SEATTLE – Molten rock that has been rising inside Mount St. Helens (search) after weeks of earthquakes and steam eruptions has finally pushed its way to the surface, forming a new lava dome just behind the existing one in the volcano's crater.
The quakes subsided as the new lava emerged Monday and cooled in the open air, suggesting molten rock from deep inside the Earth had found the path of least resistance by going around the old dome, Jon Major, a hydrologist with the U.S. Geological Survey's Cascades Volcano Observatory (search) in Vancouver, Wash., said Tuesday.
Unlike the dramatic rivers of red-hot lava (search) from Hawaii's volcano, St. Helens' extrusion of new rock was subtle and difficult to see from outside the crater. A lazy plume of steam rose slowly from the mountain for much of Tuesday.
The last dome-building activity at St. Helens began in the months after its deadly May 1980 eruption and lasted six years. Layers of emerging rock gradually formed a rocky dome nearly 1,000 feet tall at the center of the crater floor. The top of the new dome is almost level with the old one just to the north.
The mountain had been shaking since Sept. 23, with periods of sharp jolts — up to magnitude 3.3 — occurring as often as four times a minute at the height of seismic activity.
"The inference was that those were breaking a pathway" through rock, Major said.
Explosive eruptions are still possible and often follow lava extrusion, said John Pallister, a volcanologist with the USGS.
The 1980 eruption left 57 people dead, leveled trees for miles around and covered much of the Pacific Northwest with ash. It was "barely a five" on the eight-level Volcanic Explosive Index (search), Major said.
At this point, scientists believe there is a 10 percent chance of a level four or larger eruption at the 8,364-foot mountain, he said. The area immediately around the mountain is closed.
Any explosive eruption would likely go straight up, Major said, blowing ash and steam tens of thousands of feet high. That could cause concern for aircraft and cars in the area, but nothing like 1980's lateral blast.