Recently, Mjf and Tallis Rockwell commented in the café that several volcanoes in the Aleutians were undergoing unrest. After some thought, I realized that the Aleutians are within the United States territory, in Alaska, and that means they are very well monitored, and that this data is accessible. So I decided to dive into it. Turns out something really interesting is going down in this cold and remote volcanic arc. Four different volcanoes/volcanic complexes are experiencing growing seismic unrest, which started in late 2022 and has increased further into 2023.
First of all, I should say there is no need for much worry. All the volcanoes in this post are very remote, and the surrounding area is uninhabited. Only Aniakchak has a nearby population, which is Port Heiden, and for it to damage Port Heiden, it would have to go really, really big, which is highly unlikely. Even if they erupt, they shouldn’t do much damage. Additionally, they might not erupt. At this point, the activity is merely intrusive or gas/water driven. A drop of magma will not be much if you are trying to fill a whole bathtub. Recharges of magma and increases in hydrothermal activity, and degassing that might or might not be related to these recharges, are common phenomena in volcanoes. Volcanic systems have a rich internal activity that does not necessarily lead to eruptions.
In this post, I’m simply looking into a couple of volcanoes that are shaking. I will also look into their past activity, so that if they should erupt, we will know what to expect. So here we go into the four reactivating systems:
In the Katmai cluster, we have three different areas of seismic activity under four different recent volcanoes. All of these three areas have been escalating in unrest. The volcanoes are contiguous. From east to west we have Katmai, Trident, Mageik, and Martin. Katmai and Trident have their own separate seismic clusters. While Martin and Mageik are immersed within the same seismic flurry, and this topographically makes sense, given that these two cones are part of the same mountain made up of a number of old eroded overlapping volcanoes.
I have downloaded seismic data from the IRIS Earthquake Browser for each of these swarms. The three of them have monthly numbers of earthquakes that have been on the rise throughout 2022-23. The Alaska Volcano Observatory upgraded Trident Aviation Colour Code to yellow on 29 September 2022.
Earthquake depths are variable. At Trident, a large flare of micro-earthquakes averaging 24 km deep happened on August 25, followed by events averaging 16 km deep over the following 2 days, and since then events have been mostly shallow, averaging 4 km deep. Occasional deep events still continue to happen, though. At Mageik-Martin, earthquakes are very shallow, 2 km, possibly triggered by rising volcanic fluids, water, gasses and such. While at Katmai, earthquakes of variable depths are happening at the same time, with a cluster at 30 km depth, another at 16-19 km, and a third cluster from 12 km to the surface.
I also checked earlier years. Earthquakes as small as M 0.1 are located in 1997. So, since 1997, there seems to have been a very sensitive network. I don’t think this sensivity has changed much since then. The three locations seem to have experienced the strongest seismic activity in the 1997-2023 period, within 2018-2023. Trident had the greatest number of earthquakes in 2022. While Katmai peaked in 2020, although 2022 is still the second highest year, and Martin-Mageik peaked in 2019, with 2022 being its third-highest year in terms of seismic activity.
The Katmai cluster seems to have produced a dominantly effusive style, with the 1912 eruption being a rare event. The youngest major prehistorical activity is probably the one that formed Mount Martin. Enormous viscous flows of andesite-dacite lava with 62-64% SiO2 erupted from the presently ice covered cone of Mount Martin. These flows reach individual thicknesses of 70 meters and extend 10 kilometres to the northwest, infilling a broad glacial valley. The volume has been estimated in 9 km3. It has been proposed they were formed in a singular years or decades long eruption episode, which I think is likely. It happened at some point in the early Holocene, before 6000 years ago, given that ash layers of that particular age rest on top of the flows. Mageik also had a young episode of effusive activity which formed a 5.5 km3 edifice made up of 60-64% SiO2 andesite-dacite lava flows, each with individual thicknesses of 50 meters or more, this may have happened within the Holocene, but the age is not exactly known. Minor explosive eruptions may have also taken place in these systems.
