12:39 AM PST on Wednesday, March 9, 2005

By JIM PARKER and ABE ESTIMADA, kgw.com Staff

KGW photo

Large steam and ash plume from Mount St. Helens as visible from downtown Portland.

MOUNT ST. HELENS, Wash. – Mount St. Helens, relatively quiet for the past several months since awakening last fall, jettisoned a 36,000 foot plume of ash and steam that was visible in the Tuesday twilight for miles around.

The towering plume marked the most spectacular eruption in months.

"It was the most amazing ash plume -- by far the biggest I've seen since this activity started last September," said Stephanie Burhop, who witnessed the eruption from her home in nearby Cougar, Wash. It is the closest community to the mountain.

While the U.S. Geological Survey said the eruption was minor, Mt. St. Helens’ latest show awed Portland area, southwest Washington and Columbia Gorge onlookers, who saw a stunning view of the restless mountain, thanks to a clear, spring-like day. As far away as Umatilla, Ore. a dark cloud could be seen rising in the western horizon.

kgw.com/Stephanie Burhop

This picture of Mount St. Helens erupting was taken from Cougar, Wash.

The grey and white plume, mostly drifting to the northeast, threatened to drop ash over some areas of southwest Washington, prompting the National Weather Service to issue an ash advisory for portions of the reigion until midnight.

The heaviest amounts were possible in Skamania County, while Cowlitz County and Clark County faced a potential dusting of ash. None fell in Portland or Vancouver, said KGW meteorologist Dave Salesky.

"There's no change currently on the alert status on the mountain," said Rob Harper of the Washington Department of Emergency Management. He said no communities had requested any state assistance.

Flights to and from Portland International Airport weren't immediately affected by the eruption, said Steve Johnson, a spokesman for the Port of Portland.

kgw.com/Michael Robinson

The large plume, as seen from downtown Portland.

Geologists with the USGS said they were caught off-guard by the eruption, which began about 5:25 p.m. Scientists noticed quake activity ramping up hours before the eruption of steam and ash but didn’t think it was unusual, said Dan Dzurisin of the USGS.

"It didn't really look like anything large like this was going to happen until it actually occurred," added another USGS scientist, Carolyn Driedger.

The plume was preceeded by an earthquake of about 2.0 magnitude, said Bill Steele, coordinator of the Pacific Northwest Seismograph Network at the University of Washington.

The event knocked out three of the UW’s sensors in the crater, but other instruments around the rim of the crater remained intact.

kgw.com/Nicole Hymas

This picture of Mt. St. Helens was taken from a rooftop in Vancouver, Wash.

Steele said he did not believe Tuesday's eruption had increased the risk of a more 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," added Peggy Johnson, a UW seismologist.

For now, the USGS does not believe Tuesday's eruption was caused by part of the new lava dome collapsing. A new dome has been rising inside the crater beside the old dome built up since the 1980's.

"This eruption happened with some force," Driedger said. "...This one's different."

Lava also isn’t flowing on to the lava dome or crater, Dzurisin said.

Although the mountain appeared to have entered a stage of "placid dome building," geologists have warned that things could change quickly.

"We said this could change any time," Driedger said. "St. Helens has proved again she has lots of surprises."

The volcano in southwest Washington rumbled to life again Sept. 23 with shuddering seismic activity, pumping out lava and creating a huge new lava dome as the mountain nears the 25th anniversary of the May 18, 1980 eruption that killed 57 people.

Since the renewed volcanic acitivity last fall, scientists have advised that explosive eruptions, possibly dropping ash within a 10-mile radius of the crater, are possible at any time.

(The Associated Press contributed to this report.)