Southern Japan Calderas

The 5 Sisters of Kyushu

As someone who is interested in volcanology, I’ll admit that I find large eruptions and large calderas way more interesting than effusive or smaller eruptions. Regardless of size, there is a lot more intrigue with large eruptions as they’re simply more of a mystery than small eruptions.

The Ikeda Caldera is a “small” 4x3km caldera that sits inside the much larger 20km Ata caldera at the southern tip of Japan. It now forms an extremely scenic vista with the Kaimon-Dake somma volcano in the background. Photo by kazukariya (Panoramio).

Much of the mystery behind larger eruptions is due to the fact that we rarely see them. Pinatubo’s vei-6 eruption took place over 20 years ago, and even that doesn’t hold a candle to some of the larger eruptions that have occurred during the last 10,000 years. The only VEI-7 eruption in historical time that people were around to witness was Tambora, which thankfully took place in a relatively isolated area (which is why Krakatau’s much smaller eruption around 50 years later still gets more publicity).

Now imagine if you were to take a volcano that produces eruptions 40 times more powerful than Pinatubo’s 1991 eruption. Then place that volcano on an island that’s slightly larger than Taiwan. Then for good measure, copy that volcano four more times, and place those on that same island. Finally, lets make sure the island is extremely populated with high levels of population density living close to most of these volcanoes.

With that example, we arrive at Kyushu – southern Japan.

While the above example may sound extreme, it’s the reality of a live volcanic arc that overlies a graben with very thin crust (more on that later). Despite the fact that there are so many huge volcanoes in such a small area, there are other similar areas in the world such as New Zealand’s north island (Taupo Volcanic Zone).

The five major calderas in Kyushu from south to north

• Kikai: Source of what is likely the largest eruption in the last 10,000 years. Despite being south of Kyushu, the eruption that occurred approximately 6300 years ago blanketed Japan in ash, and devastated the entire south and central areas of the island. According to the GVP, the southern part of Kyushu remained uninhabited for several centuries after the eruption occurred, despite it occurring relatively far offshore. It’s believed that Kikai is the source of more than one caldera forming eruption.

• Ata: The Ata caldera lies mostly submerged at the southern tip of Kagoshima bay. It’s last major eruption was a little over 100,000 years ago, making it the least recent eruption. Despite no recent caldera forming eruptions, Ata has been active in the last 10,000 years. Mt. Kaimon forms a somma stratovolcano, and the 4×3 Ikeda caldera formed approximately 4000 years ago near the slopes of Mt. Kaimon.

• Aira: Aira is the home of the ever-erupting SakuraJima stratovolcano, which forms a landmark in Kagoshima bay as a somma volcano on the southern end of the caldera. Aira’s eruption 22,000 years ago formed the massive Ito Pyroclastic flow, has been documented as traveling in a radius as large as 90km from it’s source. To give a sense of scale, the pyroclastic flow alone from this eruption was likely larger than the entire state of Connecticut. This does not include the massive ashfall and lahars the rest of the island would have had to deal with. Around 16,000 years ago, a smaller caldera forming eruption occurred on the northern end of Aira’s caldera. The size of the caldera was approximately 6×3. Currently, SakuraJima has had quite a few larger eruptions as Karen wrote about here , although it seems mostly content with strombolian activity at present. (source)

• Kirishima: Kirishima sits north of the Aira Caldera, and is the site of a small stratovolcano and numerous monogenetic and basaltic vents. Just north of where Kirishima’s primary center of activity is sits two separated calderas, those being the Kakuto and Kobayashi calderas. These calderas are older and smaller than the other major caldera centers in Kyushu, but still are massive in their own right.

• Aso: Aso is the northernmost major caldera in Kyushu, and had a series of four massive eruptions from 300,000 to 90,000 years ago. While Aso is part of Kyushu, it sits in a different geological setting than the 4 previous volcanoes, although the overall environment is roughly similar. The final eruption in this series was likely the largest eruption in Kyushu, with Pyroclastic flow deposits traveling over 160 km from the source of the eruption. Aso has several resurgent domes and volcanic cones inside the caldera limits. (source)

Known Caldera Structures denoted in Pink

The Kagoshima Graben – Aka, a Recipe for Large-Scale Volcanism

Volcanic arcs occur around the world just about anywhere in which subduction occurs. So it’s a pretty easy question to wonder why some areas create huge calderas, while most subduction zone volcanic arcs such as the Cascade range see more standard andesitic stratovolcanoes.

