Koryaksky ended ~ 51 years without fumarolic activity with an eruption late in 2008. Activity that began on about 24 December 2008 continued through 5 March 2009 (BGVN 34:01). Fresh ash deposits in early March could be seen on both the summit, E, and SSW slopes of Koryaksky, and on the NNW slope of Avachinsky (figure 4). Between the two volcanoes, the early March ash deposits reached 1-2 mm thick.

Figure 4. (top) Ash plume from Koryaksky (peak at left) extending to the ESE in an eruption that left deposits of ash on Koryaksky and Avachinsky (peak at right). (bottom) Ash deposits on Koryaksky on the same date. Both photos from the town of Petropavlovsk-Kamchatsky on 7 March 2009 by A. Sokorenko.

Numerous plumes were observed during this reporting interval, 6 March to mid-April 2009 (table 1). A report from 8-14 April noted the presence of two vents on the NW flank. A small SO₂ plume was noted on 20 April. Seismicity often fluctuated, with considerable intervals at background level punctuated by intervals where it was elevated. Occasional tremor was recorded, but it was often weak (low amplitude).

Table 1. A compilation of eruptive plume behavior from Koryaksky based on information from the Yelizovo Airport, KVERT, and KEMSD (the Kamchatkan Experimental and Methodical Seismological Department). The data were initially compiled by the Tokyo VAAC in reports for aviators in an effort to avoid aircraft encounters with volcanic ash. Few of the plumes were considered ash rich, most were considered as gas plumes containing small amounts of ash. Courtesy of the Tokyo VAAC.

KVERT reported that seismic activity at Koryaksky was elevated on 6 and 8 March and at background levels during 7 and 9-13 March. Observers reported that gas plumes containing a small amount of ash rose to an altitude of 4 km and drifted in multiple directions during the week of 11-17 March. The plumes were also seen on satellite imagery.

An eruption on 9 April 2009 followed an increase in the number of local earthquakes and tremor during the previous month (figure 5). The total number of earthquakes during February 2008-December 2008 was 717. That was less than the total of 766 for the first four months in 2009, which were as follows: January, 58; February, 195; March, 239; and April, 274. High level gas-steam and ash emissions occurred during March-April. Some plumes extended more than 200 km (table 1).

Figure 5. Seismicity of Koryaksky (and Avachinsky, to the SE) recorded during January-April 2009. Map (left) shows location and depths of earthquakes (white line is cross-section AB; 20-km-diameter circle encloses epicenters of earthquakes plotted on histogram. Cross-section shows hypocenters projected onto the vertical plane along AB. Histogram shows daily Koryaksky earthquakes with respect to time; ascending curve is the cumulative number of earthquakes (reaching a total of 652 for the interval). Courtesy of the Kamchatka Branch of the Geophysical Service of the Russian Academy of Sciences (KB GS RAS).

On 9 April a photo was taken showing a strong ash plume emerging from fissures in the glacier on Koryaksky's NW flank. On 13 April a flight over the area enabled scientists to measure temperature around two vents (figures 6 and 7). Temperatures of the vent areas reached 400°C. This situation was attributed to glacial instability owing to melting parts of the glacier near the vents.

Figure 6. A photo taken from Petropavlovsk-Kamchatsky (~35 km N of the volcano) illustrating powerful ash-bearing emissions from the NW flank of Koryaksky. Photo taken by Sergei Ushakov 9 April 2009.

Figure 7. A photo showing two vents on the NW flank of Koryaksky. The upper plume is ash rich, the other contains only gas and steam. Photo taken by Alexander Sokorenko on 18 April 2009.

Geological Summary. The large symmetrical Koryaksky stratovolcano is the most prominent landmark of the NW-trending Avachinskaya volcano group, which towers above Kamchatka's largest city, Petropavlovsk. Erosion has produced a ribbed surface on the eastern flanks of the 3430-m-high volcano; the youngest lava flows are found on the upper W flank and below SE-flank cinder cones. Extensive Holocene lava fields on the western flank were primarily fed by summit vents; those on the SW flank originated from flank vents. Lahars associated with a period of lava effusion from south- and SW-flank fissure vents about 3900-3500 years ago reached Avacha Bay. Only a few moderate explosive eruptions have occurred during historical time, but no strong explosive eruptions have been documented during the Holocene. Koryaksky's first historical eruption, in 1895, also produced a lava flow.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanology and Seismology (IV&S), Far East Division, Russian Academy of Sciences (FED RAS), Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/); Kamchatka Branch of the Geophysical Service, Russian Academy of Sciences (KB GS RAS), Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.emsd.ru/~ssl/monitoring/main.htm); Alexander Sokorenko and Sergey Ushakov, IV&S; Kamchatkan Experimental and Methodical Seismological Department (KEMSD), GS RAS, Russia; Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).