My initial plan for this post was to look into a 2022 – 2023 earthquake swarm at Takawangha volcano on Tanaga Island, mostly because it was an Aleutian volcano I had never heard of.  But like most of these posts, the more you find out about the immediate locale, the more initial plans end up being opening bids rather than definite maps to travel by.  This one expanded to include the Tanaga volcanic cluster just W of Takawangha.  The cluster includes Sajaka, Tanaga, and East Tanaga.  Takawangha is immediately E of the cluster, 10 km from Tanaga.  All four major vents are arrayed along a 13 km E-W line.  

The four volcanoes are located on Tanaga Island, part of the Andreanof Islands.  The Andreanof Islands stretch some 440 km between Amchitka Pass and the Rat Islands to the W and Amukta Pass and the Islands of the Four Mountains to the E.  Total area of the islands is just under 4,000 km2.  Total population is 412 (2000 Census) with most of them in Adak, on Adak Island, some 94 km E.  In turn, Adak is 1,930 km SW from Anchorage.  The island was rocked with major earthquakes in 1957,1964 and 1977.

Tanaga Island measures 528 km2.  Its highest point is Mount Tanaga.  It is located some 100 km W of Adak Island.  The island is uninhabited, though there are several cabins available and Aleut villages on the E side of the island.  There are no native land mammals on Tanaga Island.  The island is part of the Aleutian Islands Wilderness and the Alaska Maritime National Wildlife Refuge. 

Adak was developed as a Naval Air Station following the end of WWII.  At its peak, it housed 6,000 navy and Coast Guard personnel and their families.  The base officially closed in 1997.  The Aleut Corporation purchased military facilities finalized in 2004.  As of 2021, the Navy is considering reopening the air bast at Adak. 

Adak and surrounding islands have a subpolar oceanic climate with persistently overcast skies, moderate temperatures, significant precipitation and frequent storms.  Winter squalls produce gusts over 100 knots.  Average temperatures over the year range from -7° C in the winter to 16° C in the summer.  Average annual precipitation is 167 cm.  December is the wettest month.  26 km of snow falls every year.  The Aleut Corporation is the current economic engine for Adak, though the US military and Coast Guard appear to be returning to the village.

Tanaga is monitored by the Alaska Volcano Observatory, AVO.  There are six seismographs on the island.  Both Tanaga and Takawangha are monitored in real time.  There is a webcam on the island.  All can be reached through the AVO webpage and the Tanaka and Takawangha pages.

Region

With the exception of Semisopochnoi, all volcanoes listed below are part of the Andreanof Islands.  Generally, they erupt andesites to basaltic andesites.  Flank collapses N into the Bering Sea are not uncommon along this stretch of the Aleutians, with at least one known from Bobrof.  Semisopochnoi is another candidate.  All distances are measured from Tanaga.

Semisopochnoi 

As with Great Sitkin to the E, 1,221 m Semisopochnoi is somewhat outside our normal limit of 100 km for regional volcanoes but forms a nice bracket on the region.  It is 152 km W from Tanaga.  Unlike the rest of the volcanic islands listed below, Semisopochnoi is located in the Rat Islands to the W of the Andreanof Islands.  We reviewed activity here in a 2015 post.  It was most recently active Dec 2022 – Apr 2023.  We took a look at activity here in a 2015 post. 

Amchixtam Chaxsxii 

Amchixtam Chaxsxii is a submarine cone rising 580 m from the seafloor some 116 km W from Takawangha.  It rises within 115 m of the surface.  The base is 4 km in diameter.  The name was selected by the local Ugangax community. 

Gareloi 

Mount Gareloi is a 1,573 m stratovolcano that occupies most of an 8 x 10 km island some 45 W of Tanaga.  The volcano has a pair of summits and a prominent SE-trending fissure formed in the 1929 eruption.  The fissure extends into the sea.  There are steep sea cliffs cut into rocks of an older, eroded center on the SW coast.  At least three flank collapse debris avalanche deposits are found offshore.

The two summits are separated by a narrow saddle, with two small glaciers extending NW and SE from the saddle.  The N peak is higher, located on the S rim of a crater 300 m across.  There are multiple active fumaroles in the crater.  13 younger craters 80 – 1,600 m in diameter are aligned down the fissure from the S summit to the ocean.  The fissure, craters and four blocky andesitic lava flows were all produced by the 1929 eruption.  That eruption also produced an andesitic tuff covering 2.5 x 5 km on the SE flank.

Gareloi was built in two periods separated by an extended period of quiet.  Some lows for the older period appear to have erupted from the flanks.  Some valleys cut in the older sequence are U-shaped, suggesting glacial erosion.  Younger rocks are uneroded.  Most of them erupted from the summit.  One of the 1929 craters held a small, milky blue green lake in 1949, suggesting acidic fumaroles were active in the crater at that time. 

