Steam Bursts Likely As Volcano Cools
An increase in barely detectable earthquakes beneath Mount St. Helens indicates a chance of steam bursts similar to the volcano’s last activity in 1989-91, scientists said Friday.
Two hiking trails north of the gaping crater in the Mount St. Helens National Monument were closed as a precaution, but the trail to the top of the crater wall on the south remained open.
“This closure will remain in effect until we have a clearer indication of … the potential for steam explosions, avalanches and mudflows,” monument officials said in a prepared statement.
Most of the quakes have been smaller than magnitude 1. They could be a precursor of more visible activity, such as volcanic ash clouds that could rise 20,000 feet above the volcano and drift as far as 100 miles, the statement added.
A Feb. 5, 1991, steam burst produced such a cloud.
Rocks as big as three feet in diameter and weighing hundreds of pounds could be blown from the top of the stadium-sized lava dome, and hot mudflows could stream out of the open northern end of the horseshoe-shaped crater, the statement issued by the Cascades Volcano Observatory of the U.S. Geological Survey said.
No rocks are likely to be hurled outside the crater walls on the north, east or west flanks if the volcano follows the same pattern as in 1989-91, said Steven Brantley, a geologist at the observatory.
During that period, he said, steam bursts often occurred about three days after significant rainfall.
The quake pattern before that activity began in late 1987, about two years before the first steam burst.
Since January the quakes have increased from less than 10 a month to about 100 a month, about the level reached in the spring of 1989, Brantley said.
Even so, the pattern does not indicate any likelihood of a dome-building ooze of lava into the crater, much less a blast like the one that shattered the once-pristine peak on May 18, 1980, Brantley said.
That’s because the quakes are occurring about one to six miles beneath the crater and are much weaker than the kind of tremors associated with more vigorous volcanic activity, he explained.
The latest development appears to be part of the continued cooling of the volcano since the giant blast 15-1/2 years ago that killed 57 people and devastated 230 square miles, Brantley said.
He explained that gas dissolved in the molten rock is released as it cools and crystalizes beneath the surface, building up pressure that finally explodes at the surface - a kind of volcanic belch.
The gas also could come from a fresh upward surge of molten rock, but that would normally be accompanied by low-frequency, long-period quakes about 4 to 6 miles below the surface. No tremors like those have been detected.