Mount St. Helens vented a new column of steam Sunday, a lazy plume that rose out of the crater of the snow-dusted volcano.
The billow of steam rose 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 southeast.
The venting reminded scientists of the volcano's activity 20 years ago, when it built the dome after its catastrophic 1980 eruption.
"It's a view . . . 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 vapor was rising and steam clouds were forming," said Willie Scott, a geologist with the U.S. Geological Survey.
The plume appeared to be mostly steam, and scientists said any volcanic ash probably was from eruptions in the 1980s.
The venting probably was produced by a combination of rain water percolating down to hot rocks and volcanic gas coming from deeper levels, scientists said.
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 Sunday, 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 U.S. Geological Survey's chief scientist for volcano hazards at Vancouver.