Growth

The oldest pre-Tanaga volcanic rocks date back at least 295 ka.  These are basalts on top of the Cable Bay Debris Avalanche deposit.  There are other debris avalanche outcrops on the N shore, but none of them are as old.  The source for the Cable Bay deposit is unknown, though likely N and W because basalts to the W are all younger.

There is a thick series of volcanic rocks underlying newer Tanaga volcanics.  These include up to boulder sized rocks in a matrix with altered and unaltered lavas.  This pre-Tanaga unit dates 251 – 91 ka.  These form a high, curved ridge E of Tanaga and E Tanaga.  It also underlies all of Takawangha.  It is absent beneath Sajaka, Tanaka and E Tanaka.  Its W exposure may mark the scar of a large debris avalanche that removed the W flank of the island. 

Exposed rocks in the steep walls of this scar are highly altered with at least one area of what appears to be conduit breccias.  The exposure may be the remnant of a volcanic center that produced some to all pre-Tanaga rocks on the NW part of the island.  There is a more recent 3 km long debris avalanche deposit formed by collapse of these altered rocks just W of the Takawangha summit.  Rocks here date 251 – 91 ka. 

There is a second ridge of steeply SW dipping basalt lava outcrops 3 km S of the Tanaga summit, the 181 – 140 ka South Ridge basalts.  The unit is cut by several dikes.  One of these is dated 118 ka.  The uppermost section of basalts may have been truncated by glaciation, significantly changing the slope of the lava flows.  A volcanic center existed near the current Tanaga 126 ka.  Undated altered pyroclastic rocks are found in the lowlands E of the Tanaga volcanic cluster.  A rugged, heavily eroded ridge E of Rough Bay on the NE part of the island may be the remnant of another volcanic center.

Orange top volcano dates 89 – 62 ka.  It is a 3 km ridge located 5 km SE of the Tanaga cone.  It is a stack of lava flows and interbedded volcanoclastic rocks rising from sea level to 700 m.  While not different in appearance from other pre-Tanaga rocks, these are much younger than ridges on either side.  The ridge is interpreted as a separate Orange Top volcano that formed in the lowland between neighboring ridges of older rock.  The N section of the Orange Top ridge is the highest.  It has two collapse scars cutting into the highly altered core of the volcano.  Alteration may be due to contact between magma and glacial ice during a past ice age.

Takawangha is the longest–lived of currently active volcanoes in the cluster, 86 ka – recent.  Lavas and pyroclastics erupted from its summit over the last 86 ka.  Much of its 1,499 m elevation is based on a thick stack (over 900 m) of pre-Tanaga rocks exposed close to the summit on the E and W. 

Takawangha was probably a much larger volcano.  Most of the lavas show interaction with ice (glaciers).  Eroded ridges high on the flanks expose lavas and pyroclastics that deep steeply from the summit.  Lower on the flanks, glacially smoothed Takawangha lavas cover older pre-Tanaga lavas and pyroclastics.  While some of these lavas date 86 – 65 ka, those on the E end of this unit are undated.

There is a glaciated basaltic trachyandesite lava field on the S flank from the summit to 274 m.  Three samples from this field date 37 – 32 ka.  There is a knob on the S flank at 216 m that may be a glaciated cone or dome.  It is compositionally distinct from surrounding lavas and should be older than 37 ka. 

Holocene activity on Takawangha formed five craters at its summit.  These erupted trachyandesite lavas to the N, E and S.  The most notable of these is a large “wrinkled rug” unit from the S summit crater.  This lava covers a large part of the S flank, older pre-Takawangha basement, and fills the upper part of the valley above Cable Bay.  A single dating attempt suggests that this lava erupted within the last few hundred years.

Two eroded ridges on the N shore of the island rise to 920 m with steeply dipping lavas.  These are the remaining portions of another vent, named the Lost Valley volcano on the site of the modern Tanaga vents.  The W ridge appears to be the cone while lavas on top of the E ridge just W of Lost Valley cover older pre-Tanaga lavas.  These lavas date around 42 ka.

