MOUNT ST. HELENS, Wash. -- Mount St. Helens vented a new column of steam yesterday from an area where a bulge of rock has been growing inside the volcano's crater.
The plume rose several hundred feet above the 8,364-foot volcano, and a light wind slowly blew it toward the south-southeast.
The steam emission followed an increase in earthquake activity over two days, with quakes of magnitude 2.4 occurring every two minutes until yesterday, when the vibrations were more frequent but weakened to a magnitude of 1 or less.
The venting reminded scientists of the volcano's activity 20 years ago, when it built the dome of rocks after 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 vapor 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 included was probably from past eruptions during the 1980s.
The venting probably was produced by a combination of rainwater percolating down to hot rocks and volcanic gas coming from deeper levels, scientists said. Scientists contend the earthquakes are a sign that magma is moving up in the volcano.
''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 Oct. 5 eruption that sent a thick gray cloud thousands of feet into the air and dusted some areas northeast of the volcano with gritty, abrasive ash.
Geologists do not expect anything similar to the May 18, 1980, blast that killed 57 people, blew 1,300 feet off the peak, and covered much of the inland Pacific Northwest with ash.
Since Sept. 23, thousands of small earthquakes have shaken the Cascade Range peak. The volcano has vented clouds of steam carrying small amounts of old volcanic ash each day from Oct. 1 through Oct. 5. Thousands of people were evacuated from areas around the mountain on Oct. 2.