A Volcano Erupted, And Despite Knowing the Exact Location The Eruption Occurred We Are Unsure Which Volcano Erupted (Patreon)

Yesterday, news broke that an unexpected volcanic eruption had occurred in the Aleutian Island chain. Specifically, on the island of Atka, which itself is about 6 times larger than the District of Columbia. Atka Island (area is approximately 1000 square kilometers) contains the Atka volcano, which on March 27th produced a weak phreatic eruption at 7:36 pm local time.

On the surface, this seems like a self-explanatory subject. After all, the only settlement on Atka island is the city of Atka, which is located 12 miles to the south of the eruption site and has a population of 51.

However, what if I were to tell you that despite knowing precisely where the eruption occurred to the nearest square meter that we are actually unsure which volcano erupted?

Let me explain.

Currently, the U.S. Geological Survey and the Alaska Volcano Observatory consider the vent which erupted known as Korovin to be part of the Atka volcanic complex. The Atka volcanic complex has been erupting intermittently for approximately 1 million years, and even formed a 5 kilometer wide caldera in a major eruption 400,000 years ago.

Since then, the complex has developed into six stratocones. 4 of these stratocones have proper names, while the other 2 are flank vents of one of the aforementioned cones. They are known as (from oldest to youngest) Konia, Sarichef, Kliuchef, and Korovin and are shown in figure 1c which is sourced from a linked scientific paper (see the image text for attribution and a link to the source paper, or just head to the bottom of this post). Each of these volcanic cones has a prominence of greater than 1,000 feet. BPD is not a proper cone but rather an assortment of mid Pleistocene age dacite flows which have been altered over hundreds of thousands of years to have a pink color. Hence BPD = Big Pink Dacite (flow).

In historic times, only Kliuchef and Korovin have erupted, 3 and 14 times, respectively.

If we look at the compositions of the old big pink dacite flows, Kliuchef, Konia, and Sarichef, you will see that they have a composition of either low silica dacite or high silica andesite. Recent modelling shows that all 3 of these cones may have pulled from the very same at depth magma chamber.

In contrast, the Korovin volcano seems to not only pull from a completely separate magma chamber, but also almost purely erupts mid to high silica basalts. In other words, an argument could be made that Korovin is actually a completely separate volcano from the broader Atka volcanic complex which includes the other 3 volcanic cones.

So, if Korovin erupted, was it truly part of the Atka volcanic complex?

To be honest, I am of the opinion that it is indeed Korovin rather than Atka which erupted, due to their distinct compositional differences. I consider the two to be separate but closely spaced stratovolcanoes.

While the U.S. Geological Survey and Alaska Volcano Observatory websites officially only list and give alerts about the Atka volcanic complex, the most recent U.S. Geological Survey volcano hazard assessment lists the two as separate entities.

In that 2018 assessment, the Atka volcanic complex and the Korovin volcano were separately listed as the 30th and 31st highest threat (active) volcanoes out of 161 monitored volcanoes in the United States.

So, what is your opinion? Do you think the two are separate volcanoes, or do you think they are part of a singular broad volcanic complex that happens to have multiple stratovolcanoes, like the Three Sisters in Oregon?

Be sure to let me know in the comments section below.

Sources:

[1] Koulakov, I.; Boychenko, E.; Smirnov, S.Z. Magma Chambers and Meteoric Fluid Flows Beneath the Atka Volcanic Complex (Aleutian Islands) Inferred from Local Earthquake Tomography. Geosciences 2020, 10, 214. https://doi.org/10.3390/geosciences10060214. CC BY 4.0.

[2] J. Ewert, A. Diefenbach, D. Ramsey, "2018 Update to the U.S. Geological Survey National Volcanic

Threat Assessment", U.S. Geological Survey, Accessed October 22, 2022. https://pubs.usgs.gov/sir/2018/5140/sir20185140.pdf

CC BY 4.0 (creative commons license link, which applies to two of the three images utilized in this post, specifically only the ones labeled with this license in the image proper): https://creativecommons.org/licenses/by/4.0/legalcode