Tanaga is a steep-sided Holocene cone, the highest point on the island.  Its lavas interlayer with East Tanaga lavas from the E.  Basaltic and basaltic andesitic lavas on the N flank reach the shoreline.  Those that flowed S fill valleys between the two ridges of glacially eroded lavas, the South Ridge basalts and Orange Top volcano.  Alternately, the flowed over the SW flank of Sajaka constructing lava deltas on the shoreline.  Tanaga lavas are young, as recent as historic, most certainly Holocene.  A basaltic andesite flow near the shoreline was dated (imprecisely) at 7.7 ka.

The East Tanaga cone is lower than neighboring Tanaga at 1,584 m.  It is not as badly eroded, relatively flat and topped with a 250 m crater.  The cone is built by basaltic to andesitic lavas that extend to the shore on the N and fill in a amphitheater to the S.  Most of these lavas are Holocene, with one sample dated 19 ka.  A young lapilli layer is found on or near the surface at several locations on the NW part of the Island.  It thickens toward East Tanaga summit and may have originated from there.

Sajaka is the westernmost volcanic center of the cluster.  It erupts the least silicic lavas of its neighbors.  The cone was built in two phases, forming Sajaka One followed by the younger Sajaka Two.  A columnar jointed basalt along the S shoreline dates around 3.2 ka. 

The W half of Sajaka One was destroyed by a sector collapse directed W.  The submarine debris avalanche deposit is visible via bathymetry 9 km W of the island.  It is not preserved on the island.  Collapse appears to have been a hot affair, taking place at the same time as pyroclastic flow deposits along the N contact between Sajaka One and Sajaka Two cones.  The pyroclastic flow unit is up to 60 m thick.  The collapse created a 290 m high wall of thin Sajaka One lavas, scoria fall deposits and pyroclastic flow deposits.

Sajaka Two grew in the amphitheater.  It is an agglutinate and scoria cone the extends to the shoreline built of multiple, steep interbedded scoria and thin, spatter-fed lavas.  There are three nested craters at the summit.  The innermost one appears to have been the source of most recent eruptions.  Associated pyroclastic deposits can be found on the summit of Sajaka One.  

Takawangha was fed by a single magma supply active over the last 86 ka.  It production is one tenth that of the W part of the Tanaga cluster.  Sajaka (5.5 km3), Tanaga and East Tanaga (27 km3) are substantial cones built since the end of the end of the last glacial maximum.  They reflect a shift in focus of magma production to the W. 

It is difficult to estimate eruptive rate for the entire 97 km3 cluster because glacial scouring, marine erosion and mass wasting have removed a significant percentage of eruptive products over its 295 – 91 ka history.  Activity has been more or less continuous over the last 150 ka. 

Flank Collapse

Most volcanoes between 176° W and 177° E show some sort of flank collapse.  These range from thin, Stromboli-style collapses to deep, major edifice failures.  Most of these are gravity driven due to rapid growth on the N edge of the 4,000 m Aleutian Ridge.  Some have been accompanied by eruptions and directed blasts. 

Tanaga Island is believed to have once been a singular stratovolcano, ancestral Tanaga.  It experienced two major flank collapses.  The first 240 – 120 ka formed when the NW portion of the cone collapsed into the sea.  It is preserved as a broad, crescent shaped scarp, concave to the W that separates Tanaga and Takawangha.  The scarp continues underwater to the N of the Island, separating slopes to the E and W.  Rock along the scarp is highly altered 3 km E of Takawangha.  Possible conduit breccias are also visible.  A second flank collapse took place 3 ka on the W flank of Sajaka. 

   To the S of the island, there is a break in the Aleutian Ridge 100 m below current sea level.  This platform aligns with Orange Top volcano and additional Holocene centers W of the platform.  This break may be the S continuation of an earlier collapse event, though no offshore debris field has yet been imaged.  It is possible the field (should it exist) is obscured by younger sediment.  The event creating this was similar to large flank failures at Kanaga and Great Sitkin.

