Mt. St. Helens adds to its dome.

Mt. St. Helens adds to its dome

Since mid-September, Mt. St. Helens in southwestern Washington had been advertising its presence. As its grumblings and gas emissions headed for a crescendo, an early-October internal avalanche belched ash over the landscape for as far as 45 miles. By Oct. 16, scientists from the U.S. Geological Survey (USGS) in Vancouver, Wash., and the University of Washington in Seattle issued a volcano advisory warning that they expected "an episode of rapid lava dome growth" within three weeks.

On the 21st, seismic levels were rated as "very high," just one notch below the maximum classificiation of "extreme." And geologists observed deformations in the lava dome, a sure signal that a lot was going on underneath. A day later, the advisory was updated: The episode of dome growth was expected to occur in three to five days. Finally, on the 23rd, lava broke through the dome, and a 100-by-200-meter front of slow-moving lava was seen on the dome's west side, where it has since stopped and settled. By early this week it was clear the dome-building eruption had ended. The mountain had regained its calm and USGS officials lifted the volcano advisory.

The seismic activity, gaseous emissions and deformation in the lava dome that previewed the eruption, says USGS geologist Patrick Pringle, mostly fit the pattern observed in the 16 or so earlier dome-building eruptions. He says the dome gained 82 feet during this latest eruption, rising to a height of 918 feet. Christine Jonientz-Trisler, a University of Washington seismic analyst, says she agrees for the most part but was surprised by a sudden disappearance of seismic activity on Oct 7. "It was peculiar that the earthquakes just shut off," she says. During the last few eruptions, it has taken increasing amounts of energy to push the magma, or molten rock, through the dome, according to Pringle and Jonientz-Trisler. Scientists are still uncertain, they say, just why this is the case.