The next major eruption is famous, and is the largest explosive eruption in the 20th century. The 1912 Novarupta eruption. This event happened from a vent on the north flank of Trident volcano. It erupted 17 km3 of pyroclastic fall material, and 11 km3 of ignimbrite (pyroclastic flows). The eruption produced two different groups of magma. First, a magma that is very rare in the world and in the Aleutians, a high silica rhyolite, with 77-78 % SiO2 and 0-5 wt% phenocrystals. And a crystal-rich andesite-dacite series, with 59-68% SiO2 and 30-40 wt% phenocrystals. The main episode of the eruption started with the high silica rhyolite, but in the latter part some dacite and andesite started to be mixed in. Two smaller andesite-dacite eruption episodes followed. The final event was the renewed eruption of high silica rhyolite that filled up effusively the bottom of the 700-900 diameter crater of Novarupta, making a small circular dome. The top of Katmai volcano collapsed during the Novarupta eruption.
A new eruption episode lasted from 1953 to 1974, birthing a volcano on the west side of Trident. Known as the West Trident volcano. The eruptions consisted mostly of ~50 meter thick lava flows with a volume 0.7 km3, and of andesite to dacite composition, similar to that of Mageik and Martin.
The next volcano is an isolated caldera system, 200 kilometres southwest of the Katmai Cluster. At Aniakchak, earthquakes as small as M 0 were first located in 2018. However, just over two months of activity in 2023, has produced more than three times the number of earthquakes recorded in four years during 2018-2021. Activity started to build up in late 2022 and rose rapidly in 2023. The Alaska Volcano Observatory upgraded the Aviation Colour Code of Aniakchak to yellow on 22 February, during its most restless recorded month since before 2018.
Earthquake depths are mostly within 6 km of the surface, and make two clusters. One cluster is under the southern side of the caldera, near the most prominent vent, which is Vent Mountain. The other under the east caldera rim, nothing special is there. Deep earthquakes extend to over 20 km underground below the caldera and along a band extending south from it.
We know little of the Pleistocene activity of this volcano, but more about what it did during the Holocene. Aniakchak erupted an andesitic ignimbrite with 60% SiO2, the Aniakchak I eruption, sometime around 9500-7000 years ago. But the volume or extent of this ignimbrite is not known. As far as I can tell, it has only been described near the present-day caldera rim, so may have been small. The volcano produced another eruption around 7000 years ago, the Black Nose Pumice, which deposited a considerable thickness of rhyodacitic pumice with 69-67% SiO2. Volume has not been calculated, but I get the impression it may have been a VEI-5 event. Then, around 3400 years ago, the 10-km wide caldera formed during the massive Aniakchak II eruption, which produced a 50 km3 bulk volume of pyroclastic flows. It started with 70% SiO2 rhyodacites and ended with 58% SiO2 basaltic andesite. About half the volume is dacitic while the other half andesitic. The pyroclastic flows reached into the Bering Sea 30 km away from the caldera, where they are exposed in cliffs along the coastline. These three are the most important eruptions of this period, but there were likely other events too.
Following the caldera formation, activity has constructed 17 vents, consisting of craters and domes of various sizes, arranged in a 6-km diameter ring within the caldera. Encircling a shallow magma chamber that is represented by a deflating area in the middle of this ring. At 2300 yr BP (years before present), a plinian eruption produced a dacite pumice fall to the north of Aniakchak. Following this event, eruptions may have been more quiet or more infrequent, until recently.
Around 380 yr BP a powerful eruption broke out from the northwest side of the caldera, violent explosions blew open a 2 kilometre wide crater, known as Half Cone, pyroclastic flows and surges swept across the caldera floor, showers of pumice extended for tens of kilometres, while fine ashfall may have reached as far as 330 km away, and a total volume of 0.75-1 km3 is estimated to have been erupted. The eruption ended with lava flows which effused from a 100-meter wide vent in the center of Half Cone and filled up the crater. The flows are relatively fluid and have a thickness of ~10 meters along the edges. Silica content ranges from 67 to 58 wt%, decreasing gradually during the explosive eruption, but the final lava flows were crystal-poor dacites with 65 wt% SiO2.
