BJ Deming

VEI 8’s: Aira Caldera, Japan

Where are the supervolcanoes?

It’s time to introduce the living, breathing, large caldera systems that “VEI 8’s, Part 2” will look at in detail.

DeSilva and Self’s list

This is the first in a series of eight brief posts on the nine active supervolcanoes that DeSilva and Self listed in their Table 1.

“Caldera” is related to “cauldron,” and like magical cauldrons in lore, supervolcanoes can brew up both trouble and great wealth. (Image: Alenini/Shutterstock)

Of note:

  • The number of Big Bads varies by researcher; I chose that one study because the paper is written by widely acknowledged experts and also because it’s so straightforward. As a layperson, of course, I have no idea whether their count is “right.”
  • “VEI 8’s Part 3” is dedicated to Supervolcano #9: Yellowstone.

    It’s doing just fine and will wait for us.

🌋🌋🌋

Sakurajima and Aira

So today, here we are in Japan.

A business in Kagoshima Prefecture — about 840 miles south of Tokyo, which is on another island — shared this view of their first sunrise in 2025, coming up over the shoulder of our old friend Sakurajima, out in Kagoshima Bay.

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Sakurajima is not a supervolcano, but it can be very dangerous — they chose Sakurajima for the Decade Volcano Program in the 1990s because its eruptions, while “normal” in size, occasionally reach plinian intensity and threaten Kagoshima City and other populated areas around the bay.

These fissures on Sakurajima’s western flank opened up first on January 12, 1914. About ten minutes later, fissures on the eastern flank cut loose. (Image: Wikipedia)

Sakurajima’s VEI 3 to VEI 4 eruptions have happened at least seventeen times over the last 30,000 years (Nishihara et al.), last in 1914 (PDF download) — a VEI 4 eruption in which fifty-eight people died.

If you look up Sakurajima at the Global Volcanism Program (GVP) website, however, you’ll find it on a page for something called Aira.

Aira Caldera underlies northern Kagoshima Bay:

We are here — the big blob is Sakurajima, and Aira forms the bay to the north.

It’s the Big Bad here, with a supereruption some thirty millennia ago — referred to variously in the references I found as the AT (Geshi et al.), Osumi-Tsumaya (DeSilva and Self), or the Ito (Global Volcanism Program; LaMEVE) eruption.

Let’s go with AT — the Aira-Tanzawa ash layer is found on sections of the Asian mainland, as well as throughout the Japanese Archipelago, and archaeologists know it as Japan’s most important Paleolithic time marker. (Ikawa-Smith; Machida)

All in all, Aira’s caldera-forming AT eruption covered almost a million square miles with 2 inches of ash or more (sometimes MUCH more!). (Tatsumi and Suzuki-Kamata)

According to Geshi et al. and Hasegawa et al., almost 400 km3 of magma blew out of what was dry land at that point in the Pleistocene ice ages, littering Kyushu Island with the:

  • Ito ignimbrite
  • Osumi pumice fall
  • Tsumaya pyroclastic flow
  • That widespread Aira-Tanzawa (AT) ashfall from the pyroclastic flows
  • Tarumizu ignimbrite

The supereruption

What was it like?

In Aramaki’s opinion (see reference in source list):

…It started with a Plinian pumice eruption (Osumi pumice fall…) followed by oxidized, fine-grained Tsumaya pyroclastic flow…, both erupted from a vent located at the present site of Sakurajima volcano, 8 km south of the caldera center. After a very short pause, violent explosive ejection of the basement rock fragments and pumiceous materials occurred at the central vent, gradually changing itself to a huge eruption column rapidly collapsing to form the Ito pyroclastic flow about 300 km3 in volume…Various textural features and monotonous petrologic character indicate that the main part of the Ito pyroclastic flow was emplaced by a simple, short-lived eruptive mechanism…Evacuation of more than 110 km3 of rhyolitic magma produced a funnel-shaped collapse structure with the center of the magma chamber about 10 km deep. Like many other Japanese Quaternary calderas, the Aira caldera is considered to have formed not by a piston cylinder-type subsidence utilizing a ring fracture but by coring and high-angle slumping of the wall rocks into a funnel-shaped central vent. The outline of the caldera was strongly controlled by the faults bounding the volcano-tectonic graben forming Kagoshima Bay…

Could that supereruption be forecast ahead of time, if it occurred today?

That’s an unanswerable question, since no one knows what precursory signals happened beforehand. Yet, given the millions of people in the area today, it’s worth asking.

Hasegawa et al. report long gaps between large Aira eruptions: a VEI 6 about 90,000 years ago; another VEI 6 roughly 50,000 years ago; and then 5,000 years of dormancy, followed by some minor eruptions for a while and then BOOM! when no one might be expecting it.

The Osumi pumice eruption in the south, per Geshi, was about 40 km3 DRE — Tambora sized.

It would have been catastrophic by itself.

Would anyone have expected that to be followed by an ash flow at least twice the size of Pinatubo’s 4-5 km3 DRE in 1991 and then the unprecedented (in our experience) 350 km3 blast from what is now Aira’s northeastern Wakamiko vent area? (Geshi et al.; JMA)

How do you forecast the apocalypse?

