BREAKING NEWS--Alaska's Mt. Redoubt volcano erupted late Monday and early Tuesday in "three large explosions," sending an ash plume an estimated 50,000 feet into the air, the Alaska Volcano Observatory reported. Geologists at the observatory say the volcano, located 100 miles southwest of Anchorage, erupted three times late Sunday and early Monday.Geophysicist John Power said, "this is a fairly large eruption, close to the larger cities in Alaska."He says no cities have yet reported any ash fall from the volcano, but noted that it's still early.Winds were expected to carry the ash plume north toward the Susitna Valley, possibly missing Anchorage to the east, the National Weather Service told the Anchorage Daily News."It looks like Anchorage might dodge the bullet," Alaska Volcano Observatory geophysicist Peter Cervelli told the paper.The volcano observatory raised the aviation color code to Red,its highest level,and the alert level to Warning after the eruption began at 10:38 p.m.local time (2:38 a.m. Monday EDT.)An official at Anchorage International Airport told the Daily News early Monday there were no immediate plans to close the airport.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. • Click to view photos of the volcano. • WEBCAMS: Monitor Alaska's volcanoes live.