Mount St Helens volcano in Washington state has erupted for the first time in nearly 20 years.
No casualties were reported
The mountain spewed ash and steam thousands of metres into the sky, although there was no sign of lava.
The sudden burst was nowhere near the scale of a 1980 eruption, which resulted in the deaths of 57 people.
The last eruption occurred in 1986, but caused little damage. Seismologists have warned of more volcanic activity in the coming days or weeks.
Fire in ice
There were no casualties reported in the latest eruption and the mountain calmed down after about 20 minutes.
Government scientists - who have been closely watching the volcano after a flurry of small earthquakes a week ago - say the alert has been raised to level three out of a maximum of four
around the volcano.
They say the lava dome formed after the 1980 eruption has swollen slightly and cracks have begun to appear across the glacier inside the volcano's crater.
"Whatever triggered this event was not magma that reached
the surface," Dr Jon Major, a US Geological Survey researcher told Reuters news agency.
"If the seismicity starts to ramp back up and goes back to
the levels we've seen prior to this event, then it is a
harbinger that we are not finished yet," he said.
But he added that if seismic activity tailed off then perhaps the volcano would return to its previous, quieter state.
The eruption nearly 25 years ago sent half a billion tons of ash into the air, leaving Portland in Oregon - the nearest major city at a distance of about 80km (50 miles) away - covered in a thick layer of the volcanic debris.