Some of the best minds on the planet are hard at work on an answer because they know that a supereruption will occur somewhere on Earth again, hopefully, at a very distant point in time. We should be as ready for it as possible.

Right now, though, there are more questions than answers.

Other points to ponder

In “VEI 8’s, Part 2,” Aira is a good example of a supervolcano that is not “about to blow” and yet has an elevated alert status (Level 3 on a four-point system) because of risk that is not supersized: Sakurajima, which has been having more or less continuous low to moderate-intensity eruptions since 1955.

Besides its usefulness in dating sites, the AT ash marker clues archaeologists in on some general impacts this supereruption had on to early and late Paleolithic humans living on Kyushu Island.

Finally — location, location, location!

Believe it or not, three of DeSilva and Self’s nine active supervolcanoes are in this Kyushu area, and they line up in a row.

We’ll explore what’s going on with that next time, when we set sail from Kyushu southward a little way to check out Kikai Caldera.

Featured image: The part of Aira Caldera (submerged) between Tarumizu City, where the photograph was taken, and Kirishima (a name shared by the distant city AND that mountainous volcanic complex behind it), by KAZU_49M/Shutterstock

Sources:

Aramaki, S. 1984. Formation of the Aira caldera, southern Kyushu,∼ 22,000 years ago. Journal of Geophysical Research: Solid Earth, 89(B10): 8485-8501. (Abstract only)

Brothelande, E.; Amelung, F.; Yunjun, Z.; and Wdowinski, S. 2018. Geodetic evidence for interconnectivity between Aira and Kirishima magmatic systems, Japan. Scientific reports, 8(1): 9811.

Brown, S. K.; Crosweller, H. S.; Sparks, R. S. J.; Cottrell, E.; and others. 2014. Characterisation of the Quaternary eruption record: analysis of the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database. Journal of Applied Volcanology, 3: 1-22.

De Silva, S., and Self, S. 2022. Capturing the extreme in volcanology: the case for the term supervolcano. Frontiers in Earth Science, 10: 859237.

Geshi, N. 2020. Volcanological challenges to understanding explosive large-scale eruptions. Earth, Planets and Space, 72: 1-10.

Geshi, N.; Yamasaki, T.; Miyagi, I.; and Conway, C. E. 2021. Magma chamber decompression during explosive caldera-forming eruption of Aira caldera. Communications Earth and Environment, 2(1): 200.

Global Volcanism Program. 2024. Aira.

Hasegawa, T.; Mochizuki, N.; Shibuya, H.; Nishihara, A.; and others. 2024. Paleomagnetic study of the 30 ka Aira caldera-forming eruption and 60–45 ka Iwato pyroclastic flow deposits, southern Kyushu, Japan. Earth, Planets and Space, 76(1): 161.

Hata, M.; Uyeshima, M.; Handa, S.; Shimoizumi, M.; and others. 2017. 3‐D electrical resistivity structure based on geomagnetic transfer functions exploring the features of arc magmatism beneath Kyushu, Southwest Japan Arc. Journal of Geophysical Research: Solid Earth, 122(1): 172-190.

Ikawa-Smith, F. 2022. Over the water, into and out of the Japanese Archipelago, during the Pleistocene: Humans, obsidian, and lithic techniques. In Maritime Prehistory of Northeast Asia: With a Foreword by Dr. William W. Fitzhugh, pp. 51-71. Singapore: Springer Nature Singapore.

Japan Meteorological Agency (JMA). __ Wakamiko. https://www.data.jma.go.jp/vois/data/tokyo/STOCK/souran_eng/volcanoes/089_wakamiko.pdf (PDF download)

Kuritani, T. 2023. Geochemical constraints on the evolution of the magmatic system leading to catastrophic eruptions at Aira Caldera, Japan. Lithos, 450: 107208.

LaMEVE. 2024. Aira entry,

Machida, H. 2002. Impact of tephra forming eruptions on human beings and the environment. Global Environmental Research-English Edition-, 6(2): 61-68.

Moriwaki, H.; Shinto, K.; and Kobayashi, T. 1991. Quaternary tephra studies in the Kyushu District, Southern Japan with special reference to gigantic pyroclastic flow deposits. The Quaternary Research (Daiyonki-Kenkyu), 30(5): 329-338.

Nishihara, A.; Geshi, N.; and Naruo, H. 2022. Long-term change of the eruption activities of Sakurajima volcano, Japan, inferred from the fallout tephra deposits. Frontiers in Earth Science, 10: 988373.

Tatsumi, Y., and Suzuki-Kamata, K. 2014. Cause and risk of catastrophic eruptions in the Japanese Archipelago. Proceedings of the Japan Academy, Series B, 90(9): 347-352.

Yamaoka, T.; Ikeya, N.; Miyoshi, M.; and Takakura, J. 2022. New perspectives on the behavioral patterns of early modern humans from the Japanese Islands. Mitteilungen der Gesellschaft für Urgeschichte, 31: 41-70.

Yokoyama, I. 2022. The 1815 Tambora eruption: Its significance to the understanding of large-explosion caldera formations. Geofísica internacional, 61(1): 5-19.

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