Before we tackle this question, it’s important to understand how magma is created. In a fairly simplistic manner, there are three basic mechanisms which influence magma generation.

1. Heat: If you heat rock up to a high enough temperature, it will melt just like any other material in the universe. With that said, rock does not melt naturally until way far deep inside the earth. If heat were the only factor in magma generation, we would likely have no volcanoes, since the rock would be way too deep in the earth when it melts to ever see the surface.
2. Water: Water is the main reason you see volcanoes around subduction arcs. Water has a way of changing the melting temperature of rock. When you saturate rock in water then heat it up, it suddenly requires much less heat to melt the rock. Because ocean crust is saturated in water, it naturally forms magma pockets as it sinks beneath less dense continental crust.
3. Pressure: Much like water, pressure changes the temperature in which rocks melt at drastically. The lower the pressure, the easier it is to melt rock. Given extremely high pressure, rock can stay in a somewhat solid state incredibly deep in the earth. Given extremely low pressure, rock will melt at a surprisingly low temperature. If you ever hear the term “decompression melting”, it’s generally referring to magma formation due to a lower volume of pressure in an area of crust. Low pressure is typically caused by crustal stretching and rifting. This stretching thins the crust greatly, which allows magma to well up from depth with ease, and gives greater access of heat towards the shallow bedrock.

So… how does this relate to Kyushu volcanism?

Most subduction arcs generate magma through mechanism 1 and 2. Just add heat and water, and voila, you have a volcanic arc. But what happens when you see decompression melting in the same zone as a volcanic arc? The answer can be found just about anywhere you would find a string of very large caldera systems: you see large rhyolitic caldera complexes pop up.

Decompression volcanism in subduction arcs is most likely the predominant formula for what drives the creation of what the media refer to as “supervolcanoes”. If you look at most known “supervolcanoes” outside Yellowstone, they sit in crustal rift basins or grabens above subduction zones. Keep in mind, there are other very large volcanoes and calderas that aren’t in rift zones, but this seems like the most common way in which massive caldera volcanoes are created.

For Kyushu specifically, the primary source of decompression is what is known as the Kagoshima Graben. A graben is a block of crust that has sunken down into the mantle between two faults. The Kagoshima graben can be easily discerned, as it forms the trough of Kagoshima bay itself (which contains Aira and Ata Calderas). The graben runs north south, and continues beyond the bay north until approximately the Kirishima Volcanic Center, and continues south into the ocean as it runs through Kikai Caldera. The crust in the Kagoshima graben is extremely thin (source), which serves to lower pressure below the graben, and allows very hot magma to well up to extremely shallow depths. This likely underplates the already thin crust with fresh hot magma, resulting in the melting of granitic bedrock, and the formation of large rhyolitic magma complexes. The actual process of magma creation in zones like the Kagoshima Graben can be more complex, but this should provide at least a basic understanding of how it works (I encourage any experts to chime in here and offer additional input or corrections).

While Aso is not part of the Kagoshima graben, it is part of another similar graben in Northern Kyushu, which is aligned with Unzen and Kuju Volcanoes. Aso also overlies

Sakurajima sits at the southern boundary of the Aira Caldera in Kagoshima Bay – Image courtesy of Wikimedia commons

Implications for future volcanism: How do these volcanoes affect Kyushu and Japan?

I believe most people have the common sense to realize that any large VEI-7 volcanic eruption would be quite a problem in terms of a disaster. Environmental impact has been well documented from eruptions such as Tambora, and it’s unknown how it would affect the world food production chain.

With that out of the way, If we’re limiting our eruption size to VEI-7, I can’t think of any worse place for an eruption to occur in terms of disaster mitigation. Most of the caldera forming eruptions in Kyushu devastated large portions, if not the entirety of the island. With a population of over 13 million living on the island, and very few ways to organize a mass evacuation (only one bridge to mainland japan), a large caldera eruption in Kyushu would be a disaster without anything close in historic scale. Outside local devastation of Kyushu, assessing global risks to the economy, environment, and other variables such as nuclear meltdowns which would be impossible to stop if they were to start could affect the world (as we now realize is a reality based off the 2011 tsunami).

Luckily, like most very large volcanoes, the large eruptions are extremely far apart, and we’re not likely to see any Aira style eruptions for quite a long time. There is also no sign of impending eruption from any of these calderas outside small-scale activity.

Currently, the greatest risk are from smaller eruptions from somma volcanoes like Sakurajima, which sits close to highly populated areas such as Kagoshima City.

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