Alaska's Mount Redoubt is rumbling again and geologists at the Alaska Volcano Observatory increased the official alert level to orange, the stage just before eruption.
A significant eruption did not appear imminent, geologists said Sunday, but they cautioned that conditions could evolve rapidly.
The 10,200-foot Redoubt Volcano is about 100 miles southwest of Anchorage.
Ash from the volcano could harm engines and is especially dangerous for aircraft. Ash blown to cities also can cause respiratory problems.
Residents of south-central Alaska have kept a close eye on Redoubt since the observatory on Jan. 25 warned that an eruption could occur at any moment. The alert level was downgraded last week after nearly two months.
Just after 1 p.m. Sunday, however, seismic activity picked up again.
"We got a return of this stuff we call volcanic tremors," said geologist Chris Waythomas. "Think of the phenomenon that produces sound in an organ pipe."
Instead of sound waves in a pipe, geologists detect movement of magma within cracks and fractures of the mountain that resonates and produces a distinct signal.
"We think it's associated with the hydrothermal system there. It's being reinvigorated," Waythomas said.
The tremors lasted about four hours and then settled down.
An observatory flight Sunday reported that a steam and ash plume rose as high as 15,000 feet (4,600 meters) above sea level and produced minor ash fall on the upper south flank of the mountain. Later reports indicated the plume had changed into mostly steam.
Ash emission had not been seen before, Waythomas said, and until samples are taken, geologists will not know whether it's new magma or, more likely, old ground-up material from previous episodes.
Other signs that a volcano could erupt are deformities in the landscape and the mix of gases escaping from vents on the side of the mountain.
Alaska volcanos typically explode and shoot ash upward, sometimes to 50,000 feet (15,000 meters), high into the jet stream. An eruption of Redoubt on Dec. 15, 1989, sent ash 150 miles (240 kilometers) away into the path of a KLM jet, stopping its engines. The jet dropped more than two miles (three kilometers) before the crew was able to restart engines and land safely at Anchorage.