A volcano on Alaska's Aleutian Islands has sent up an ash cloud which prompted scientists to increase the alert level for commercial air traffic.
The Alaska Volcano Observatory said satellite images showed the Cleveland Volcano had spewed ash 15,000ft (4,572m) into the air in a cloud which moved east-southeast. US Geological Survey (USGS) 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.
But the event drew strong interest from airlines. "Any time you put an ash cloud up into the atmosphere, the airlines, the air carriers, air freight companies - it's a major concern," Mr Power said.
The ash cloud 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 5,675ft (1,729m) peak on uninhabited Chuginadak Island about 940 miles south-west of Anchorage. 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 gases escape.
Radar images earlier this month showed the dome had cracked and subsided, Mr 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 20,000ft (6,096m) threshold, as Cleveland Volcano has done in the past.
Cleveland Volcano's last major eruption was in 2001. It has had bursts of activity nearly every year since then, and the ash cloud today was not out of character.