Smithsonian GVP Bulletin reports list a pair of smallish ash eruptions Aug and Sept 1980.  Satellites imaged another plume Jan 1982.  A flow like feature was imaged on the N crater rim Sept 1987.  Final Bulletin was a PIREP of a minor ash and steam eruption from Gareoli Sept 1996. 

Bobrof 

738 m Bobrof is a small volcanic island located some 49 km E of Tanaga.  The island is surrounded by a massive submarine deposit likely created by a flank collapse event to the NE.  Bobrof has andesitic pyroclastic flows.  Activity was both explosive and effusive.  Volcanic materials are similar to those of Moffett 50 km E.  Geology of the island and its volcanic history are unknown. 

Kanaga 

Kanaga is a 1,307 m andesitic – basaltic / andesite stratovolcano located in the Kanaton caldera on the N tip of Kanaga Island, 68 km E from Tanaga.  It most recently showed unrest 3-9 Jan 2024 following an explosive event the previous Dec.  We took a look at possible future flank collapse in a 2016 post.

Moffet   

Moffett is the largest and youngest of three volcanoes on the N part of Adak Island.  From W – E, these are Moffett, Andrew Bay and Adagdak.  It is located 97 km E from Tanaga, 10 km NW from the town of Adak itself.  The 1,196 km stratovolcano erupts andesites to basaltic andesites to basalts. 

Moffett is heavily glaciated, constructed from thick andesite flows, flank domes and a substantial parasitic cone constructed of multiple thin basalt flows.  There is a scoria covered, block dome on the S flank of Moffett that may be the youngest volcanic features of this center and the entire island.  The Heart Lake lava flow W of Sweeper Cove on the SE side of the volcano is another young feature.  There are recent lava flows down much of the W side of Moffett.  Several Holocene ash layers on Adak Island are attributed to Moffett.

Its proximity to Adak Naval Air Station makes it one of the most accessible of all Aleutian volcanoes.  Kiguga Warm Springs are located at Cape Kiguga, W of Moffett.  The N part of Adak Island has been investigated for geothermal power production.

Andrew Bay Volcano 

Andrew Bay Volcano appears to be the oldest and perhaps smallest of three volcanoes making up the N part of Adak Island.  It is located some 105 km E from Tanaga, 10 km ENE from neighboring Moffett.  It is a smallish, highly eroded volcanic center of 40 km3.  The 350 m structure has been eroded to the point where volcanoclastic debris and the occasional thin andesite flow are all that remains.  Much of this volcano was obliterated by caldera formation, sea level rise following the end of the last glacial, and glacial erosion.  Much of its W half is located today below sea level. 

Adagdak 

The 645 m Mount Adagdak is the third relatively recent stratovolcano located on Adak Island, 108 km E from Tanaga, 13 km NE from Moffett, 3 km NE from Andrew Bay, and 14 km NNE from Adak itself.  Like Moffett, both volcanoes are smallish when compared to other large Aleutian volcanoes, with several flank domes.  The summit cone has been mostly destroyed by glacial erosion.  It was filled with an andesite plug.  There are another five domes scattered on the flanks.  The Andrew Bay Hot Springs are located along the coastline W of Mount Adagdak. 

Adagdak grew in three stages starting with thick andesitic lava flows.  This was followed by construction of two composite cones dated 2.2 and 1.2 Ma.  Final activity built three domes 201 ka.  There is a summit crater with a well-defined S rim has an eroded andesite plug.  The plug may have been at one time a Pelean spine.  Large blocks of the spine are scattered across the shield.  There is a late-stage basalt dome on the SE flank.

There is a suspected N directed explosion and ashflow that may have breached the summit crater before dome emplacement.  This shows up in Google Maps Satellite View as a pair of nested amphitheaters open to the N.  The larger one is 1.5 km across.  The smaller one is 1 km across.  Either or both could have been easily created by a directed blast or flank collapse. 

Great Sitkin 

While Great Sitkin is somewhat outside our normal regional review at 142 km E from Tanaga, we took a look at it as a source of a flank collapse in a 2016 post.

Tanaga  Volcanic Cluster 

The Tanaga volcanic cluster (TVC) is on the N half of Tanaga Island.  It currently has three active volcanoes, Sajaka, Tanaga and Takawangha.  There are several older inactive eroded volcanic centers.  The cluster is unique among volcanoes in the central Aleutians in that it includes four volcanoes in close proximity that may share a common parent magma.   Volcanoes on Tanaga Island erupt low silica magmas, generally basalts and basaltic andesites and are less explosive than other island volcanoes.

Tanaga Island hosts long-lived volcanism.  The oldest rocks under the N part of the island are at least 250 ka.  They mark the beginning of relatively continuous volcanic activity from the cluster and Takawangha.  Because of erosion, glacial scouring and hydrothermal alteration, little is known about the volcanoes that produced the earliest lava flows and rock on the NW part of the island.  The volcanic structure was much larger than today, probably centered between Tanaga and Takawangha. 

