Mount St. Helens, Wash. ? After eight days of seismic rumbling, Mount St. Helens on Friday gave scientists and tourists what they were waiting for: a giant belch of steam and ash that rose 16,000 feet into the air before dissipating into a hazy cloud.

The event thrilled hundreds of tourists at the Johnston Ridge Observatory, five miles north of the crater, but did little to disrupt life in southwest Washington. Airplanes flew around the plume, cars whizzed down Interstate 5, and locals went about their business.

Officials kept the alert level at “volcano advisory,” the second of a three-step scale, with the third level indicating a major volcanic event.

Seismologists groped for words to describe the event — calling it a hydro-thermal explosion or a seismic burp — because it didn’t fit the definition of a classic eruption involving lava and mudflows.

At 12:03 p.m., without detectable noise or tremor, a small puff of steam that looked like a cotton ball emerged from a spot inside the crater.

The puff grew in slow motion into a column, and continued to grow for another 20 minutes. The wind blew the column toward Portland, Ore., 50 miles southwest, but within an hour, the column was gone and scientists doubted any ash would reach the city.

As the cloud cleared, scientists in helicopters were able to photograph the “source hole” where the steam punched through. The hole, about 100 feet in diameter, was on the southeast edge of a 900-foot-tall lava dome inside the crater. The dome, made of layers of once-molten rock, slowly has been building since a series of eruptions in the 1980s.

The event culminated more than a week of small but increasingly intense earthquakes that originated deep inside the crater. The largest quakes occurred Thursday and Friday morning, reaching a magnitude 3.3 on the Richter scale.

Scientists differed on what might happen next. University of Washington seismologist Tony Qamar said that even if small quakes continued, it was possible the most dramatic part of this episode might be over because most of the pressure had been released.

A column of steam and ash starts to rise from the crater of Mount St. Helens in Mount St. Helens, Wash., at the start of a predicted eruption. The plume rose for 20 minutes before blowing away in the wind.

But Jon Major, a geologist with the U.S. Geological Survey, which runs the observatory, said, “My guess is this is the opening salvo. We’ll probably see more like this.”

The prospect of more explosions likely means a continuing flow of tourists to the mountain, which sits in the middle of the Gifford Pinchot National Forest. The number of visitors to Johnston Ridge has increased steadily since the volcano began quaking Sept. 23. Forest Service workers say they are considering keeping the observatory open beyond Oct. 31, when it normally closes for the season.

For much of the week, scientists had been predicting a small to moderate eruption, which attracted tourists who wanted to witness relatively safe volcanic activity.

“I came here to watch it blow up!” said an exuberant Kay Robb, 66, who traveled to the observatory with her husband, Jim, from Kennewick, Wash. The couple were joined at the observatory by hundreds of other tourists, including numerous busloads of students from the Seattle area.

The increased traffic has been welcomed by businesses along the 50-mile stretch of Highway 504, which runs between I-5 and the observatory. More traffic means more customers for places like the Silver Shores Resort Bar.

The bar sits right off the highway, and offers a distant but unobstructed view of the volcano. When the explosion occurred Friday, more than 15 carloads of people stopped, took pictures from the deck, and many had drinks to mark the event.