Mount St. Helens (search) released a towering plume of ash on Tuesday. The volcano has vented ash and steam since last fall, when thousands of small earthquakes marked a seismic reawakening of the 8,364-foot mountain.

Late afternoon television footage showed the plume billowing thousands of feet into the air, then drifting slowly to the northeast.

The ash explosion happened around 5:25 p.m., about an hour after a 2.0 magnitude quake rumbled on the east side of the mountain, said Bill Steele, coordinator of the Pacific Northwest Seismograph Network (search) at the University of Washington.

Steele said he did not believe the explosion had increased the risk of a significant eruption and noted that recent flights over the volcano's crater did not reveal high levels of gases.

"We don't expect another explosion," said Peggy Johnson, a UW seismologist.

Steele said the ash burst may have been triggered by partial collapse of a lava dome (search) in the crater, which has been growing steadily over the last several months.

"Until we get a better view in the crater we won't know," Steele said.

Johnson said there had been no increase in quake activity before the explosion.

"The seismicity had been continuing just as it had been," she said.

On May 18, 1980, the volcano 100 miles south of Seattle blew its top, killing 57 people and covering the region with gritty ash.

Mount St. Helens rumbled back to life Sept. 23, with shuddering seismic activity that peaked above magnitude 3 as hot magma broke through rocks in its path. Molten rock reached the surface Oct. 11, marking resumption of dome-building activity that had stopped in 1986.

Scientists have said a more explosive eruption, possibly dropping ash within a 10-mile radius of the crater, is possible at any time.