Sajaka

Sajaka, 1,354 m is named for Cape Sajaka on the Extreme NW of Tanaga Island.  It is a compound volcano, with the older Sajaka One cone to the E partly destroyed by a debris avalanche into the ocean the last few thousand years.  The new 1,312 m Sajaka Two cone grew into the amphitheater.  This cone is built by steeply dipping, interbedded scoria and thin, spatter-fed lava flows.  The new cone is steep with slopes reaching to the shoreline.  Three shallow explosion craters are found on its summit.  The saddle between the two cones has a small glacier.

There are no debris avalanche deposits visible on land.  There is a pyroclastic flow deposit on top of Sajaka One indicating the collapse may have been a hot, directed blast.  The new Sajaka Two cone mostly fills the Sajaka One amphitheater.  The Sajaka One collapse is thought to be around 5 ka.  Eruptions from Sajaka Two produce thin, runny lava and spatter flows interbedded with scoria layers, a strombolian eruptive style. 

Tanaga

The 1,806 m Tanaga is a steep, classic stratovolcano with a small crater on its summit.  Lava from Tanaga flowed from the summit several kilometers down the N and S flanks, reaching the ocean on both sides.  Another series of thick lava flows wrap around the E side of a ridge of older rock and fill a glaciated valley.  The cone is mostly snow-free during the summer.

Tanaga and East Tanaga are much younger and share similar eruptive styles.  Most eruptions produce blocky lavas with explosive deposits confined to areas near the vent.  One exception is a young tephra layer at several sites around the N part of the island.  This probably erupted from E Tanaga and is several centimeters thick close to the vent.

There is a broad, crescent shaped scarp, concave to the W that separates the current Tanaga and Takawangha cones.  This ridge is thought to be the residual rim of an amphitheater formed as the older structure collapsed into the sea.  The collapse took place 240 – 118 ka.  Similar collapses took place for other Aleutian volcanoes on the N edge of the underwater Aleutian ridge.  There is no widespread eruptive deposit yet associated with this collapse.  It was either removed by erosion or the collapse was entirely gravity driven.

Following the collapse, the current volcano grew near the current location of Tanaga.  It was active 118 – 40 ka.  The older volcano remnants are deeply eroded and covered by the modern Tanaga and East Tanaga cones. 

East Tanaga

East Tanaga, 1,584 m is located on the NE shoulder of Tanaga.  It has a 650 m crater on its summit and two more ice-filled craters on its NE flank.  These three craters are aligned with the summit crater on Tanaga.  E Tanaga’s N slopes are similar to those of Tanaga, with lava flows down the steep flanks to the coast.  Lava flows down the S flank filled a glacially carved basin.  Lava flows from both Tanaga and E Tanaga overlap.  The cones were active during the same period.  Upper flanks are blanketed with ash and scoria.

Takawangha

1,449 m Takawangha is a separate stratovolcano located 10 km E from the Tanana volcanic complex.  It is a broader volcanic structure than the ones of the Tanaga cluster.  Much of it consists of older basement rock beneath Takawangha lava.  It is built of both glaciated and unglaciated lavas, and older than the Tanaga cluster volcanoes.  The summit is mostly ice covered.  There are five young craters on the summit that erupted ash and lava flows over the last few thousand years.  Parts of Takawangha’s cone are altered, with orange-yellow outcrops of older rocks on its flanks. 

Takawangha is the oldest currently active stratovolcano on the island.  It has been active over the last 60 ka.  Many lava flows from it have been eroded by glacial action until uncovered by the most recent glacial retreat some 10 ka here.  Recent eruptions here produced blocky lava flows from a shallow crater at the summit with minor tephras.

Part 2 will take a more detailed look into how the Tanaka Volcanic Cluster grew and then move into eruptive history, current activity, tectonics and conclusions. 

Additional information

Mount Takawangha, Wiki 

Tanaga Island, Wiki 

NASA JPL, Takawanga Volcano, Alaska, Dec 2022

Volcanic and tectonic sources of seismicity near the Tanaga volcanic cluster, Alaska, Lally, et al, Jun 2023 

Geologic database of information on volcanoes in Alaska (GeoDIVA), GroPlatform.gov

2017 volcanic activity in Alaska – summary of events and response of the Alaska Volcano Observatory, Scientific Investigations Report 2020 – 5102, USGS

2018 update to the US Geological Survey Nation Volcanic Threat Assessment, Scientific Investigations Report, 2018 – 5140, USGS

Regional controls on volcano seismicity along the Aleutian Arc, Buurman, et al, Mar 2014

Preliminary volcano – hazard assessment for the Tanaga volcanic cluster, Tanaga Island, Alaska, Scientific Investigations Report 2007 – 5094, USGS

Preliminary geology of the Tanaga Island volcanic cluster, western Aleutians (Alaska), Coombs, et al, Dec 2004

Geology and 40Ar/39Ar geochronology of the medium- to high-K Tanaga volcanic cluster, western Aleutians, Jicha, et al, May 2012