Japan’s largest volcano lies hidden. It can be found, if you know where to look, by starting from the southern island of Kyushu. Find the active volcano Sakurajima. (We will come back to it later. But this is not that volcano.) Sakurajima lies in-between two bays, the twin calderas Aira and Ata: follow the bay to the south. When reaching the ocean, after some 30 km, turn south-southwest and keep going. Beware, the waters here can be wild. After traveling another 30 km or so, two small islands appear, Takeshima on the left and Satsuma-Iojima on the right. The skyline of Satsuma-Iojima is dominated by a steaming volcano. It is called Iodake, and it rises to 700 meters above sea level with a 400-meter wide, 150-meter deep summit crater. This also is not that volcano. But we are getting close.

(As an aside, in case you wonder, I am using a less common spelling of Iojima here in order to avoid confusion with the other Iwojima.)

Both islands are clearly volcanic. Takeshima is dormant, but Satsuma-Iojima has been rather active recently.

An off-shore eruption in 1934/35 formed a small island in-between Satsuma-Iojima and Takeshima, called Showa-iojima, which is just visible on the far right in the satellite map below.

Volcanoes of Satsuma-Iojima

There are two volcanoes on Satsuma-Iojima. Iodake is the dominant and active one. The eroded sides show that larger summit eruptions of Iodake must be rare, but the bare slopes also show that vegetation is quickly killed. The second volcanic cone on the island is green and much smoother-looking, and is called Inamuradake.

Satsuma-Iojima has a surprising variety of lavas. The main cone of Iodake is rhyolitic, as is the new island that formed off-shore in 1934/35. But the green cone of Inamuradake is basaltic/andesitic. That difference is surprising, given that both cones have erupted in the past 7000 years. The Inamuradake cone formed 3900 years ago. Iodake is older, at 6300 years, but remains more active. It has had explosive episodes, probably involving lava domes, on average once per 1000 years. The most recent such event occurred 500 to 600 years ago, based on charcoal C14 dating, although there is no historical record of this eruption. The next oldest eruption from Iodake happened about 1200 years ago.

Iodake was fairly quiet during the 20^th century. This changed in 1996 when a widening crack in a road was found, perhaps indicating inflation. In 1997, a strong fumarole formed in the central crater. Earthquake swarms followed in 1998, and some minor ash falls were found. In the subsequent years, explosions occurred in the crater which left ash deposits in the region. The activity declined after 2004 but became more frequent again in the past decade. The most recent explosion was reported on 17 May this year. These eruptions have been VEI 1, to VEI 2 on occasion, dangerous for people within the vicinity of the crater. The SO2 emissions at Iodake are high, indicating an active magma chamber and providing another reason to avoid climbing to the crater. The gas emissions make the summit dangerous and off-limits.

The 1934/35 eruption which formed Showa-iojima occurred from a vent on the sea floor, 300 meters below the surface. Earthquakes had been felt for several months before the eruption started. In September of 1934, floating pumice was found. The largest pieces of pumices were measured at 10 meters across. The eruption reached the surface in December, before an explosion on 30 December destroyed the emerging cone again. It reformed in January when lava was seen and a new dome grew. A second cone broke the surface 50 m off the new islet. The island continued to grow and after March, when activity declined, it reached a size of 500 by 300 meters and a height of 55 meters. The cone subsided by 30 meters within months but stabilised after that. Some of Showa-iojima has since been lost to erosion but the main structure has survived.

The Showa-iojima eruption had a lava volume of around 0.1 km³. The composition is mainly rhyolitic with a high silicate content, similar to Iodake.

Kikai caldera

The two (or three) volcanic islands turned out to be the tip of an iceberg. When bathymetry became available in the region, a whole different world was found. A crater-like hole appeared, with a central dome. The hole surrounding the dome measures 15 by 17 km, with a maximum depth of 600 meter below sea level. It overlaps with a second hole measuring 19 by 24 km, centred a few kilometers further east and with a maximum depth of some 400 meters. The deeper caldera is nested into the larger, shallower caldera.

The two islands are located on the northern rim of the caldera. The southern rim is located towards the Okinawa trough, where the ocean is deeper, and is therefore submerged. Only the northern rim reaches sea level.

