A small explosion from the summit lake region of Korovin, one of the volcanoes at the Atka volcanic complex, was detected at 1926 on 20 February, prompting the Alaska Volcano Observatory (AVO) to raise the Volcano Alert Level to Watch (the second highest level on a four-level scale) and the Aviation Color Code to Orange (the second highest color on a four-color scale). The event was brief (less than 10 seconds), probably phreatic, and likely ejected a small amount of ash that was deposited within the immediate vicinity of the lake, though ash was unconfirmed. A meteorological cloud deck at altitudes of 2.4-3 (8,000-10,000 ft) a.s.l. obscured direct satellite observations, but no ash emissions were observed above the clouds. Satellite data acquired later that evening showed no evidence of hot material at the surface. There were no further explosive events detected and no noticeable change in seismic activity; at 1114 on 21 February AVO lowered the Volcano Alert Level to Advisory and the Aviation Color Code to Yellow. This type of singular explosion was common at Korovin. Several small local seismic events were detected during 21-23 February, though the overall level of seismicity was very low. Satellite and webcam observations were obscured by weather clouds.

Geological Summary. The Atka Volcanic Complex consists of a central shield and Pleistocene caldera and four notable volcanic cones. A major explosive dacitic eruption accompanied formation of the caldera about 500,000 to 300,000 years ago; approximately half of the caldera rime remains, open towards the NW. The Sarichef cone, ~5 km ESE of the caldera rim, retains a symmetrical profile, unlike most other heavily eroded features outside the caldera to the S and W. The Kliuchef stratovolcano grew within the caldera and exhibits five eruptive vents striking NE, including two at the summit, that have been active in the Holocene. A 700-m-diameter crater 1 km NE of the summit may have been the source vent for a large 1812 CE eruption. Hot springs and fumaroles are located on the flanks of Kliuchef and in a glacial valley to the SW. The most frequently active volcano of the complex is Korovin, at the NE tip of Atka Island about 5 km N of Kliuchef. An 800-m-diameter crater on the SE side of the summit contains a deep circular pit that sometimes contains a crater lake thought to be the source of phreatic ash explosions. The smaller Konia cone, slightly offset to the E, lies between Kliuchef and Korovin. Most of the lava flows in the complex are basaltic, though some dacitic flows are also present.

Source: US Geological Survey Alaska Volcano Observatory (AVO)