S Ridge basalts dated 180 – 118 ka are W of the scarp separating Tanaga and Takawangha.  Younger, pre-Tanaga lavas and Orange Top volcano are to the E.  There is some discussion as to whether this was an actual flank collapse due to the lack of debris field on the sea floor N and W of Tanaga.  Additionally, the collapse would have taken place 91 – 41 ka.  An alternate explanation would be that the abrupt transition from older to younger volcanic rocks E – W is simply explained by extreme glacial erosion and W migration of volcanic activity.

In contrast, it does appear that smaller, Holocene sector collapse took place on the W side of Sajaka.  It was accompanied by a lateral blast similar to Bezymianny (1959) and St Helens (1980).  The primary debris field is located offshore.  There is a pumice-bearing pyroclastic flow on Sajaka One lavas.  The collapse may have been relatively recent 3.2 – 0.8 ka. 

There are a significant amount of hydrothermally altered rock in the pre-Tanaga edifice.  These weakened zones have been the site of at least one small failure W of Takawangha.  They are likely to be sites for future landslides. 

Activity

Information about historic activity on Tanaga island is sparse due to its remote location.  There have been at least five episodes of possible activity since western ocean vessels started traveling past the island in the 18^th Century.  The first of these was 1763 – 1770 when the volcano was described as constantly active.  Observations of smoking were reported in 1791 and 1829.  These are more likely fumaroles at the summit of one or more cones.  The final activity report was 1914 when a lava flow was reported.  Note that none of these reports specify an individual peak.

Tanaga is listed as the 61^st largest threat volcano in the US.  Takawangha is listed as the 84^th largest threat volcano in the US.   Both volcanoes are grouped as moderate threat for future eruptions.  The first seismic station installed at Tanaga Aug 2003.  The array was completed Aug 2003 and added to the AVO monitored list Jun 2004.  All four volcanic centers on the island have erupted over the last thousand years. 

The last known eruption of Tanaga took place in 1914.  There are no known historic eruptions from either Sajaka or Takawangha.  Both Tanaga and Takawangha had earthquake swarms in the last 25 years.  The 2023 Lally, et al paper noted that they had successfully located over 3,000 earthquakes 2003 – 2017, with a total over 5,680 2003 – 2023.

Not all seismic events on and around Tanaga Island are associated with the Tanaga volcanic complex.  There are a few tectonic quakes in the W portion under Sajaka and central Tanaga island.  A July 2007 swarm took place N of Cape Sudak, some 25 km E of Takawangha at 6 km deep.  There was an M 6.6 May 2008 followed by a robust aftershock sequence of over 1,500 events.  A second major event took place May 2009 with a M 4.0 earthquake followed by a two-month sequence of aftershocks.  The W coastline of Tanaga Bay and S of Tanaga Island were both seismically active.  These quakes are all considered to be regional tectonic activity.

The only fumarolic activity visible from the sea was on the NW flank of Sajaka Two near the shoreline.  Temperature and composition of emitted gases are unknown.  Blocks of native sulfur near one of Takawangha’s vents indicate fumarole activity in the recent past.  There is a small thermal spring on the E coast of the island in Hot Springs Bay.  The spring has some deposited minerals with a temperature that is below boiling. 

Tanaga Activity

AVO Bulletin reports on Tanaga carry two seismic swarms, the first in 2005 and the second in 2023.

The 2005 swarm began Oct 1 with 15 – 68 earthquakes daily.  Background level is one per month.  These were centered 2 km NE of the summit at 10 – 20 km below sea level.  Most quakes were around M 1.0.  There was a marked increase in seismicity 5-7 Oct, with the largest M 1.9.  Depth decreased to 6 – 12 km below the summit.  At this point, AVO changed the Aviation Color Code to Yellow, noting that the swarm did not indicate imminent eruptive activity.