Clearly, Iodake is only a minor volcano, compared to the size of the Kikai caldera, and its eruptions are an afterthought. But in this hole we have found our missing volcano. It is like the ghost in the Antigonish poem, hiding, and also not there anymore.

The crater-like basin is known as the Kikai caldera. Here happened Japan’s largest eruption of the holocene, one which changed its history. But how did this caldera form? Why is it double? And what is the large dome at the centre of the caldera, almost making it look like an impact crater (which it isn’t)? And why is there a volcano on its rim?

Sakurajima

We may compare Iodake to Sakurajima, where our story started. Both volcanoes rise from the rim of a large caldera. Sakurajima is on the edge of the large Aira caldera, which formed 30,000 years ago. After a caldera collapses, it is not unusual for new volcanoes to form on the rims, making use of the caldera fault for their magma transport. Sometimes those volcanoes remain minor and the real action later resumes at the centre – an example is Krakatau. In other cases, the rim volcano grows to dominance and may itself eventually explode and leave its own caldera.

Iodake appears to be an example of the former. Sakurajima is more like the real deal. It is frequently active, had a large eruption a century ago and the next large event is expected (by us) by 2044.

Name

Some Japanese words are more easily translated than others. The word ‘jima’ (also rendered shima) means island. ‘Dake’ derives from a word for summit or peak. But what about ‘Kikai’?

The name ‘Kikai’ roughly means ‘demon world’. It refers to an ancient (12th century) Japanese epic called The Tale of the Heike. It details a struggle for supremacy between two rival clans, with all the intrigues and battles that entails. Think of it as a Japanese version of Hamlet, with as main theme ‘how the mighty have fallen’. In the story, the Buddhist monk (and samurai) Shunkan is exiled to a southern island called Kikai-ga-shima where he later dies of starvation.

Which island is meant is not known – the epic does not come with a map! Satsuma-Iojima is sometimes identified as the original Kikaijima, which is certainly plausible since just the sulphurous air would justify the name. But there is another island further south in the archipelago with that name and it also lays claim to the Heike heritage.

The Kikai eruption

The Kikai caldera is worthy of its name. Once, this demon underworld rained hell on much of the country. The evidence is found far and wide, as an ash layer that is present across southern and central Japan and South Korea, called the Akahoya (meaning reddish) ash. The layer is thickest in Kyushu and the southern islands, and is absent only from Hokkaido. The volume of the eruption that caused the widespread ash is estimated at >150 km³. The layer has been carbon dated, but it is also sufficiently recent that there are dates from neolithic archaeology. In a lake near Kyoto, the ash is embedded in a series of annual layers. Counting of these layers in a lake indicates that the eruption occurred only 7300 years ago. Both the distribution of the ash and the composition show that the Akahoya ash stems from the Kikai caldera. And the date shows it was recent – well after people had arrived in Japan.

This was likely the largest volcanic explosion on Earth during the holocene! And going back much further, it is the 3rd or 4th largest ash layer in Japan over the past 100,000 years.

The Kikai caldera is therefore related to a very large and very recent eruption. The caldera is also a repeat offender, as one of the other Japanese ash layers of the past 100,000 years is also from Kikai: the older is dated to 95,000 years ago and has a similar volume to the Akahoya ash layer. The holocene eruption is known as the Kikai-Akahoya eruption.

The best carbon-14 date (calendar years) for the Kikai-Akahoya eruption is 7303-7165 yr BP. The ice core data yields 7180+-30 BP.

Onset

The Kikai-Akahoya eruption had three main phases. They are in common with other (smaller) caldera-forming eruptions such as Krakatau, Pinatubo and Hunga Tonga.

Eruptions at Kikai had resumed around 45,000 years ago. Not long before the Akahoya ash, a rhyolitic lava flow occurred at the western rim. Sometime after this, the first phase of the main event started. There were small explosions, possibly phreatomagmatic, which may have continued for some time, days to months. Similar events were seen at the other three volcanoes: the increasing heat flux triggers steam explosions which may become magmatic.

Around this time, there was a large earthquake which affected southern Kyushu. The evidence for this comes from sand boiling and liquefaction found at archaeological sites, which occurred just before the tephra fall. It is hard to tell whether the earthquake was related to Kikai, either caused by or triggering the events there.

