Sayonara, Japan? The Aira Caldera

Update: I have followed this at my other blog with posts on predicting eruptions and on the published paper by the Kobe University volcanologists.

Image of Mount Fuji by Swolib.  Taupo image by NASA

Image of Mount Fuji by Swollib. Taupo image by NASA

Some stark headlines have come out recently, along the lines of “Major Volcanic Eruption Could Make Japan ‘Extinct,’ Study Warns.”

The fuss is over news of a study on Japanese volcanoes that’s due out on November 11.

Kobe University volcanologists Yoshiyuki Tatsumi and Keiko Suzuki-Kamata studied 120,000 years’ worth of eruptions at Aira caldera and other Japanese volcanoes. Some of these have been supereruptions. They discovered that the country faces a 1% risk of such an eruption in the next 100 years.

That doesn’t sound like much, but it’s exactly the same statistical risk that the city of Kobe faced for a major earthquake the day before a M6.9 earthquake happened there in 1995.

Now, that’s what I think the scientists are saying. The news release is in Japanese. The Google-translated version is here, but it’s difficult to sort out details.

While we’re waiting over the next couple of weeks for the study to appear in the Japan Academy Bulletin (hopefully, in English), let’s look at Japan’s volcanoes and then see what progress scientists are making towards predicting eruptions. Then, on November 16, we’ll examine, as amateurs, what Doctors Tatsumi and Suzuki-Kamata have learned.

This ought to be interesting because a lot of geochemistry is involved – that subject thwarted my bid for an undergraduate degree in geology back in the 1980s. Fortunately, volcanologist Clive Oppenheimer, author of Eruptions That Shook The World, has a knack for describing hard science in easily understood English. There are also plenty of authoritative and easily understood resources available online. We can do this!

First off, though…

Gases drive eruptions.  Holes like these are part of the evidence they leave behind.  Image source

Gases drive eruptions. Holes like these are part of the evidence they leave behind. Image source

Why do volcanoes erupt?

In Eruptions, Dr. Oppenheimer says that magma – a combination of molten rock, crystals, and dissolved gases – forms around 30-60 miles underground where there is intense pressure from the weight of the overlying rock.

This stuff is more buoyant than its surroundings and so moves upwards through cracks and faults until it reaches a point, around from 1 to 6 miles below the surface, where rock is just as dense as the magma.

And that would be the end of it…if not for the fact that six miles of rock weigh a lot less than sixty.

Now the magma isn’t under so much pressure. Think of what happens when you release pressure on bottled soda pop by loosening the cap. Bubbles appear. That’s dissolved gases coming out of solution, and the same thing is going to happen underground.

Our magma ends up with a bunch of gas bubbles floating around in it (which scientists can “hear” with special equipment, alerting them to potential trouble from the neighborhood volcano). The freed gas makes it less dense, so it starts to rise again. The result of this is even less pressure on it. More bubbles form…you get the idea. Things could develop into a self-feeding cycle that eventually leads to an eruption.

Or not. Maybe the magma doesn’t have enough oomph to make it out of the volcano. It could stop, slowly cool off, and eventually harden into place.

Devils Tower, in Wyoming, formed this way. Scientists aren’t sure if any of its lava erupted. In any event, the softer rock around this hard basalt then eroded away.




Volcanoes don’t follow rule books, but generally speaking, if our magma does erupt, it might come out as runny red lava in a Hawaiian-style eruption. However, if there are a lot of dissolved gases, and especially if there is a lot of silica (the mineral silicon oxide) in the melt, the stuff may explode right out of the conduit or form a sticky plug that collapses in spectacular pyroclastic flows. Such a flow killed over 40 people at Mount Unzen in 1991.

Japan has a many types of volcanoes, but we’re going to concentrate on the explosive ones because the Kobe University study involved one of the explosive giants.

Aira caldera and the Kagoshima graben

Meet Sakurajima, an active cone in the Aira caldera (PDF). It erupts hundreds of times every year and occasionally, every few centuries, has large eruptions (generally VEI 4 or 5 – Mount St. Helens/Pinatubo range). In this video, posted by Photo Volcanica in 2013, the initial blast of gray tephra comes out faster than the speed of sound:

Now meet Sakurajima’s big daddy, Aira:


Kagoshima City, there in the lower left, has a population of over 600,000 and sometimes gets ashed by Sakurajima. In the upper right is Kirishima City, home to 128,000 people. Image source

Sakurajima is 13,000 years old – a new kid on the block, in geologic terms. Its most recent big eruption, in 1914, threw out enough material to weld its cone to the nearby shore.

Aira Caldera is pretty young, too – just some 22,000 years old. Its most recent big eruption covered the southern part of Kyushu Island with up to 525 feet of ash.

But wait…there’s more. Meet Aira’s family!

That's Japan's southern Kyusu Island in the upper right.  Source

That’s Japan’s southern Kyushu Island in the upper right. Source

Why, oh why is this volcanic gigantism happening?

All of Japan’s volcanism happens because the country sits in a complex subduction zone that involves four tectonic plates.

In subduction generally, a heavier basalt seafloor plate sinks beneath a more buoyant granitic continental plate. That sea floor rock, says Dr. Oppenheimer, melts into a magma loaded with volatiles, which is why subduction zone volcanoes are often explosive. This magma starts rising and eventually erupts at the surface, forming a volcanic arc.

Churikova, Figure 1

Like this. (Figure 1, “Dehydration and melting in a subduction zone.” Churikova)

The nation of Japan has thousands of islands, but geologically speaking, it only has five island arcs:

  • the Kuril Arc
  • the Northeast Japan Arc
  • the Izu-Bonin Arc
  • the Southwest Japan Arc
  • the Ryukyu Arc

With all the complicated plate tectonics going on, these arcs don’t always match up with the modern shapes of Japan’s islands. The modern island we’re going to focus on is Kyushu.

Aira Caldera and some of its buddies may be located in southern Kyushu, but as far as geologists are concerned they sit on the northern end of the Ryukyu Arc.

This Ryuku volcanic arc is colliding with the Southwest Japan Arc and the ground underneath Aira and the other calderas is slowly being pulled apart, forming a sinking topographic feature geologists call a graben.

So, take that subduction zone picture up above and start stretching out the land on top where those volcanoes sit. Cracks form and the land gets thinner and thinner. As this happens, more lava can and will erupt there.

How much more? Well, here’s a figure from the Kobe University study showing the sort of eruption they have in mind (I added the island names and Tokyo’s location):

Note:  The study authors say this is the worst-case scenario.

Note: The study authors say this is the worst-case scenario.

Yeah, that’s headline worthy.

So how likely is this worst-case scenario? That depends on a lot of things, including the current state of the fine art and science of predicting eruptions.

Next week, we’ll see how much progress volcanologists are making toward being able to predict a volcano’s future behavior. And then, as mentioned, on the 16th, hopefully (if it’s published in English), we’ll try to find out just what the Kobe University team has found out about the chances for a supereruption in Japan.

This has been cross-posted from my other blog, Clear Sight

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