At some point before or after the major Half Cone eruption, Vent Mountain became active on the southern side of the caldera. Vent Mountain makes a elegant 500 meter tall cone with an 800 meter-wide crater at the top. The cone seems to be made up of layers of spatter and ash. At the end of its activity, lava ponded within the crater. It was possibly formed by repeated intense vulcanian explosions, adding layer upon layer of pyroclastic material. The last spatter deposits look more youthful than the Half Cone lavas (less ash on them). After the last activity of Vent Mountain, a 2 km long fissure opened across the southern foot of the cone. Vents all along the fissure emitted fluid looking crystal-poor dacites with 65% SiO2, which formed a series of lava flows. These flows seem to be less than a few meters thick on steep slopes near the vents, while the largest coulee thickens to merely 10-20 meters along its distal end in the flat caldera floor.
The last eruption came in 1931. It happened from the west side of the caldera. Explosions took place for 6 weeks. The climax was 10 days after the start of the eruption, when for several hours heavy ashfall produced total darkness within a radius of 100 km around the volcano. It erupted a volume of 0.9 km3 and formed a crater 700 meters wide. Lava ponded inside the crater at the end. As some other Aniakchak eruptions, it started with more silicic stuff and ended with a more mafic melt, shifting from 69 to 56% silica.
Okmok is a rather quiet one. Seismically speaking. That’s why the relatively minor earthquake activity since late 2022 is significant. The oldest earthquakes with less than M1 are from 2007, but maybe a dense seismic network was in place before that year, only that small earthquakes did not happen. The most active years were 2008 and 2009, 2008 was the last eruption of Okmok. The third most active year is 2022, and 2023 is on its way to surpass it.
The earthquakes are concentrated within 6 km of the surface under the southwest corner of the caldera, where Cone A is located, the most active historical vent. Deep earthquakes extend to 25 km depth to the south of the caldera. The deeper activity was dominant from 2009 to 2020. Instead, in 2021-2023 activity has been mostly shallow. Some minor shallow activity in 2007-2008 preceded the last eruption.
Despite Okmok being a near-continuously inflating volcano, unlike other Aleutian systems, its seismic activity seems very subdued. A GPS on the western side of the caldera has recorded about 1 meter of uplift since the 2008 eruption. Uplift will have been a bit higher towards the center of the caldera. Inflation accelerates and deccelerates repeatedly. But seismic activity has been below that of other volcanoes considered here.
The past eruption history of Okmok is very interesting. The volcano has undergone two recent caldera-forming ignimbrite eruptions. The first was at 12000 yr BP, and the second at 2000 yr BP. Both eruptions were very similar in size and style, and emitted at least 25 km3 of material each, but possibly as much as 50 km3. The 12000 yr event consisted of a minor initial rhyolite-andesite plinian eruption, which was then followed by a massive basaltic-andesite ignimbrite of 55 % SiO2. The 2000 BP event similarly started with rhyodacite, and then erupted the same basaltic-andesite ignimbrite. During the basaltic-andesite stage, pyroclastic flows covered the entire region within 10 km of the caldera rim in more than 10 meters of scoria, while dilute pyroclastic currents reached to almost 40 km distance, beyond the sea, into nearby Unalaska Island.
Post-caldera activity is reminiscent of Aniakchak, but more mafic. More than 12 craters form a 7 km wide ring around the beating heart of Okmok. Some of the craters are larger, up to 1 km, formed in stronger explosive eruptions, while others are small, 200 meters wide, and formed in low-level strombolian activity. Cone A, at the southwest end of the caldera, erupted 9 times from 1938 to 1997. Eruptions were basaltic, 52% SiO2. Several channelized aa lava flows, around 5 meters thick each, have flowed to distances of 5 kilometres from Cone A. While strombolian eruptions must have taken place at the vent itself.
