SEATTLE, Washington, Oct 1 (AFP) - US volcano Mount St. Helens roared to life Friday, spewing a giant plume of steam and ash into the sky and raising fears of a new larger eruption.
After letting off steam from its lava cap in a spectacular natural display, the rumblings stopped and the huge cloud quickly dispersed.
But the burst of activity sent scientists scrambling to establish whether the volcano, which killed 57 people when it erupted dramatically 24 years ago, was holding another more powerful eruption in store.
US Geological Survey scientists said the eruption at 11:57am (1857 GMT) marked a "small explosion" following a week of intensifying earthquakes in the area.
"This was small explosive eruption from the lava dome that lasted for 24 minutes and generated a plume that rose up to a height of just under 10,000 feet (3,000 meters)," said USGS volcano and hydrology expert John Major.
"The eruption was predominantly steam but containing a little bit of ash," he said adding that there were no lava or mud flows from the crater and that the alert level in the region had not been raised.
The steam cloud poured from the southern edge of a 300-meter tall (1,000-foot) lava dome in the volcano's crater that has risen in recent days.
An area of two-to-four hectares (five to 10 acres) on the lava cap rose and fractured during the eruption, releasing glacial water and the column of steam, Major said. The last explosion from the dome took place in 1986.
The steam cloud drifted southwest from the mountain peak in southern Washington state and aviation officials warned aircraft in the area to avoid the plume, Federal Aviation Authority officials told AFP.
Seismic experts scrutinised the volcano from aircraft and from the mountainside to determine whether further eruptions were expected from the volcano that has been extremely active for more than 4,000 years.
"We are going to keep a very close eye on it," Major told journalists in Seattle, adding that the level of earthquake activity in the area would be a crucial indicator of what is to come.
"If the seismicity in the area does not ease off, it's a sign that it's not over yet. If we do not see anything over the coming days, then there is a good chance that that might be it," he said.
But another USGS expert Jeff Wynn warned that while the volcanic activity had ceased for the moment, it could come roaring back at any time.
"The history of this volcano suggests that this could be the opening salvo of more eruptions," he said adding that the small 1986 eruption was similar to the latest one.
Volcanic 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 told AFP, adding that it was unclear what the rumbling mountain would do next.
But scientists stressed that any larger explosion would not be as serious as the 1980 eruption that covered much of the northwestern US region in a thick blanket of ash.
"In 1980 things happened a little bit faster," Major said. Other officials said they did not believe there was any real danger to people who live in the region.
Trails up the mountain were closed to climbers and hikers after quakes began eight days ago, and the area that would likely be affected by a slightly bigger eruption is largely uninhabited.
Quakes began around the volcano on September 24 and rose in magnitude through the week until they measured between 3.0 and 3.2 on a modified Richter scale and were taking place four times a minute on Thursday and Friday.
The mountain top lies around 88 kilometers (55 miles) northeast of the Oregon's main city of Portland, which was covered by the ash in the 1980 eruption.