The Alaska Volcano Observatory said that persistent “thermal anomalies” had been observed in satellite data, indicating a possible rise of lava to near the surface.
No real-time seismic equipment is installed on the Aleutian island of Chuginadak, where the volcano is located.
This means that vulcanologists depend on nearby observers and satellite data to determine what Cleveland is doing.
Its ash eruptions have the potential to disrupt air transport across the North Pacific.
Cleveland produced three significant eruptions in 2001 that sent ash soaring to 39,000 feet into key international air routes. Hot ash and lava flows reached the sea at that time.
Minor bursts of ash occurred in September 2010 and July 2007.
Photo: Doug Dasher-Max Hoberg