Korovin is a stratovolcano located on Atka Island in the central Aleutian Islands; its most recent reported activity ended in 2007. This report summarizes and contains new information on activity from 1998 to 2007 by drawing on information primarily from the Alaska Volcano Observatory (AVO) and their cited publications. Much of the summary takes the form of a table at the end of the report.

Atka volcanic complex. According to Myers and others (2002) Korovin is a part of the 360 km² Atka volcanic complex, found on the northern part of Atka Island. It is the largest modern complex within the central Aleutians (Myers and others, 2002). The ancestral Atka volcano, in the complex, was described as a large shield volcano consisting of basaltic and basaltic andesite flows, which was subsequently surrounded by a series of satellite vents (Myers and others, 2002).

A caldera forming eruption at the Atka shield volcano occurred ~300,000-500,000 years ago, creating a 5-km-diameter caldera. Associated with that event was the eruption of a large dacitic flow, called Big Pink. Regarding the composition of Big Pink, Myers and others (2002) said, "It consists of pumiceous and glassy units but is not associated with any ash flows." After the caldera formation, the volcanic centers of Korovin, Kliuchef, Konia and Sarichef formed. Figure 4 is a topographic map showing the location of these four volcanic centers and the location of the Atka caldera. These structures all comprise the Atka volcanic complex.

Figure 4. Topographical map of the northern part of Atka Island, located in the central Aleutian Islands. The map highlights the locations of the Atka caldera, the Korovin, Kliuchef and Sarichef volcanoes and the Konia vent, which all comprise the Atka volcanic complex. Image created by the Alaska Volcano Observatory (AVO) and U.S Geological Survey (USGS) using BigTopo 7 software and AllTopo 7. Image taken from the AVO website.

Korovin volcano. Korovin is located 21 km NE from the town of Atka (figure 4). It is the largest and tallest volcano of the post-caldera volcanic centers within the Atka volcanic complex. According to Myers and others (2002), Korovin shows little evidence of glaciation, unlike Kliuchef, located ~5 km S of Korovin. Regarding Korovin' edifice and age, Myers and others (2002) say "Its uneroded form suggests the volcano is mostly Holocene in age."

Korovin has a basal diameter of ~7 km and two summit vents located 0.6 km apart (Myers and others, 2002). The NW summit vent has a small crater and is the lower of the two vents. The SE summit has a 1 km wide crater, with steep walls and a depth of several hundred meters (Myers and others, 2002). The SE summit crater sometimes contains a crater lake and is considered Korovin's active crater. Figure 5 is an aerial photo of Korovin, highlighting its two summit vents.

Figure 5. Photograph of Korovin volcano taken from an aircraft flying at 9.1 km altitude on 5 August 2007. The view is oblique and from the N (i.e. looking S). Steam is rising from the active crater (SE crater). The summit of Kliuchef volcano is partially visible at the top of the image; it sits ~5 km S of Korovin. Photograph taken by Burke Mees, Alaska Airlines. Photograph from McGimsey and others (2011).

During the summer of 2004, AVO installed a network of seismic stations throughout the northern part of Atka Island. Data from the network was accessible in March 2005; however, it wasn't until December 2005 that Korovin was considered seismically monitored. On 2 December 2005, Korovin was also officially assigned the Level of Concern Color Code Green after "a sufficient period of background seismicity had been recorded" (McGimsey and others, 2007). Before, AVO had listed Korovin as UA (unassigned) during periods when no significant activity was noted. AVO assigns volcanoes UA when there is no real-time seismic network in the area that can be used to define background levels of seismicity.

In addition to being seismically monitored, Korovin is also monitored through ground-based, aerial, and satellite imagery and photographs. Korovin and its plumes are often photographed by residents of Atka village (figures 6 and 7), which are then sent to the AVO. Figure 8 provides examples of photos of Korovin taken from satellites. Images from figures 6-8 furnish various kinds of evidence, from steaming (i.e. non-eruptive cases, figure 7), ash-bearing plumes (figure 6), and the result of ash-bearing eruptions (ash on the snow surface seen in satellite views, figure 8). Evidence of these kinds is summarized in next section.

Figure 6. Photographs showing the progression of a steam plume that developed over Kovorin around 1900 on 23 February 2005. Plume was observed drifting to the E, and ash was seen falling out near the base of the plume. These photos were taken in Atka village and are courtesy of Louis and Kathleen Nevzoroff. Photos were taken from McGimsey and others (2008).

Figure 7. Photograph of a steam column rising from Korovin on 27 July 2007. Steam was estimated to reach ~215-245 m above the crater. The photo was captured by Louis Nevzoroff from Atka village. Taken from McGimsey and others (2011).

Figure 8. Two satellite photographs showing ash deposits on the upper E flank of Korovin in 2002 (top) and 2004 (bottom). The source of these ash deposits is thought to be intermittent, minor phreatic eruptions through the hot, roiling lake within the SE summit crater of Korovin (McGimsey and others, 2007). Top image was taken on 5 July 2002 and produced by the Image Analysis Laboratory, NASA Johnson Space Center. Bottom image was captured on 4 July 2004 and is an Ikonos near-infrared color composite, copyrighted by Space Imaging LLC. Both images originally published in McGimsey and others (2008).

Activity during 1998-2007. During this interval (table 1), activity ranged from eruptive cases to those that were considered non eruptive.

Activity was often reported to AVO by Atka village residents and pilots in the area. Korovin was also monitored through satellite imagery, when weather conditions were favorable. During this interval, the highest plumes were observed on 30 June 1998 and reached an altitude of ~9.1 km. As activity varied, the Aviation Color Code (ACC), the Volcanic Activity Alert Level (VAAL), and the Level of Concern Color Code (LCCC) were changed to reflect Korovin's activity status.

AVO presented general information on reported activity from 1998-2007 on their website. For each of the events within this interval, AVO cited information from several sources, some of which included the following: McGimsey and others (2003), which discussed activity in 1998; McGimsey and others (2008), which discussed 2005 activity; Neal and others (2009) that looked at 2006 activity; and McGimsey and others (2011) that detailed 2007 activity. AVO also referenced several past Bulletin reports, which highlighted Korovin activity (BGVN 23:06, and 31:02).

Our summary in table 1 summarizes the following: (1) the basic information on Korovin's activity from the AVO website and (2) additional information from some of AVO's cited references. Greater detail can be found on AVO's website and in their cited references.

Table 1. Condensed descriptions of key events during both eruptive and non-eruptive periods during 1987-2007. The data sources are stated in the table. The Remarks column generally contains the following: (1) "AVO:" This presents a very brief synopsis of the summary that AVO provides on each of their Korovin reported activity web pages (as accessed in May 2015). (2) Below that, we present a succinct timeline of Korovin activity created using on information found in some of AVO's cited references. The 2005 activity is in two sections to highlight different periods of activity during that year; 2006-2007 is considered one period of activity. Where AVO cited references are augmented by past Bulletin reports, the information has been [bracketed]. Times are all local, unless otherwise stated. The term 'resident(s)' refers to resident(s) of Atka village. Abbreviations used are as follows: Village Public Safety Officer, VPSO; above sea level, a.s.l., and Interferometric synthetic aperture radar, InSAR; Aviation Color Code, ACC; Volcanic Activity Alert Level, VAAL; Level of Concern Color Code, LCCC; unassigned activity (UA); and satellite-based Ozone Monitoring Instrument, OMI.

References. Alaska Volcano Observatory, the U.S. Geological Survey, BigTopo 7, and AllTopo 7, Topographic shaded relief image of the northern part of Atka Island (Image 2906), accessed on 14 April 2005, (URL: http://www.avo.alaska.edu/images/image.php?id=2906).

McGimsey, R. G., Neal, C. A., and Girina, O., 2003, 1998 volcanic activity in Alaska and Kamchatka: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Open-File Report OF 03-0423, 35 pp, (URL: http://pubs.usgs.gov/of/2003/of03-423/).

McGimsey, R.G., Neal, C.A., Dixon, J.P., and Ushakov, S., 2008, 2005 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2007-5269, 94 pp, (URL: http://pubs.usgs.gov/sir/2007/5269/).

McGimsey, R.G., Neal, C.A., Dixon, J.P., Malik, N., and Chibisova, M., 2011, 2007 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2010-5242, 110 pp, (URL: http://pubs.usgs.gov/sir/2010/5242/).

Myers, J.D., Marsh, B. D., Frost, C. D. and Linton, J.A., 2002, Petrologic constraints on the spatial distribution of crustal magma chambers, Atka Volcanic Center, central Aleutian arc, Contributions to Mineralogy and Petrology, vol. 143, issue 5, pp. 567-586, DOI 10.1007/s00410-002-0356-7 (URL: http://link.springer.com/article/10.1007/s00410-002-0356-7).

Neal, C.A., McGimsey, R.G., Dixon, J.P., Manevich, A., and Rybin, A., 2009, 2006 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory: U.S. Geological Survey Scientific Investigations Report 2008-5214, 102 pp, (URL: http://pubs.usgs.gov/sir/2008/5214/).

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.

Information Contacts: 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; Game McGimsey, AVO.