But Mount St. Helens teased the Sunday crowds of spectators, and many volcano watchers headed down the mountain at sunset without having seen a major blast.

"There's a significantly greater chance of gas-rich magma moving toward the surface," said geologist John Pallister of the U.S. Geological Survey Cascades Volcano Observatory in Vancouver, Wash., about 50 miles south.

With shallow earthquakes of magnitude 3 about every five minutes, the seismicity or shaking was "basically back up to those relatively high levels that we saw before the last eruptions," Pallister said Sunday night.

"We do see evidence of rise of magma to shallow levels inside the crater area," Pallister said. More steam and ash eruptions could occur at any time, the update said.

"There is also an increased probability of larger magnitude and more ash-rich eruptions," the update said.

The volcano alert remained at Level 3 since it was raised Saturday, meaning that a volcanic eruption appeared imminent.

The Ape Cave, near Cougar south of the mountain, was closed by the U.S. Forest Service late Sunday after a rock was found dislodged from the lava tube's roof. Also closed was the Mount Margaret back country located north of Spirit Lake.

Most air traffic was prohibited within a 5-mile radius of the volcano.

Scientists were considering lowering their alert from a Level 3 "volcano advisory," which indicates eruption is imminent, to Level 2 "volcanic unrest," which indicates an eruption is possible.

Many spectators couldn't wait out the mountain, which runs on geological time rather than by the human clock. Sunset brought a mass exodus off the mountain.

"Our attention span is about like this," said James Wilder, 25, of Aberdeen, holding his forefinger and thumb about one-quarter of an inch apart. "We've been here five hours and we need to leave pretty soon."

The Coldwater Ridge Visitors Center, 8.5 miles from the mountain with a straight-on view into the crater, closed at 6 p.m. as usual and vehicles had to leave the parking lot.

Earlier in the day, Nick Racine, 25, a U.S. Forest Service ranger from Chicago, held court with his laptop computer, logging onto seismographs at the University of Washington and showing the eager crowd some real-time seismicity.

"This is generating a lot of excitement. We're thrilled (that) you guys are thrilled," he told the crowd.

He wasn't bothered that some visitors were impatient. "It's human nature," he said with a shrug.

Nearby, a female ranger led the crowd in an "Eruption Dance," shaking like an earthquake, moving her arms to act out a possible eruption.

Scientists have said they do not expect anything close to the devastation of the May 18, 1980, explosion, which killed 57 people and coated much of the Northwest with ash.

The main concern has been the possibility of a significant ash plume carrying gritty pulverized rock and silica that could damage aircraft engines and the surfaces of cars and home.

Winds were from the east and southeast Sunday night, meaning that any ash clouds would drift to the west and northwest, the USGS said. The closest community is Toutle, 30 miles west near the entrance to the park.

The degree of explosivity in the magma could vary widely, depending on the gas content of the magma and conditions, said geologist Willie Scott at the observatory. Cascade range magma is very viscous with a consistency like toothpaste, so lava is not expected to escape the crater.

Volcanic tremors detected Saturday and Sunday were the first since before the 1980 eruption.

Most of the action has occurred beneath a 1,000-foot lava dome that has been building up on the crater floor - mostly with lava releases between 1980, after the eruption, and 1986.

The dome essentially serves as a plug on the rift in the Earth that connects the mountain and magma miles below the surface. The dome is filled with lava that came up during 1998 earthquakes but never surfaced. New lava may be coming up as well.

The mountain took scientists on a "rollercoaster ride" early Sunday when instruments detected the second extended volcanic vibration in two days - 25 minutes long compared to Saturday's 50-minute vibration.

The 1980 blast obliterated the top 1,300 feet of the volcano, devastated miles of forest and buried the North Fork of the Toutle River in debris and ash as much as 600 feet deep.

Besides lava emission, ash flows and rock-throwing, an eruption could cause melting of the volcano's 600-foot-deep glacier and trigger debris flows to the barren pumice plain at the foot of the mountain.

The monument is in the Gifford Pinchot National Forest about 100 miles south of Seattle.

For More Information:

Video of Friday's eruption at www.komotv.com/qt/st_helens_video.mov

St. Helens Info -- vulcan.wr.usgs.gov.
Live Web Camera Of Mt. St. Helens -- www.fs.fed.us
UW Real-Time Seismology Graphs Of Mt. St. Helens -- www.pnsn.org
What To Do In Case Of Ash Fall -- vulcan.wr.usgs.gov