Small earthquakes resumed within hours of the blast, suggesting pressure inside the mountain was rebuilding. Scientists said there could be more steam eruptions soon.
The noontime eruption cast a haze across the horizon as the roiling plume rose from the nearly 1,000-foot-tall lava dome, forcing Alaska Airlines to cancel flights and divert others around the ash.
Scientists with the U.S. Geological Survey told reporters that at 3:03 ET, there was a small explosion on the south side of the lava dome, along a previously identified crack. A plume of steam and some ash was released, rising 10,000 into the far. The event lasted 24 minutes.
"It was such a thrill!" said Faye Ray, a retired teacher who watched from an observatory near the mountain. "I just felt we would see something today and we did."
As scientists monitor the ominous rumbles inside Mount St. Helens, they admit that every jolt of the seismograph brings a jolt of adrenaline, reports CBS News Correspondent John Blackstone.
"It's very exciting for those of us who study volcanoes to be studying a restless volcano and try to understand what's going on," said Dr. Willy Scott of the U.S. Geological Survey.
Scientists had been predicting just such an eruption for days because of thousands of earthquakes and signs that the rock inside the crater was expanding rapidly.
The eruption was nowhere near what happened 24 years ago, when 57 people were killed and towns up to 250 miles away were showered with rock and ash.
About 20 minutes after Friday's eruption, the mountain calmed and the plume began to dissipate. The ash appeared to pose no threat to anyone, but scientists warned that people living southwest of the mountain might notice a fine film of ash on their cars. No evacuations were ordered, and there was no sign of any lava oozing from the volcano.
A few hours later, small earthquakes had started again at a rate of about one every 4 minutes. Within an hour they hit a one-per-minute pace, said Bill Steele at the University of Washington seismic laboratory, which is working with USGS. A couple were larger, exceeding magnitude 2.
He said there are likely to be a few more steam explosions "until enough debris is cleared, and then there is a significant chance that lava could be extruded at the surface."
Few people live near the mountain, about 100 miles south of Seattle. The closest structure is the Johnston Ridge Observatory, about five miles from the crater.
"It wasn't lava-y, so I wasn't scared," said Lorain Weatherby, who was working a snack bar down the road from St. Helens. "It was like a big white cloud."
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. Steam frequently rises from the crater, but the 8,364-foot peak had not erupted since 1986.
"This is exactly the kind of event we've been predicting," said U.S. Geological Survey scientist Cynthia Gardner.
The earthquakes quit after the eruption, said Jeff Wynn, another USGS scientist.
He called the eruption a "throat-clearing."
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 in Seattle, said the plume had reached 16,000 feet in altitude.
Alaska Airlines canceled five flights scheduled to take off from Portland International Airport in Oregon, but quickly resumed its normal schedule, said spokesman Sam Sperry.