The size and frequency of more than 2,000 tiny earthquakes that have hit Mount St. Helens since Thursday suggest some sort of explosion is possible.
“We don’t really know where this is going, but the chances of having some kind of eruption are now significant,” said Steve Malone, director of the Pacific Northwest Seismographic Network.
Early tests of gas samples collected above the volcano by helicopter Monday did not show unusually high levels of carbon dioxide or sulfur.
“This tells us that we are probably not yet seeing magma moving up in the system,” said Jeff Wynn, chief scientist with the U.S. Geological Survey in Vancouver, Wash. He noted additional tests are necessary to better define whether there’s magma moving under the mountain’s crater.
Scientists initially thought the volcano was just blowing off some steam, something it has done occasionally since the devastating eruption of 1980, when 57 people were killed.
In 1986, Mount St. Helens was hit with the last in a series of post-1980 eruptions that built a 925-foot tall dome of hardened lava over the mouth of the volcano.
Since then, steam explosions have occurred as heavy rain or melting snow works its way under the dome.
When the water hits hot rocks just below the surface, it turns to steam. The steam pressure builds under the lava dome, building up enough to explode, sending rocks flying around the volcano’s crater.
This summer’s wet ending could have provided the moisture needed for just such an explosion.
“Past explosions have launched rocks up onto the crater rim,” said Todd Cullings, assistant director of Mount St. Helens National Volcanic Monument’s Johnston Ridge Observatory.
Steam explosions are not considered big threats.
But the size and frequency of the earthquakes have some seismologists believing that a bigger event is possible.
About two earthquakes per minute shook the mountain on Sunday and Monday, hitting magnitude 2 to 2.8.
“This all happened after we had a ton of rain come into the system. What we’re thinking is, if anything is going to come out of the mountain, it’s going to be one of these steam explosions,” said Seth Moran, a seismologist with the U.S. Geological Survey’s Volcano Observatory in Vancouver.
Carbon dioxide and sulfur gas samples collected above the volcano will help scientists figure whether the rash of earthquakes is the result of water seeping into the mountain or magma moving beneath its crater.
In either case, scientists will continue to watch it from the Cascade Volcano Observatory, located about 50 miles away.
A helicopter carried scientists and instruments over the crater Monday afternoon, to assess the gases and ground deformation that would indicate pressure building below the dome.
Measurements of ground movement “will tell us whether there’s any new magma coming into the system,” Moran said.
The USGS issued a notice of volcanic unrest on Sunday, citing “an increased likelihood of a hazardous event.” U.S. Forest Service officials closed hiking trails above the tree line at 4,800 feet on the 8,364-foot mountain, though the visitor’s center and most other trails at the Mount St. Helens National Monument remained open.
In the event of an explosion, concern would be focused within the crater and on the upper flanks of the volcano, survey geologist Willie Scott said. A five-mile area, primarily north of the volcano, could receive flows of mud and rock debris.
The Associated Press contributed to this report.
Reporter Lukas Velush: 425-339-3449 or lvelush@heraldnet.com.
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