Highly variable activity continued throughout August-December 1999. Typical daily activity observed during clear weather consisted of a small fumarolic plume rising 50-200 m above the crater and extending a few kilometers downwind, usually E or SE. Seismicity was generally at background levels, consisting of shallow earthquakes with some periods of tremor. However, higher gas-and-steam plumes were frequently seen and two episodes of increased seismicity were detected. The volcano was frequently obscured by clouds.

Tremors and shallow earthquakes were registered during 9-15 August. Typical small fumarolic plumes were seen on 9-10, 13-14, 16, 21-26, and 28 August, and 2, 4-5, 7-8, and 12 September. On 30-31 August a gas-and-steam plume rose 500-1,500 m above the crater. On 15 September a gas-and-steam plume rose 600 m, and on 16 September the plume rose 200 m extending 5 km E. Mainly shallow earthquakes were registered from 19 September through 24 October. Gas-and-steam plumes rose up to 500 m during 19-26 and 28 September, and 3, 5, 7, 11, 20-21, and 24 October, extending as far as 5 km E or SE. During the afternoon of 15 October there was a 6.5-hour-long series of shallow earthquakes. On 22-23 October a fumarolic plume rose 700-1,000 m and extended 5-20 km to the E and SE.

Seismicity, consisting of shallow earthquakes and tremor, was above background levels during much of the period from 25 October until 17 December. Only small fumarolic plumes 50-300 m high were seen on 25 and 27 October, but on 26 October a plume rose 1,000 m above the volcano and extended 40 km NE. Small fumarolic plumes to 300 m extending 5 km SE were seen on 29-31 October and 4 November, with smaller typical plumes on 5, 7-8, and 10-11 November. Shallow earthquakes and volcanic tremor were recorded especially on 15, 21, and 25 November, when a gas-and-steam plume rose 1,000 m and extended more than 7 km NE. Typical smaller fumarolic plumes were seen on 12, 16, 18-19, 22-24, 26, and 28 November, and on 1, 3, and 10 December. On 29 November and 1 December gas-and-steam plumes rose 1,500 m above the volcano and extended more than 20 km SE. A fumarolic plume on 8 December rose 2,500 m.

During December 17-29 seismicity at the volcano returned to background levels. Small plumes were recorded on 17, 19-21, 25, and 28 December. Another plume on the 23rd rose 700 m.

Geological Summary. Klyuchevskoy is the highest and most active volcano on the Kamchatka Peninsula. Since its origin about 6,000 years ago, this symmetrical, basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during approximately the past 3,000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 and 3,600 m elevation. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. These eruptions over the past 400 years have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.