MOUNT ST. HELENS, Wash. - A thick column of steam drifting above the rim of Mount St. Helens reminded scientists of the volcano's dome-building activity 20 years ago.

"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," Willie Scott, a geologist with the U.S. Geological Survey, said Sunday.

The billow of steam rose early Sunday from the south side of the volcano's crater from the area where a large upwelling of rock has been growing rapidly.

Scientists believe the steam was created when part of the bubble on the south side of the dome broke off, taking some of the glacier with it. The ice melted, the water seeped down and that most likely caused the steam, said USGS geologist John Pallister.

From an airplane, the crooked plume of steam could be seen drifting at least 500 feet above the rim Sunday afternoon, dissipating a mile south of the 8,364-foot volcano.

Scientists said Sunday's steam cloud had no new ash but may have included some old ash from the 1980s.

Scott described the emission as a "very lazy conductive rise of this warm, moist air," unlike previous weeks' bursts characterized by more vigorous jetting that threw up ash, large pieces of rock and glacier ice.

Scientists made helicopter flights to collect gas-level samples and get a better look inside the crater on Sunday.

The bubble of upwelling rock has risen to at least 330 feet since scientists first spotted it Sept. 30, and has become almost as tall as the dome's 1,000-foot summit, said Pallister.

Earthquakes were less frequent and weaker by Sunday afternoon. Over the previous two days, earthquake activity had increased with quakes of magnitude 2.4 occurring 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 seismologist Tony Qamar at the University of Washington's seismic lab in Seattle.

It indicates that pressurization of the system is very uniform, which may suggest magma is constantly moving up into the system, he said. "The pressure will build up, the rock will break, and then you'll get an earthquake," he said.

Scientists cannot say exactly where the magma is, however.

The level alert remained at "volcano advisory" Sunday. Scientists have said an eruption would have to be imminent or occurring to raise the alert.

However, officials have said an eruption could occur with very little warning.

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.

Thousands of small earthquakes have shaken the peak in the Cascade Range since Sept. 23. The volcano spewed clouds of steam mixed with small amounts of old volcanic ash each day from Oct. 1 through Oct. 5.

The alert level was lowered Oct. 6. The downgrade indicated the probability of a life-threatening eruption had decreased significantly since Oct. 2, when thousands of people were evacuated from areas around the mountain.

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 paralyzed much of the inland Pacific Northwest with gritty volcanic ash.

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On the Net:

U.S. Geological Survey regional site: http://vulcan.wr.usgs.gov/Volcanoes/Cascades/CurrentActivity/