Guest post for VolcanoCafé by Greg S (Aka Cbus05)
One thing I have come to realize after years of reading about and following volcanoes, is that there is a considerable bias in people’s interest towards certain volcanoes. I used to think there was a western bias, but this phenomenon is just as prevalent in Japan and Indonesia, with many articles written in their native tongue about famous volcanoes such as Fuji or Merapi.
Realistically, the bias is more such that volcanic regions that are extremely remote get very little attention. This is understandable, why should people worry about or care about volcanoes that are way far away from civilization? It also makes more sense that funding and monitoring goes towards volcanoes that have a higher likelihood of impacting people, so some of these remote regions simply do not get as much love as they potentially deserve.
While I don’t blame the average person for not spending lots of time understanding distant volcanic arcs that probably will never affect them, I have found an extremely valid reason why those of us who are more volcanically inclined should pay a bit more attention to these remote regions.
I will get back to that reason further down this post.
Caldera Eruption Findings
Over the past 12,000 years, the period referred to as the Holocene, there have been a variety of very large volcanic eruptions and volcanic events that have been well-studied. On the VEI-scale, there have been no VEI-8 sized eruptions, but there have been a few VEI-7 sized eruptions, similar to Tambora’s blast in 1813 AD.
According to the Global Volcanism Program, there are 9 listed and verified VEI-7 eruptions, coming from the following Volcanoes:
- Tambora – 1813 AD
- Rinjani / Samalas – 1257 AD
- Changbaishan / Baekdu – 969 AD
- Taupo – 230 AD
- Santorini – 1610 BC
- Cerro Blanco – 2300 BC
- Kikai – 4350 BC
- Mt. Mazama / Crater Lake – 5677 BC
- Kurile Lake – 6440 BC
My Personal Research Into Caldera Eruptions
On my own blog; Big-Volcanic.com, I wrote a post that goes into detail on a different way to track large eruptions. In that post, I listed the largest volcanic eruptions of the Holocene period based on the size of the calderas being formed.
My reasoning for doing this was the fact that large eruptions are likely highly under-represented in the current record. My simple reasoning is that caldera size has a direct relationship with eruption size, and there are many large calderas out there that formed in the Holocene that we haven’t really accounted for.
Luckily, we have some good measuring sticks – those being the well-measured and researched VEI-7 eruptions of the last 1200 years (Tambora, Changbaishan, Samalas/Rinjani). We know with much greater accuracy how much ash was ejected at these volcanoes, and we can also accurately measure the size of their respective calderas.
As a result, it’s fairly reasonable to assume that volcanoes that created a caldera of similar or larger size likely came from eruptions that were of a similar size or larger. There are some important caveats here, and this is not by any means a perfect system, but it DOES give us a much more accurate ballpark to determine how many very large eruptions have occurred during the Holocene.
What I Found was Quite Interesting
By scouring every source I could to find known dates of large caldera formation, I was able to round up a pretty sizable list – significantly larger than the current list of 9 volcanoes that have gone VEI-7 in the Holocene. The list currently sits at 27 volcanoes that have created Tambora sized calderas or larger, with a few additional volcanoes that are more uncertain.
This dramatically changes current expectations of caldera eruption frequency for VEI-7 range events.
It has previously been thought that VEI-7 eruptions occurred approximately once per 1000 years as a very rough estimate. This is wrong.
With 27 known large caldera eruptions over the last 12,000 years, this provides an approximation of a large caldera forming eruption every 444 years (note that I won’t say VEI-7 since that can’t be verified). If you account for the fact that there are likely quite a few large eruptions that have not been discovered or dated, we would see this interval fall down into the 300’s, which is way more frequent than many would have thought.
Back to Volcanic Regions…
When compiling this list, something basic stood out to me like a sore thumb. There were three regions that accounted for over half of the total volcanic eruptions (15 total).
The most notable region was the Aleutian Arc in remote Alaska, which had 6 known large caldera eruption events during this time period, and also had numerous nested caldera events or smaller caldera eruptions (like Katmai).
Second most active in terms of large caldera creation came the remote island of New Britain (and islands north of New Guinea) with 5 events, although we know considerably less about the eruptions here. This is particularly impressive given the fact that this is not a particularly large volcanic arc. No region has had a higher rate of caldera formation per volcano than New Britain has.