Okmok erupted again in 2008, but this time from the north side of the caldera. A more evolved basaltic andesite with 55% silica came out. At the onset, an ash plume rose to 15 km high, and the total size of the event is usually considered being a VEI-4. The eruption was completely unexpected. Despite the presence of 13 seismic stations on the island at the time of the event, no precursory activity was detected up until 3 hours before the eruption. The only precursor visible in the seismic catalogue seems to be 17 M 0.5-2.2 quakes that happened from July 2007 to April 2008. Which is so low that it couldn’t be considered alarming. Then, the eruption itself, which was in July 2008, came after 3 months of complete seismic silence. Only a few hours before the explosion did activity pick up in the area where the vent opened. But the immediate precursory sequence itself was very weak, with less than 20 earthquakes detected, none of them exceeding M 2.4. As a result, the advisory level was risen directly from green to red when the eruption was already underway.
Takawangha and Tanaga
As I write this article, a powerful swarm rages under Tanaga Island with large earthquakes of up to M 4.1, below the twin volcanoes of Takawangha and Tanaga.
Earthquakes of less than M 1 have been located near Takawangha volcano since at least 2008, however the number of earthquakes in 2022-2023 is nearly three times the total number of events located during all previous years. Activity increased dramatically at Takawangha volcano in November 2022 and has persisted up until now. Then, on 4 March, activity skyrocketed at nearby Tanaga, producing 350 earthquakes within merely six days, and has kept going. AVO raised Takawangha to yellow on 18 November 2022 and did the same for Tanaga on 7 March 2023.
These two overlapping volcanoes consist mainly of crystal-rich basalts, basaltic andesites, and their alkalic equivalents. Takawangha is mostly eroded and little activity must have taken place in the Holocene. However, four youthful craters 200-700 meters in diameter crown the old edifice. They must have produced recent low-level explosive activity. Some lava flows have issued from the craters and reached down to near the sea. The fronts of these flows seem to be around 10 meters thick. These young lavas must be Holocene. Their composition is trachy-basaltic andesite with 54-56 % SiO2.
Tanaga is younger. Three prominent cones, with a combined volume estimated in ~32 km3, are aligned in a 7 kilometre E-W line. East Tanaga, Tanaga, and Sajaka. Tanaga towers nearly 2 kilometres above the ocean, and another 2 kilometres above the seafloor. It is partly built on the shoulders of Takawangha, though. There is very little erosion, and at least two of the three cones have erupted within the Holocene. The youngest lavas are at the westernmost cone, which is called Sajaka. Sajaka is made of basalt, with 47-51% SiO2. As a result of this mafic composition, it seems like an overgrown version of Stromboli. A small 200 meter crater at the top probably features frequent strombolian activity, and maybe occasional fountaining, fluid flows of lava, less than 10 meters thick, extend downslope on precipitous slopes, these at times will surely crumble into pyroclastic flows, and the cone itself might be at risk of avalanching. In fact, at some point, part of Sajaka seems to have collapsed into the ocean, leaving a horseshoe depression filled by a younger cone.
So, four volcanic systems have become restless across the Aleutians. Coincidence? A lot of volcanoes are going in an out of volcanic unrest over time in this archipelago, so it being a coincidence is possible. Regardless, it is an exceptional situation to have so much unrest. As I mentioned at the start, there is not much reason for alarm, though. These four volcanoes are all remote, uninhabited. Other than flight disruption, a VEI-6 of Aniakchak is about the only thing that could do serious destruction, and that is obviously something very unlikely.
If the volcanoes do erupt, the scenario to expect should be similar to the earlier activity of the volcanoes. Four volcanoes/volcanic complexes undergoing unrest mean there are more chances of an eruption than in a more typical situation. But the activity at this point just shows an increased input of magma or hydrothermal/degassing activity, it may not lead to eruptions. Okmok and Sajaka/Tanaga have the best chances, I think. For big fireworks, Aniakchak. And if something happens in the Katmai Cluster, I think it will probably be a small ash emission, or a reactivation of Trident, but who knows. I will monitor the situation and write a post if there is a new major development.
Links to sources of information
IRIS earthquake browser
Eruption history of the Katmai Cluster:
Eruption history of Aniakchak:
Eruption history of Okmok:
Eruption history of Takawangha and Tanaga: