Korovin is the most recently active member of a volcanic complex on the NE tip of Atka Island. It tops out at 1,533 m with two craters on its summit, the largest of which is about a kilometer in diameter. There are multiple vents associated with the complex. Konia volcano is on the SE flank and has a fresh-looking cinder cone. Mount Atka is about a kilometer to the S. The largest part of the complex is Mount Kliuchef, another 4 km to the S. Konia, Atka and Kliuchef are all eroded to varying degrees.
The system is dominantly basaltic with varying amounts of andesites and dacites. Both Korovin and Konia have recent dacite lava flows on their flanks.
Atka Island is the largest of the Andreanof Islands in the Alaskan Aleutians. It is located some 1,800 km SW Anchorage. The island is around 1,050 km2 in area. Neighboring Amlia Island, located immediately to the E, is 74 km long with nearly 450 km2 of rough terrain. Amlia is the second-largest uninhabited island in the Aleutians.
The largest local village is Atka, with a population of 61 – 81 Aleuts (Unangas). It has been populated for at least 2,000 years. The first contact with Russians took place in 1747 and the village became an important trade site and safe harbor for the Russians. As with most events during that point in history, dealings with the Russians and Atka residents turned ugly when a number of hunters were forcibly relocated to the Pribilofs to harvest seals in 1787. The current townsite was settled in the 1860s.
Excess hunting crashed sea otter population by the end of the 1800s after which Atka had no viable economy. Reindeer were introduced in 1914. Fox farming provided a decent economy during the 1920s. After the Japanese attacked Unalaska and seized Attu and Kiska in 1942, the US evacuated the village and burned it to the ground to prevent Japanese occupation.
The community was rebuilt after the war and now supports a subsistence economy and a commercial halibut fishery with both onshore and offshore fish processors. A 45-boat fleet operates out of the local port, a deep protected anchorage in Nazan Bay. The port has good access north into the Bering Sea and south into the North Pacific. For a small village, its facilities and infrastructure are relatively modern, though always in need of improvement. The reindeer herd on the Island now numbers over 2,500, sufficient to feed the inhabitants.
Local climate is considered maritime, with short, cold, windy summers and long, cold, windy winters. It is overcast year-round with summer fog. Temperature range is generally -1 C – 13 C. Around 150 cm of precipitation falls with 170 cm of snow yearly. Water temperatures are quite cold, generally no warmer than 9 C.
Korovin is monitored by Alaska Volcano Observatory (AVO) with a pair of webicorders, KOFP and KOKL in operation. I have not found any webcams for Atka.
Volcano
The northern lobe of Atka Island was built by the Atka volcano, the largest volcanic center in the central Aleutians at 200 km3, covering 360 km2. There is nothing larger to the W and the closest larger center is Umnak, some 300 km E. Seguam is the closest volcano to the E. Sergeif on the W end of Atka Island, Koniuji, Kasatochi and Great Sitkin lie to the W.
The overall structure is a broad central shield with a central cone (Atka Volcano) ringed by 7 -8 smaller satellite cones. It was built by predominantly basalt and andesitic lavas. Over time, the magmas evolved to a more silicic type before the caldera forming event. The central cone collapsed creating a 5 km diameter caldera some 500 – 300 ka. The caldera collapse erupted a massive dacitic lava flow called Big Pink. Interestingly, there is no associated pyroclastics or ash flows from this event. A number of intrusive dikes cut the shield at about the same time.
Subsequent activity started around 100 ka from the satellite vents, building four major volcanic cones, Korovin, Kliuchef, Konia, with Sarichef being the youngest. Kliuchef volcano grew on the N rim of the ice-filled caldera with at least five vents. The two main summit vents and easternmost vent appear to be fresh. Korovin was built next, overlapping considerably with Kliuchef. Konia is midway between Korovin and Kliuchef and is about the same age as Korovin.
Korovin continues to be the most active volcano and tallest on Atka. Its base is 7 km in diameter topped with a pair of summit vents. The NW vent is a symmetric cone with a small crater. The SE summit vent is about a kilometer in diameter and several hundred meters deep. The bottom hundred meters are nearly vertical and consist almost entirely of lava flows. This vent generally contains a crater lake which disappears before phreatic eruptions. There was at least one pilot report that It was full of lava. This vent is the source of most recent eruptions.
All volcanoes in the system are generally built of basaltic lavas interbedded with scorias and increasing amounts of pyroclastics, breccias and lahar debris. The summit of Kliuchef is mostly dacite, as are some recent flows on Korovin and Konia. There are thick lahar aprons filling many glacial valleys. Many of these have been cut by dikes spanning the volcanic center, apparently emplaced at the time of the caldera collapse.
The W side of Sarichef and the E flank of Konia are extensively eroded. Konia has a fresh cinder cone on its W flank. Sarichef is thought to be a satellite cone of the older Atka Volcano. Korovin and Kliuchef are not strongly eroded and may be post-glacial.
There are some volcanic connections between Korovin and Kliuchef. While they appear to be fed by separate magma reservoirs, they share a common hydrothermal system which may explain reports of synchronous phreatic eruptions from both volcanoes. The March 1987 eruption had simultaneous ash plumes from Korovin and several Kliuchef vents suggesting some connection with their hydrothermal systems.
Starting in 2004, AVO’s eight installed seismometers started collecting data on seismic anomalies, identifying a pair of shallow low velocity zones beneath Korovin and Kliuchef. These zones are 2 – 5 km beneath the summit of both volcanoes. There is a two-level structure of these anomalies, with the deeper one thought to be an intrusion of juvenile, hot magma and the upper being a region of partial melting and mixing between juvenile and evolved magmas. In addition to Korovin and Kliuchef, there are remains of at least six volcanic edifices that may have been active pre-caldera. Some of their active conduits may still exist and are visible via seismic tomography. The complex appears to have multiple magma reservoirs, visible to at least 26 km below the surface. The deepest one is mostly juvenile magmas with minimal crystallization. Crystallization took place 7 – 17 km where the magma evolved to more silicic basalts, basaltic andesites and dacites. The shallow reservoirs were tapped by the most recent eruptions of both volcanoes.
While Sarichev volcano at the eastern end of the island has a regular conical shape, there is little information about its recent eruptive activity. It is also at the edge of the model developed to image low velocity zones beneath Korovin and Kliuchef. That being said, there is a low velocity zone beneath it that may indicate an active magma conduit or hydrothermal fluids beneath it.
There are three hot spring areas associated with Korovin, Kliuchef and the Atka caldera. There is a fourth hot spring some 7.5 km W of Kliuchef. Hot springs and fumaroles are on the S and W flanks of Kliuchef and near the head of a glacial valley 6 km SE of Korovin.
Eruptions
Due to its remoteness, there are no reports of activity from Korovin before the 19th Century as the Aleuts did not have a written tradition. Since then, however, there were at least two reported eruptions in the latter stages of the 19th Century and one close to the turn of the 20th Century. Starting in 1951, there have been at least 11 eruptions. The largest of these was a VEI 2 in 1987, followed 11 years later by a VEI 3 in 1998. For the most part, recent eruptions have been in the VEI 1 range.
AVO installed a network of seismic stations on the northern part of Atka Island in summer of 2004. While data was available the following March, though it wasn’t until Dec 2005 that sufficient observations were logged that the volcano was considered to be seismically monitored. It is also monitored via local photography from Atka, passing air traffic, and satellite imagery.
Later in 2005 and 2006 – 2007 newly installed seismographs recoded increased seismicity. The 2006 activity recorded volcanic tremor in Sept – Oct. In Oct, the crater lake disappeared. By the end of Oct, white vapor plumes were observed. These coincided with strong tremor. Satellite imagery picked up hot spots in the crater and ash on the E flank of the SE crater in Nov. This ash was not present three days later. Sporadic activity continued through 2007, with inflation that began June 2006 tapering off in early 2007. Tremor bursts, steam plumes, ash deposits, and volcanic earthquakes were all observed through the end of summer 2007.
There were minor eruptions observed in 2002, 2004 and 2005 all of which ejected ash plumes. The first two were discovered via satellite photography. The 2005 eruption was observed from Atka.
AVO lists a series of reports of a weeklong ash eruption the first week of July 1998.
Satellite images of three distinct plumes from Korovin were observed March 1987. These came from the SE vent and two other vents at lower elevation on the flanks. The third vent was 10 km NE the summit vents. All reports were from passing pilots. Local residents reported sulfur smells but did not notice any unusual eruptive activity.
A substantial steam plume was observed May 1986 shortly following a M 7.7 earthquake some 100 km SSE. The 1987 simultaneous eruption took place 10 months after this. Steam from the crater was observed by a pair of climbers July 1983.
Tectonics
The basic tectonic driver of volcanism in the Central Aleutian and Andreanoff Islands is the subduction of the Pacific Plate beneath the North American Plate. The collision is slightly oblique in this section of the Aleutians and is proceeding just under 9 cm/yr. There are five other volcanic islands in this portion with different compositions and eruptive styles: Kanaga, Adak, Great Sitkin and Seguam. Atka Island is the largest of these. Aleutian volcanoes stay relatively fixed to one location for a long time, 5 Ma. This means that stable magma pathways were established beneath the volcanic centers.
Atka Island has two geographically distinct parts. The elongated SW part is 85 km long and up to 10 km wide. It and its neighbor Amlia Island to the E are composed of igneous and volcanoclastic materials with no evidence of recent volcanism. The NE portion of the island is a distinct volcanic center covering 360 km2 and connected to the main island by a narrow isthmus separating Korovin and Nazan Bays. This portion is recently volcanic with total erupted volume around 200 km3.
The age of the oldest erupted rocks in Atka is 6.6 Ma. The circular portion of the island was formed 1 – 2 Ma by basaltic and andesitic flows from a single eruptive center, eventually building the Atka volcano with a base diameter of 20 km and a peak likely reaching 2,200 m.
Aleutian volcanic activity dates as far back as 38 Ma. It proceeded in three pulses: 38 – 29 Ma, 16 – 11 Ma, and 6 – 0 Ma, the most recent pulse. The magma supply proceeds up a long-lived established conduit which has controlled the activity of the complex for over a million years.
Conclusions
Atka Island is home to a large active volcanic complex with Korovin and Kliuchef being the most recently active vents. It has an active hydrothermal system, at least two shallow magma reservoirs, and a deep magma conduit that has been active 1 – 2 Ma. Continued subduction of the Pacific Plate ensures a continuing magma supply and megathrust earthquakes from time to time. While remote, expect this system to remain active for the foreseeable future.
Additional information
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