Third Most Active is Kamchatka and the Ryukyu Island Arc with 4 caldera eruptions. Including the Ryukyu arc may be cheating to an extent however, since it could just as well be claimed by Japan. Either way, Kamchatka has had numerous other smaller or nested caldera events as well, and has been the most active region in the world for caldera formations over a longer period of time than just the Holocene.
The Unfrozen North
When you consider the fact that Alaska and Kamchatka are right next to each other, it becomes even more interesting that there were so many large caldera eruptions that came from remote regions close to the northern pacific.
Alaska Caldera Formations in the Holocene Period
Aniakchak: Aniakchak’s 10×10 caldera formed during a major eruption 3400 years ago according to the GVP. Pyroclastic flows from this eruption travelled over 50km to reach the Pacific ocean, where they were large enough to cause a significant tsunami. Aniakchak has also been highly active even after its major blast, with many other sizable eruptions that have occurred since.
Veniaminof: 8 x 11 km, glacier-filled caldera that formed around 3700 years back. The detail in the GVP isn’t quite as high on the caldera eruption here, although there is likely other literature that can be found on this volcano.
Fisher: 11 x 18 km, the Fisher Caldera is possibly the largest caldera that has been created in the last 12,000 years. Kikai is technically bigger, but that’s only because it is a nested caldera, whereas Fisher formed in one fell swoop around 9400 years ago. It has been researched and most studies state that it was a VEI-6 eruption based on measurements of the eruptive products, but I’m not buying that. Pinatubo-sized eruptions don’t create Long-Valley sized calderas.
Okmok: 10 x 10, Okmok is a monster in another way – it created a 10 x 10 caldera not once, but twice in the Holocene, once at the very start of the Holocene, and a second time at approximately 2050 years back. If we were to count each individual caldera formation here separately, there would be 7 large caldera formations in the Aleutians, and not 6.
Seguam: With a size not entirely known (I estimate around 6×7 based on measurements), Seguam has two calderas, but there is one particularly notable caldera that seems to be larger than what it’s listed as (going off measurements made using Google Earth). This caldera is probably more of a collapse scarp, where we don’t truly know how long it goes as the scarp heads into the ocean.
Semisopnochoi: This is a very little-studied caldera far out in the western Aleutian islands that is 8km wide. It is thought to have formed in the Holocene period.
Why Are the Aleutians so Active During this Period?
While I won’t say anything definitive, there is one fairly obvious answer – that being of deglaciation. Just like Iceland saw an increase in large effusive eruptions after glaciers went away, it’s not surprising that the northernmost subduction arc in the world also had a period of greatly increased volcanic activity after the glaciers retreated.
How long will the effects of deglaciation affect volcanoes? That’s tough to say, but considering we were still seeing big caldera eruptions as recent as 2000 years ago, and have seen smaller VEI-6 caldera events such as the eruption of Novarupta, it’s likely that heightened Aleutian activity is not yet over.
Were these eruptions VEI-7 in size? Why aren’t they listed in the GVP as such?
First off, not all of these volcanoes have been extensively studied. It takes a lot of money to undergo an expedition to a remote island volcano in the frozen north. Furthermore, it’s necessary for scientists to justify the reasoning for that study, so we don’t see nearly as much research on these types of volcanoes as we do on volcanoes such as Mt. Shasta, or Vesuvius for example.
For the volcanoes that DO have a lot of research performed on their eruptive history, the Aleutians are particularly challenging when it comes to measuring eruptive deposits. Not only do they sit in a region where there was likely rapid weathering and lots of glacial activity, they also are surrounded by the ocean, which is quite effective at hiding eruption deposits.
As such, it would make sense that some eruptive output estimates are extremely conservative, or simply far off base due to the erosion of those eruptive deposits. That’s not to say all of these eruptions definitively created VEI-7 sized eruptions, but we should at least realize that these are NOT small calderas. All of these are larger than Tambora, some of which dwarf Tambora’s caldera, which we recognize as a definitely large eruption which affected the world.
Well, this was an exceptionally long-winded post that basically states that the Aleutians are the most active volcanic arc in the Holocene, and the most likely to produce a large caldera forming eruption.
We don’t pay much attention to the Aleutians because they are out of sight and out of mind, but another large eruption would certainly have some ramifications that would affect people far outside the Aleutians.
Greg S (Aka Cbus05)