Unusual rhythmic tremors shook Mount St. Helens and the volcano spat bigger plumes of gas and ash into the sky yesterday, convincing scientists that a lava eruption is likely.

In Olympia, Gov. Dixy Lee Ray declared a state of emergency last night, saying rapid evacuation might be necessary because of ominous growlings from the volcano.

"We do know the magma [molten material] is moving" beneath the volcano, geophysicist Bob Christiansen of the U.S. Geological Survey told reporters. "The harmonic tremor [recorded late Wednesday] makes it seem likely we will go to a magmatic eruption.

"We've always said that was a likely possibility. It seems even more likely now, although not necessarily today or tomorrow. It could be a week or a period of weeks or more."

He said scientists cannot tell exactly where the hot, molten rock is in the 9,677-foot mountain, which began erupting on March 27 after 123 years of quite. University of Washington scientists estimated the lava could be about three miles below sea level.

A lava flow could burn trees on the flanks of the mountain. Depending on its location, a flow might also melt snow and ice and trigger mud flows.

Meanwhile, one of the many small airplanes full of sightseers that have flown over the volcano was forced to crash-land about 10 miles northwest of the mountain, injuring all four occupants, one of whom suffered a broken ankle.