effect. Johnston Ridge is usually closed for the winter by the end of October.
Geologists have said there is little chance of anything similar to the blast that blew 1,300 feet off the top of the peak, killed 57 people and paralyzed much of the inland Pacific Northwest with gritty volcanic ash on May 18, 1980.
Even if a major eruption did occur, Mastin said, it would not do as much damage as the 1980 blast. ”The volcano has already fallen down,” Mastin said.
On the Net:
U.S. Geological Survey regional site: http://vulcan.wr.usgs.gov/Volcanoes/Cascades/CurrentActivity/
