Pillowy clouds of steam gushed out of Mount St. Helens on Monday in the volcano's largest eruption since it woke up 10 days ago from years of slumber.

Scientists said that molten rock is pushing upward, like a piston, inside the volcano. It is venting hot gases that are vaporizing ice and snow, causing periodic plumes of steam and some ash. A large eruption of rock and ash is possible at any time, they said, but it will almost certainly be smaller and far less destructive than the 1980 eruption at Mount St. Helens that killed 57 people.

"We know magma is close," Tom Pierson, a geologist with the U.S. Geological Survey, told reporters. "We know it is deforming the crater floor in a drastic way. We are detecting gases that give away the presence of that magma."

There has been a steady buildup of pressure inside the mountain in the past few days, Pierson said. But he and other scientists say there is no way to be certain when -- or if -- magma will explode through the 1,000 feet of loose volcanic rock that forms a dome and plugs the top of the volcano's mile-wide crater.

On Monday, there was also a small earthquake beside Mount Hood, a towering volcanic peak near Portland, Ore., and about 60 miles south of Mount St. Helens. Seismologists from the University of Washington said the earthquake was unrelated to the flurry of seismic activity at Mount St. Helens, which is one of about two dozen volcanoes in the Cascade Range.

But the tremor in Oregon was a reminder that the Pacific Northwest is a geologically unstable region where several huge volcanic mountains -- nearly all of them draped in snow and glacial ice -- loom near population centers.

In that row of volcanic peaks, which extends 1,000 miles from northern California to southern British Columbia, the most active is Mount St. Helens. But it is in an unpopulated region of southwest Washington, and the cataclysmic eruption of 1980, which blew away nearly a half billion tons of earth, essentially gutted the volcano and defanged much of its destructive potential.

By far the greatest volcanic threat to human life along the entire West Coast is presented by Mount Rainier, a 14,410-foot monster of a mountain that has more glacier ice on it than all the other Cascade volcanoes combined.

Rainier is just 87 miles from Seattle and 65 miles from Tacoma, which together form the largest population center in the region. Even more at risk are about 150,000 people who live in a chain of several smaller towns built on or near historic mudflows from previous eruptions on Mount Rainier.

"Any kind of eruption from Rainier would always cause more concern than what is happening at Mount St. Helens," said Tony Qamar, a seismologist at the University of Washington.

USGS scientists have been warning in recent years that Rainier, which is an active volcano but hasn't produced a major eruption for about 500 years, has an unpredictable record of spontaneous activity that triggers massive mudflows, or lahars.

The shoulders of the mountain are believed to be fragile, with a high potential for a collapse, either from an earthquake or a volcanic eruption. That could send huge volumes of snow, ice and mud all the way down to ports on Puget Sound, scientists warn.

In the town of Orting, about 30 miles downstream from Rainier, residents would have about 40 minutes to get to higher ground before mudflows would sweep the town away, the USGS predicts.

In recent days, though, volcano experts in the Northwest have turned all their attention to Mount St. Helens, which, in its current state, offers the relatively benign prospect of a sizable eruption without sizable human consequences.

Qamar said he went for a hike last fall on the volcano's dome. "It is generally an unstable pile of rocks, and you pass by steam vents every once in a while," he said, referring to the structure that was formed during the 1980s in a series of eruptions that followed the big blow.

In the past 10 days, pressure from rising magma and gas deep in the volcano has pushed up the south corner of that unstable pile of rocks about 100 feet, according to the USGS. Under that pressure, the dome has been creaking and cracking and has destroyed some USGS measuring devices. "We believe that about 10 million cubic yards of rock has been uplifted," Qamar said. "That is roughly equivalent to about 1 million cement trucks worth of stuff being pushed up."

Qamar and other scientists say they do not believe that anywhere near that much rock (the cooled magma from previous eruptions) is likely to be blown out of the mountain's crater.

If there is a major eruption, scientists said, the most likely consequence for residents of the Pacific Northwest would be a large plume of ash that could drift over cities and towns, depending on wind direction.

The eruption at Mount St. Helens was mostly steam and ash. The area is essentially uninhabited.