As if on cue, Mount St. Helens on Tuesday morning spat up another towering cloud of steam and ash, just as it had the previous morning during a government news briefing about the volcano.

As the volcano spewed and a U.S. Geological Survey scientist spoke -- both of them live on split-screen television for the second day running -- it became clear that the Pacific Northwest was in for an anxious season of intermittent small eruptions, ash alerts and a wait of unpredictable length for something more spectacular.

"People need to get used to the concept" of the mountain erupting, Jacob B. Lowenstern, vulcanologist for the USGS, said at the briefing. "Mount St. Helens has started into a period of activity, and that period may last weeks; it may last months."

He said that there probably will be a significantly larger eruption than has occurred since Mount St. Helens woke up 11 days ago, after nearly six years of quiescence. Since last Friday, there have been several steam and ash eruptions, the largest of which occurred Tuesday.

Scientists continue to assure the public that it is highly unlikely that the mountain, the most active volcano on the West Coast, could again explode with the cataclysmic force that in 1980 killed 57 people and sprayed ash across much of the United States. Such an explosion is all but impossible, since much of the mountain has been blown away.

Instead, Lowenstern spoke Tuesday of lesser consequences: a narrow column of rising magma, or molten rock, that seems to be moving toward the volcano's dome and is creating steam when it encounters water, snow and ice.

Pressure from the rising magma has deformed about 20 percent of the volcano's dome, he said, adding that "it is very possible that things are breaking up" in the 1,000-foot-deep dome. That dome was formed by about six years' worth of relatively small eruptions of magma that occurred inside the mountain's mile-wide crater after the 1980 explosion.

"We may be in for a period of dome-building," Lowenstern said, explaining that at some point, the rising magma may extrude through the existing dome, cool and turn to rock.

That would take time.

Mount St. Helens has had several recent eruptions, the largest of which occurred Tuesday.