A volcano in Alaska's Aleutian Islands sent up an ash cloud Thursday that prompted scientists to increase the alert level for commercial aircraft traffic.
The Alaska Volcano Observatory said satellite images at 4:02 a.m. Alaska time showed Cleveland Volcano had spewed ash 15,000 feet into the air in a cloud that moved east-southeast. U.S. Geological Survey scientist-in-charge John Power called it a small explosion.
"It's not expected to cause a disruption to big international air carriers," he said.
However, it was significant enough to raise the alert level from yellow, representing elevated unrest, to orange, representing an increased potential of eruption, or an eruption under way with minor ash emissions or no emissions.
Cleveland Mountain is a 5,675-foot foot peak on uninhabited Chuginadak Island about 940 miles southwest of Anchorage. The nearest village is Nikolski on another island about 50 miles east. Previous eruptions of Cleveland Volcano were not considered a threat to Nikolski and its 18 permanent residents.
Scientists in July noted increased activity in the crater at the summit of the volcano. Satellite images showed lava building and forming a dome-shaped accumulation.
Chris Waythomas of the USGS said in September that lava domes form a lid on a volcano's "plumbing," including the chamber holding the magma. When they grow big enough, lava domes can become unstable and will sometimes collapse. When the magma chamber decompresses it can lead to an explosion as the conduit inside the volcano suddenly becomes unsealed and gasses escape.
Radar images earlier this month showed the dome had cracked and subsided, Power said.
The Federal Aviation Administration and the airline industry get concerned for trans-Pacific flights when an ash cloud has the potential to exceed the 20,000-foot threshold, as Cleveland Volcano has done in the past.
Alaska's Redoubt Volcano blew on Dec. 15, 1989, and sent ash 150 miles away into the path of a KLM jet carrying 231 passengers. Its four engines flamed out and the jet dropped more than 2 miles, from 27,900 feet to 13,300 feet, before the crew was able to restart all engines and land the plane safely in Anchorage.
Cleveland Volcano's last major eruption was in 2001. It has had bursts of activity nearly every year since then and the ash cloud Thursday was not out of character.
"It's not unexpected for a volcano like Cleveland to do things like this," Power said. "Unfortunately, Cleveland is one of those that is so remote, we have no on-ground monitoring or instrumentation there, so it's hard for us to pinpoint things any more accurately than we can do with satellite imagery."
The event Thursday drew strong interest from air carriers.
"Any time you put an ash cloud up into the atmosphere, the airlines, the air carriers, air freight companies -- it's a major concern," Power said.
The observatory is working with the University of Washington to monitor lightning above Cleveland Mountain, which could signal a major ash plume.
"Any time you put up a big ash cloud, you induce a lot of lightning activity," he said. "It's like having a big thunderhead go up."
The cause is linked to the interaction of ash and warm air.
"There's a whole lot of hot air and it rises through the atmosphere very quickly," Power said. "All the ash particles rub together and develop electrical charges, and that discharges as lightning.
The eruption Thursday was not large enough to create lightning.
Seismic activity at nearby volcanoes could also signal an explosion at Cleveland Mountain, the observatory said.
The observatory is a joint program between the USGS, the University of Alaska Geophysical Institute and the state Division of Geological and Geophysical Surveys. It was formed in response to the 1986 eruption of Mount Augustine.
The observatory Thursday morning had no satellite images of the crater after the eruption.