Mt St Helen's vibrates with weak quakes
Mount St Helens has vented a new column of steam, a lazy plume that rose out of the crater of the snow-dusted volcano.
The billow of steam rose yesterday from an area where a large upwelling or bulge of rock has been growing on the dome-shaped formation of rock in the crater. The plume rose several hundred feet above the 8,364-foot volcano, and light wind slowly blew it toward the south and south-east.
The venting reminded scientists of the volcano’s activity 20 years ago, when it built the dome following its catastrophic 1980 eruption.
“It’s a view very, very reminiscent of the years in the 1980s during dome-building and a few years after when the system was hot and water was being heated and vapour was rising and steam clouds were forming,” said Willie Scott, a geologist with the US Geological Survey.
The plume appeared to be mostly steam, and scientists said any volcanic ash that was included was probably from past eruptions during the 1980s.
Scientists believe the steam was created when part of the bubble of rock on the south side of the dome broke off, taking part of a glacier in the volcano with it. The ice melted, the water seeped down and that most likely caused the steam, said USGS geologist John Pallister.
Scott described the emission as a “very lazy conductive rise of this warm, moist air”, unlike previous weeks’ bursts characterised by more vigorous jetting that threw up ash, large pieces of rock and glacier ice.
The steam emission followed an increase in earthquake activity over the previous two days, with quakes of magnitude 2.4 occurring every two minutes until yesterday, when the vibrations were more frequent but weakened to magnitude 1 or less.
“What has been peculiar about these earthquakes is that there seems to be a disproportionate number of them that are uniform in size,” said seismologist Tony Qamar at the University of Washington’s seismic lab in Seattle.
It indicates that pressure in the system is very uniform, which may suggest magma is constantly moving upward, he said. “The pressure will build up, the rock will break, and then you’ll get an earthquake,” Qamar said.
“Exactly where the magma is, since we don’t have visuals, we just can’t say,” said Jeff Wynn, the US Geological Survey’s chief scientist for volcano hazards at Vancouver.
Seismic activity on Saturday was equal to or higher than levels during the October 5 eruption that sent a thick grey cloud thousands of feet into the air and dusted some areas north-east of the volcano with gritty, abrasive ash.
Activity is expected to ebb and flow, and the most likely scenario now is weeks or months of occasional steam blasts and possibly some eruptions of fresh volcanic rock. Officials have cautioned, however, that an eruption still could occur with very little warning.
Geologists do not anticipate anything similar to the May 18, 1980, blast that killed 57 people, blew 1,300 feet off the top of the peak and covered much of the inland Pacific North-West with ash.
Since September 23, thousands of small earthquakes have shaken the peak in the Cascade Range. The volcano vented clouds of steam carrying small amounts of old volcanic ash each day last week. Thousands of people were evacuated from areas around the mountain on October 2.
