Scientists raised the alert level for a remote Aleutian volcano on Monday after an explosion was detected on the mountain and heard by residents of a tiny village some 45 miles away, a monitoring website said.
Cleveland Volcano, a 5,676-foot peak on the uninhabited Chuginadak Island, about 940 miles southwest of Anchorage, was raised to orange from yellow by the Alaska Volcano Observatory, indicating "heightened or escalating unrest with increased potential of eruption."
A red code is issued when an eruption is imminent or underway.
The observatory said that an explosion was detected on Cleveland by infrasound and seismic data and heard by residents of Nikolski, a small community on Umnak Island about 45 miles to the east.
Infrasound instruments measure air pressure around the volcano.
Scientists said clouds obscured Cleveland's peak in satellite images but that no evidence of an eruption cloud was detected at an altitude of 28,000 feet.
Previous explosions have spewed ash emissions, according to the observatory. The volcano is one of the most active of Alaska's scores of volcanoes.
It forms the western portion of Chuginadak Island and has been intermittently producing small lava flows and explosions since 2001, the observatory said.
Chuginadak Island is part of Alaska's Aleutian archipelago, a geologically active chain of volcanic islands that is part of the Pacific Ring of Fire and is very prone to earthquakes.
Earlier this year Mount Pavlof on the Alaska Peninsula erupted with little advanced warning, spewing an ash cloud up to 20,000 feet high that prompted aviation warnings across the region.
Pavlof is currently at yellow on the alert scale, meaning that it is "exhibiting signs of elevated unrest" but not erupting, according to the observatory.
The Alaska Volcano Observatory is a joint program of the U.S. Geological Survey, the Geophysical Institute of the University of Alaska Fairbanks and the State of Alaska Division of Geological and Geophysical Surveys.