AVO reported that during 3-9 September seismic activity from Kasatochi, detected by stations approximately 40 km W on Great Sitkin island, was low. Vigorous steam-and-gas plumes that rose above the crater and drifted up to 32 km downwind were observed on 3 and 4 September by passing mariners. On 4 September the Volcano Alert Level was lowered to Advisory and the Aviation Color Code to Yellow. Clouds mostly prevented satellite image observations; weak thermal anomalies were detected on 5 and 7 September.

Geological Summary. Located at the northern end of a shallow submarine ridge trending perpendicular to the Aleutian arc, Kasatochi is small 2.7 x 3.3 km island volcano with a 750-m-wide summit crater lake. The summit reaches only about 300 m elevation, and the lake surface lies less than about 60 m above the sea. A lava dome is located on the NW flank at about 150 m elevation. The asymmetrical island is steeper on the northern side than the southern, and the crater lies north of the center of the island. Reports of activity from the heavily eroded Koniuji volcano to the east probably refer to eruptions from Kasatochi. A lava flow may have been emplaced during the first recorded eruption in 1760. A major explosive eruption in 2008 produced pyroclastic flows and surges that swept into the sea, extending the island's shoreline.

Source: US Geological Survey Alaska Volcano Observatory (AVO)