USGS: Mount St. Helens could erupt within 24 hours
Observatory 3 miles from volcano's base evacuated
The afternoon sun shines Saturday on Mount St. Helens.
Mount St. Helens rumbles to life in a spectacular seismic display.
Observatory near Mount St. Helens is evacuated.
A geologist explains an imminent eruption is likely.
VANCOUVER, Washington (CNN) -- Scientists warn that Mount St. Helens could erupt within 24 hours, and with more force than previously expected.
Saturday afternoon, the U.S. Geological Survey issued a Level 3 Volcano Alert, indicating an eruption could occur within the next day, said Tom Pierson with the USGS. That level alert is the third of four -- with the fourth being eruption.
The alert was issued after scientists detected the movement of magma, or underground lava, the USGS said.
At noon, scientists began measuring a 50-minute long "harmonic tremor," or steady, even vibration, that indicates magma rising to the surface, Pierson said.
Scientists were weighing whether the movement involved new magma or magma from a 1998 eruption, Pierson said. New magma releases more gas and is more explosive.
"The data suggests that ongoing, intense earthquake activity has weakened the rock dome, increasing the likelihood of an eruption either in the form of more explosions or perhaps lava flow from the dome," U.S. Interior Secretary Gale Norton said.
"The greatest public safety concern at this point is an ash plume and the spread of ash itself," she said. "That might come from an explosion. This is a concern for aircraft travel, and that is the primary concern."
An observatory three miles from the base of the mountain was evacuated Saturday. Bumper-to-bumper traffic snaked down the road from the observatory after the order.
Scientists have been closely watching Mount St. Helens since a small eruption spewed a harmless plume of steam and ash thousands of feet into the air Friday. It was the end of a week in which the number of earthquakes near the volcano grew significantly.
One scientist described the eruption, the biggest in 18 years, as a "hiccup."
Seismic activity decreased shortly after the noon (3 p.m. ET) eruption but picked up again within hours. Peter Frenzen, a scientist with the Mount St. Helens National Volcanic Monument, said a 2.0 magnitude earthquake was detected.
A small explosion was detected on the south side of the volcano's lava dome, where cracks had been detected in a glacier, said John Major of the U.S. Geological Survey.
Scientists said the presence of magma could indicate the potential for a more serious eruption.
Molten rock is called magma before it reaches the surface, where it is called lava.
Scientists had been predicting a minor eruption after swarms of small earthquakes were detected, and the mountain's volcanic dome shifted three inches since Monday.
In anticipation of an eruption, the mountain was closed to hikers, and the media and sightseers gathered at a visitors center 5 miles away.
According to Pierson, "We realized this morning that we had had more energy released than at any time all the way back to May 18, 1980."
That was when an eruption blew more than 1,000 feet off the top of the mountain. The 1980 eruption killed 57 people, left deep piles of ash hundreds of miles away and caused $3 billion in damages.
After that disaster, small eruptions continued at Mount St. Helens until 1986, when the volcano finally went quiet. Major said Friday's eruption was comparable to the minor eruptions seen during that period.
Mount St. Helens is about 50 miles northeast of Portland, Oregon.
CNN's Kimberly Osias contributed to this report.