Seismicity continued 7-14 Oct, though the rate decreased slightly.  This was the highest recorded rate since the seismic network was installed in 2003.  Earthquake rate continued to decline but remained above background levels.  Weak volcanic tremor was recorded Oct 24.  Activity continued to decline though Nov.  AVO returned the Concern Color Code to Green by Nov 25.

The second swarm began Mar 4, 2023, with events up to the M 2 range centered 9-18 km deep.  Seismicity continued to increase to 2-3/minute.  AVO raise the Aviation Color Code to yellow once again.  Seismicity continued and rose to 8-9 Mar.  The largest event was M 3.9 under Tanaga.  Events under Tanaga and Takawangha took place at several events per minute at the peak, 10-11 Mar and decreased slightly12-14 Mar.  Earthquake magnitudes were up to M 4 at the peak.  Activity started decreasing following the peak and continued to decrease though July.  Satellite imagery did not detect any deformation on the island.  Aviation Color Code was lowered to Green July 18. 

Takawangha Activity

There are no historic eruptions from Takawangha.  The edifice is hydrothermally altered and may be unstable, creating localized debris avalanches from its flanks.  Summit is ice-covered with 5 young craters that erupted ash and lava flows in the last few thousand years. 

AVO Bulletin Reports for Takawangha carry two seismic swarms, the first of these in 2017, with the second starting before the combined swarm under both Takawangha and Tanaka in 2023.  Note that there were three previous swarms recorded in same area Oct 2005, May 2008, and Jun 2009. 

An energetic swarm began on Tanaga Island Jan 23, 2017 located 6-7 km ESE of Takawangha near Gusty Bay.  Aviation Color Code was raised to Yellow. The swarm peaked Jan 24 with 190 events.  They steadily decreased through Jan 27.  Most of the events were shallow at 7-8 km.  The swarm continued to decrease to background levels by Feb 10 when they declined to background levels. AVO returned the Alert Level to green Feb 10.

The second swarm was detected a few days before 17-18 Nov 2022.  They intensified 17-18 Nov when the Aviation Alert Level was raised to Yellow.  Quakes were shallow at 3-6 km below sea level with the largest M 2-3.  Intensity was variable during late Nov.  The swarm started to decline Mar 1, but abruptly increased Mar 8 when the previously described Tanaga swarm Mar 4 – Jul 18, 2023, swarm began.  That swarm was attributed to both volcanoes. 

Tectonics

The Aleutian volcanic arc a result of the ongoing collision between the Pacific and North American Plates.  It is a chain of active volcanoes that extends from the Alaska mainland to Buldir Island at 175° E.  E of 166° W, the arc has been constructed on oceanic crust.  The curved shape of the Aleutian ridge coupled with the convergence vector over the last 46 Ma means that the rate of convergence varies along the arc.  Near Tanaga at 178° W, the convergence is 7.1 cm/yr.  The crust near Seguam Island is 25 – 35 km thick.  There is very little thickness or velocity data available W of 173° W.  Magma compositions transition W of Adak Island from andesites to more primitive basaltic andesites to basalts.  The andesites may reflect slab melting. 

The oblique convergence angle and regional plate coupling between the Pacific and North American Plates near the Andreanof Islands transports the region about 1 cm/yr relative to the North American Plate.  This causes portions of the forearc to deform as rotating, fault-bound blocks.  Clockwise rotation of the crustal blocks in the forearc is responsible for multiple large strike-slip earthquakes, impacting orientation and magnitude of local stress fields. 

Conclusions

Eruptive activity of the Tanaga volcanic cluster has been more or less continuous over the last 150 ka.  Glacial ice has been present throughout that history.  Ice does not appear to have inhibited eruptions from taking place for any significant time.  The cluster suffered at least one sector collapse and one deep-seated edifice failure that dramatically altered the shape of the cluster.  The continuing threat of future eruptions and mass-wasting events will pose local and regional hazards for the foreseeable future. 

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