The 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 at the University of Washington.

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

Hours after the burst, the Weather Service received reports of light ash falling across the Yakima Valley and across Yakima and Kittitas County, inlcluding the cities of Ellensburg, Yakima and Toppenish.

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.

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 Tuesday's 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.

"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.

Geologists have said there is little chance of anything like the massive explosion that blew off the top of the peak on May 18, 1980, 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.

UW seismologists said Tuesday's explosion destroyed three sensors in the volcano's crater, but that other instruments around the rim of the crater remained intact.

Several messages with U.S. Geological Survey scientists were not immediately returned Tuesday.