Mount St. Helens lets off steam
'There's a heck of a lot going on underneath,' scientist says
VANCOUVER, Washington (CNN) -- Geologists warned late Monday that two small eruptions of steam and ash from Mount St. Helens earlier in the day were likely precursors of a much larger explosion that could soon occur.
An eight-mile radius around the volcano has been cleared and a Level 3 alert is in effect, the highest issued by the U.S. Geological Survey.
Tourists are being kept away from Mount St. Helens National Volcanic Monument, a large park surrounding the peak.
"What we are expecting is a blast of ash that will rise quickly up in the air tens of thousands of feet, form an ash column and then a big expanding ash cloud that will drift with the wind," said Tom Pierson, a USGS scientist.
The volcano gave off a burst of steam and ash for about 40 minutes Monday morning and was followed in the afternoon by another small eruption.
The volcano has been increasingly active in recent days.
The plumes unleashed Monday were larger and darker than those of Friday's steam eruption, rising to about 10,000 feet above sea level, roughly 1,600 feet above the volcanic peak, according to geologists and pilots in the area.
And unlike in Friday's steam eruption, which did not contain ash, minor earthquake activity that has been going on since September 23 did not stop during Monday's eruption. The earthquakes indicate releases of seismic energy and magma movement.
Officials have said they do not expect a catastrophic eruption on the scale of the one in 1980, when 57 people were killed and ash blanketed much of the Pacific Northwest.
But officials are concerned that magma -- known as lava when it comes to the surface -- is pushing up closer to the surface, and they believe it is rich in super-compressed carbon dioxide gas bubbles that could unleash a powerful explosion.
"There's a heck of a lot going on underneath," USGS geologist Carl Thornber said. "It's building up something."
Geologists said the lava dome inside the volcano is now deforming, or growing, by tens, maybe hundreds, of feet. Surrounding it is a glacier containing 80 million cubic meters of water and ice.
Increased concentrations of carbon dioxide and hydrogen sulfide were detected in the air, signs of fresh magma rising.
The USGS clocked harmonic tremors last weekend, and one that lasted 90 minutes Sunday night. Such tremors are continuous, rhythmic quakes that often precede eruptions, and 90 minutes is the longest one clocked so far.
The clouds that billowed from the volcano Monday were the result of the intense heat from inside the volcano interacting with melting waters from the glacier above.
Willie Scott of the USGS said there was little to worry about in Monday's activity.
"What we're more concerned about is what's following behind it," he told reporters.
As Bill Steele, a seismologist with the University of Washington, put it: "Something's driving this train, and I think there is a growing consensus that at least fresh gases have reached older magma that is shallow in the volcano, activating it."
Steele said Monday's activity was unlike anything since the buildup to the 1980 eruption.
"We really have a good basis for concern," he said, adding that a catastrophic event is not expected.
He said officials expected a large ash plume if the volcano blows.
"The ash plume can travel, and the hazard to aviation is real," Steele said. "To individuals, it's more of an annoyance -- could be an extreme annoyance if a lot of ash comes out."
The volcano, 95 miles south of Seattle, lay dormant for 123 years before the 1980 eruption, which knocked more than 1,000 feet off the peak in a violent explosion of rock, steam and ash.
The mountain is very different from how it was 24 years ago, said Bonnie Lippitt of the U.S. Forest Service.
"It's quite a bit shorter," Lippitt said. "There's a lot less volume to it as a result of the 1980 eruption."
Before 1980 the volcano was 9,677 feet high; now it is 8,364 feet high, the USGS said.
Several smaller eruptions took place through 1986 and in 1998 and 2001.