Mount St. Helens (search), the volcano that blew its top with cataclysmic force in 1980, erupted for the first time in 18 years Friday, belching a huge column of white steam and ash after days of rumblings.
"This is exactly the kind of event we've been predicting," said U.S. Geological Survey (search) scientist Cynthia Gardner.
Still, the eruption was nowhere near what happened 24 years ago, when 57 people were killed and towns 250 miles away were coated with ash.
About 20 minutes after Friday's eruption, the mountain calmed and the plume began to dissipate.
The eruption was so short-lived that the ash appeared to pose no threat to anyone. No evacuations were ordered, and there was no sign of any lava pouring from the volcano.
Few people live near the mountain, about 100 miles south of Seattle. The closest structure is the Johnston Ridge Observatory (search), about five miles from the crater.
The eruption occurred at 12:03 p.m. PDT, when there was a small explosion, followed by the steam and ash cloud that rose from the southern edge of the nearly 1,000-foot-tall lava dome. Steam frequently rises from the crater, but the 8,364-foot peak had not erupted since 1986.
For the past week, scientists have detected thousands of earthquakes of increasing strength — as high as magnitude 3.3 — suggesting another eruption was on the way.
The earthquakes quit after the eruption, said University of Washington seismologist Tony Qamar.
"That makes us think this is the end of the eruption," Qamar said. "All this buildup was leading to that relatively small eruption."
But USGS seismologist Bob Norris said magma could be moving underground and he would not be surprised to see more explosions in the next days or weeks.
"The monitoring will definitely continue on a very intense scale until we can determine that the thing has really gone back to sleep," said Tom Pierson, a USGS geologist.
Mike Fergus, a spokesman with the Federal Aviation Administration (search) in Seattle, said the plume had reached 16,000 feet in altitude, but did not know whether any planes would need to be rerouted.