MOUNT ST. HELENS, Wash. (AP) - Mount St. Helens gave off more steam Monday morning, with a small cloud drifting up from the snow-dusted crater as the sun was rising.
Within the mountain, however, seismic activity remained relatively low, said Jeff Wynn, chief scientist for volcano hazards at the U.S. Geological Survey's Cascades Volcano Observatory.
On Sunday, a crooked plume of steam rose at least 500 feet above the rim of the crater, dissipating as it drifted about a mile south of the 8,364-foot-high volcano.
Scientists believe the steam was created when part of a bulge or bubble broke off on the south side of the dome-shaped rock formation inside the crater, taking along some of the glacier in the area. The steam was mostly likely created as water from the melting ice seeped down to hot rock layers, said USGS geologist John Pallister.
Scientists said Sunday's steam cloud may have carried some old volcanic ash from the 1980s, the last time the mountain erupted.
Willie Scott, a geologist with the U.S. Geological Survey, described Sunday's emission as a "very lazy conductive rise of this warm, moist air," unlike previous weeks' vigorous bursts that threw up ash, large pieces of rock and glacier ice.
Scientists 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 paralyzed much of the inland Pacific Northwest with gritty volcanic ash.
The bubble of upwelling rock has risen to at least 330 feet since Sept. 30, Pallister said.
Earthquakes were less frequent and weaker on Sunday, following two days of increased activity when quakes of magnitude 2.4 happened every two minutes.
"What has been peculiar about these earthquakes is that there seems to be a disproportionate number of them that are uniform in size," said University of Washington seismologist Tony Qamar.
It indicates that pressurize 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," he said.
Scientists cannot say exactly how far the magma has risen.
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.
On the Net:
Mount St. Helens National Monument: http://www.fs.fed.us./gpnf/mshnvm/
U.S. Geological Survey: http://vulcan.wr.usgs.gov/Volcanoes/MSH/framework.html
Volcano Web Cam: http://www.fs.fed.us/gpnf/volcanocams/msh/