Mount St. Helens Belches Towering Plume of Ash

Mount St. Helens (at left) releases ash on March 8, 2005, as seen from Portland, Ore. AP Photo/Greg Wahl-Stephens

Updated 7:45 a.m. ET, March 9

MOUNT ST. HELENS, Wash. (AP) _ Mount St. Helens made its most significant emission in months, sending an ash cloud drifting slowly to the northeast.

The National Weather Service issued an ashfall advisory after pilots reported spotting ash higher than 30,000 feet, said National Weather Service meteorologist Danny Mercer.

The weather service received reports of light ash falling across the Yakima Valley, roughly 75 miles east of the mountain. Late Tuesday, the ash cloud continued to slowly drift east, across Yakima and Kittitas County.

The explosion happened Tuesday, 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 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.

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.

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

Peggy Johnson, a university seismologist, said there had been no increase in quake activity before the explosion.

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.

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