During 1 January to mid-April 2004 (BGVN 29:04), ash-and-gas explosions and gas plumes were observed and seismicity remained generally above background levels. From May to the beginning of September 2004, seismic activity remained above background levels, varying over this time from 100-800 small shallow earthquakes per day. Ash-and-gas explosions and gas plumes to a maximum height of 7.5 km were frequent. On 1 September 2004 an increase in activity led the Kamchatka Volcanic Eruptions Response Team (KVERT) to raise the Concern Color Code from Yellow to Orange. From September to December 2004, seismicity remained above background levels, and ash-and-gas explosions and ash plumes were frequent. On 12 November the hazard status was lowered to Yellow.

Increasing seismicity, rock avalanches and possible ash plumes to 2.5 km altitude led KVERT to raise the Concern Color Code to Orange again on 7 December 2004. On 28 December, an observed eruption at Karymsky produced a plume composed primarily of gas and steam, but with some ash, that rose to ~ 1 km above the crater. Thermal anomalies were also visible on satellite imagery on 27 and 28 December. On 30 December the Tokyo VAAC reported that a plume was present up to ~ 8 km altitude extending SW.

There were no seismic data from 12 December 2004 till late January 2005. Through January and February thermal anomalies were frequently visible on satellite imagery. Seismicity remained above background levels from February 2005 through July 2005.

Through March and April 2005, ash-and-gas explosions and gas plumes were frequent. Ash deposits extended 10-15 km S and SW of the volcano. On 20 April, volcanic bombs rose to 50 m above the crater, and ash fell to the NE on 21 April. On 26 and 27 April, Strombolian activity was seen in two of the volcano's craters; volcanic bombs rose to ~ 300 m above the craters. Ash fell to the SE on 22-23 April and pyroclastic-flow deposits were seen on the NNW flank of the volcano. During May 2005, ash-and-gas explosions and plumes were again frequent, and a thermal anomaly continued to be visible on satellite imagery.

Due to a decrease in seismic and volcanic activity during 3-10 June, KVERT decreased the alert level from Orange to Yellow. Seismic activity increased starting on 22 June. Ash explosions up to 3,000 m altitude traveling SW were observed by pilots. According to seismic data, about 10 ash-and-gas plumes and avalanches occurred at the volcano. On 23 June KVERT increased the alert level to Orange. Satellite imagery of Karymsky showed a narrow ash-and-gas plume at a height of ~ 3.5 km altitude on 30 June. Based on interpretations of seismic data, ash-and-gas plumes may have reached 3 km above the crater.

The Tokyo VAAC posted four messages on Karymsky during the 90 days prior to 8 August 2005; in each, ash was not identifiable from satellite. The earliest, 18 May was similar to the last one, on 23 June. Both noted a reported plume to FL100 ('flight level 100' signifies 10,000 feet; 3.05 km altitude). Reports on 22 and 24 May both noted ash to FL 120 (3.65 km altitude).

Geological Summary. Karymsky, the most active volcano of Kamchatka's eastern volcanic zone, is a symmetrical stratovolcano constructed within a 5-km-wide caldera that formed during the early Holocene. The caldera cuts the south side of the Pleistocene Dvor volcano and is located outside the north margin of the large mid-Pleistocene Polovinka caldera, which contains the smaller Akademia Nauk and Odnoboky calderas. Most seismicity preceding Karymsky eruptions originated beneath Akademia Nauk caldera, located immediately south. The caldera enclosing Karymsky formed about 7600-7700 radiocarbon years ago; construction of the stratovolcano began about 2000 years later. The latest eruptive period began about 500 years ago, following a 2300-year quiescence. Much of the cone is mantled by lava flows less than 200 years old. Historical eruptions have been vulcanian or vulcanian-strombolian with moderate explosive activity and occasional lava flows from the summit crater.

Information Contacts: KVERT (URL: http://www.kscnet.ru/ivs/kvert/); Tokyo Volcanic Ash Advisory Center (VAAC), Japan Meteorological Agency, Tokyo Aviation Weather Service Center, Haneda Airport 3-3-1, Ota-ku, Tokyo 144-0041, Japan (URL: https://ds.data.jma.go.jp/svd/vaac/data/).