Geologists believe there will be a major eruption of Washington's Mount St. Helens volcano in the weeks ahead that could signal the start of an eruptive period that lasts for 20 years.

This belief is based on signs that a giant pool of molten rock (magma) as hot as 2,000 degrees Fahrenheit has begun to stir inside the mountain and may already be moving upward inexorably pushing aside the rock that's in its way.

"What we see now tells that the magma is moving within subsurface passageways," Dr. Robert Chistiansen of the U.S. Geological Survey said by telephone from Vancouver, Wash., 40 miles from the volcano. "We don't know where this reservoir of magma is or how big it is but we know it's there."

The telltale signs of moving magma are the "harmonic tremors" in the earth near the volcano that have been picked up by seismic listening devices since late last Tuesday, five days after Mount St. Helens erupted for the first time in more than a century.

Harmonic tremors are rhythmic motions in the earth that involve an almost continuous release of seismic energy. Distinctly different from earthquakes, which strike in episodes and then stop, harmonic tremors go on for long periods, oscillating the earth at a steady frequency.

Christiansen says harmonic tremors are always the result of magma movement and usually are the first signs of an impending major eruption. The harmonic tremors at Mount St. Helens are growing more frequent, Christiansen said, another sign that the magma is not only moving but moving closer to the surface.

The tremors's accelerating pace is "all the result of ground motion related to the subterranean movement of magma," Christiansen said.

Geologists say that harmonic tremors are almost always the early warning signals of a major eruption, one that could pour hot lava through cracks in the flanks of the volcano or explode hot rock and gas at terrifying speeds through new breaks in the dome of the mountain.

"This volcano has had lava flows and explosive rock eruptions," said Dr. Meyer Rubin of USGS, who has done exhaustive analysis of the eruptive history of Mount St. Helens. "If it's a nice lava flow nobody gets hurt, but if it's a real bang we could be in for some trouble out there."

An explosive eruption would involve hot gases like sulfur and carbon dioxide carrying extremely hot pumice, a light, sponge-like rock pocked with holes left by gas bubbles. The pumice could easily cover the flank of the volcano, which themselves are covered with snow 16 feet deep.

"We could have high-speed pumice flows that would melt lots of snow and ice in the region," said Dr. Donal R. Mullineaux of USGS, who is in Washington to study the possible hazards of a major eruption of Mount St. Helen. "We could have floods and mudflows that could present some serious hazards to the region near the volcano."

Luckily, Mullineaux said, two large water reservoirs that lie at the foot of Mount St. Helens are not full right now because of routine maintenance schedules.

Should a major eruption involve lava flows, the principal hazard will be heat as the lava oozes out over the volcano's flanks. It will melt snow and ice and cause mudflows but less violently than an explosive eruption. It could pose a fire hazard to forests, though it will have cooled quite a bit by the time if reaches the mountainside timberlines.

So far, the eruptions that have shaken Mount St. Helens have poured clouds of gas and ash into the air but nothing else. The ash has gone no farther than a mile and the mudflows produced by melting snow are still on the volcano flanks.

What concerns geologists most is the change last Tuesday night from the spasmodic earthquakes that preceded the ash eruptions to the harmonic tremors that signify the movement of magma. How big is the subterranean lava pool? How hot is the Lava? How fast is it moving?

Geologists say they have no idea of the pool's size but think it's big because of the steady increase in the frequency of the tremors.

Its heat depends on the type of molten rock that makes up the pool. It could be as "cool" as 1,650 degrees or as hot as 2,000.

"All we know about its rate of movement is that the magma is moving up through old fractures in the valcano, forcing its way up," said Dr. Robert I. Tilling, director of the Volcano Hazards Program at USGS. "It's literally fracturing rock, and forcing all these earthquakes and seismic motions we've seen, to make room for itself."

No matter how fast the magma is moving, Geologists think a major eruption is not far off. Says the survey's Rubin: "There's little question the lava pool is surging, moving up and down. The critical question is what kind of eruption will the pool trigger as it moves toward the surface."

The only town threatened by a major eruption is Cougar, 10 miles southwest of the summit. The tiny village is besieged by curious tourists and campers who have more than doubled its population.

When and if a major eruption comes, it could easily be the start of a long eruptive period that could last for months and be the first in a series of eruptive episodes that last for years. The last seriously active period of Mount St. Helens began in 1831 and didn't end until 1857.

"History suggests there will be many episodes," Christiansen said, "that will keep Mount St. Helens active for 20 years or more." Adds Rubin: "The size of the magma pool determines that. If it has a decent-size pool, this thing could chug away for 20 to 30 years."