The US Geological Survey (USGS) said the volcano, which killed 57 people during a major eruption in 1980, was shaken by a "tremor burst" for 25 minutes. Eathquakes are rumbling at a rate of one or two per minute, it added.

Scientists on Saturday saw an increase of fumaroles, a hole from which hot gases and vapors emanate, in St. Helens' lava dome and detected carbon dioxide, the USGS said.

After 18 years on inactivity, St. Helens roared back to life Friday with a giant plume of steam and ash.

"All this suggests that a second eruption is imminent," said USGS spokeswoman Catherine Puckett. "It could happen right now while we are talking, in the next few hours or tomorrow."

Friday's eruption brought hundreds of curiosity seekers to the area who were evacuated on Saturday from a five-mile (8 kilometer) radius around the mountain, including the USGS Johnson Ridge Observatory.

The Federal Aviation Administration on Friday warned aircraft to avoid the area around the volcano.

The USGS has raised its alert to the maximum third level.

Volcano experts have been warning of a possible imminent eruption of Mount St. Helens for several days as seismic activity in the area grew.

"This is exactly the eruption we predicted," the USGS's Stephanie Hanna said Friday when St. Helens erupted.

On May 18, 1980, after weeks of quakes, slides and tremors, of belching smoke, steam and ash, Mount St. Helens, a major peak in the Cascade mountain chain, erupted.

Sandy McComb, 56, of Portland, Oregon, 60 miles (100 km) south, helped in search and rescue operations after St. Helens' devastating eruption, and she came to see the volcano come back to life this weekend.

"We had to come back here when this happened," she said.

The 1980 eruption blew off the upper third of the mountain, leaving a gaping crater 2,050-foot (400-meter) deep, 1.7-mile (2.7-km) long and 1.3-mile (2-km) wide.

The explosions shaved off 400 meters (1,300 feet) of the mountain's top, reducing it from 2,950 meters (9,680 feet) to 2,549 meters (8,363 feet).

The blast sent clouds of volcanic ash high into the atmosphere, where jet streams carried it as far as the US eastern seaboard 3,000 miles (4,800 km) away.

Towns and cities throughout the Pacific Northwest were buried in gray ash.

The flash snow melt on the mountain triggered floods and mudslides, sending streams and rivers over their banks and destroying infrastructure for hundreds of miles around.

The Columbia River, the main inland shipping channel from the Pacific Ocean, was clogged with volcanic sediment and eventually had to be dredged.

Molten lava flows devastated forests and mountain meadows, creating a vast wasteland of mud, magma and ash.

Mount St. Helens erupted again in 1986, but with nowhere near the same intensity.

Although geologists said the probability for another eruption is high, they predict it would not be anything like that of 1980.

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