The second phase started when the vulcanian precursors gave way to two much larger plinian eruptions. During this phase, eruptions intensified over time. The plinian explosions came from the western area of the caldera, perhaps not too far from the modern Iodake. The plume height is estimated at 15-20 km for the first eruption and up to 35 km in the second one. The ejecta from the phase reached the southern tip of Kyushu, carried by what appeared to have been a southwesterly wind.

In Hunga Tonga, this phase was brief, lasting a day. At Pinatubo it was several days. For Krakatau it is harder to tell because the island had become too dangerous to approach. The intensification with time during this phase is notable. Not all eruptions show this. In the case of rift eruptions, the number of vents tends to be highest at the start, before the eruption focusses on one or a few. In the case of Krakatau, the number of vents had instead been increasing, with the last reliable observations mentioning six. For Kikai, we do not know this but we do know that the first plinian eruption was much weaker than the second one and that both strengthened over time.

The first plinian eruption was followed by some pyroclastic flows. These finished before the larger explosion occurred – this shows that there was a pause between the two eruptions. The second eruption caused much more massive pyroclastic density currents which covered Satsuma-Iojima with a thickness of up to 20 meters in the original valleys. At Takeshima, the layer reached 10 meters depth.

The volume of the plinian tephra deposits of the second phase have been estimated at between 5 and 17 km³. The large uncertainty comes from the fact that the volume has been extrapolated from deposits on-land and there is not much land around Kikai caldera! In dense rock equivalent (DRE, magma volume) it becomes 1-4 km³. The pyroclastic deposits should be added to this. Based on a 20-meter thickness at 15 km, a volume of 14 km³ has been derived for this but this is likely to be a significant overestimate. Together, this plinian phase of the eruption may have reached up to 10 km³ DRE – a large VEI 6. This is significant but eruptions of this size are not that uncommon for Japan!

Collapse

After this plinian eruption, there was a pause, as shown by the layered deposits. This pause may have lasted hours or days. Now, disaster struck during a third phase. Starting at the western side, the roof of the magma chamber (2-3 km thick) began to fail. Quickly, the entire roof followed, and the caldera collapsed. This collapse triggered the main climactic explosion. While these events unfolded, Kyushu was hit by a second earthquake, less strong than the first.

Hunga Tonga, Pinatubo and Krakatau all showed the largest explosion(s) at the end. After these, the eruptions sharply declined and soon ceased entirely. Kikai is likely to have shown the same pattern, though with a much larger final phase.

The resulting explosions of the third phase deposited thick ash over much of Japan and South Korea, reaching 1000 km distance. Much of Kyushu and southern Honshu was covered in 20 cm of ash, as far as 400 km from Kikai. For comparison, Pinatubo deposited 5cm of ash 60 km away, sufficient to cause chaos and severe damage. A Kikai-size eruption would have done this over the entire Philippines. The database of Japanese volcanic ash falls (Sudo et al. 2007, https://www.jstage.jst.go.jp/article/bullgsj/58/9-10/58_261/_article/-char/ja/) gives a volume of the tephra of 374 km³, or a minimum of 249 km³ when excluding regions where no measurements existed. There may have been a large tsunami but this is not well known.

The pyroclastic flows of the third phase reached Kyushu, after flowing 80 km over water, which added to the destruction.

Part of the pyroclastic flows occurred under water. They can flow along the bottom of the sea when the water is more than 40 meters deep. The sea floor around the caldera shows evidence of these submarine density flows. Seismic profiles show two separate flows. The top one has a thickness of 20 to 30 meters near the caldera rim and 3 meters at 40 km away. (The 3-meter thickness is the limit of the resolution of the seismic profiles.) The bottom flow has similar thickness but is less smooth, containing some large sliding blocks. The flows cover an area of 4500 km². The top flow is associated with the Akahoya eruption, as shown by the rhyolitic composition of glass fragments retrieved from it. The origin of the lower unit remains unclear but it may be associated with the older eruption.

The bulk volume of the upper flow amounts to 71 km³. The volume of the pyroclastic currents which reached Kyushu should be added to this. This is 5 km³, giving a total of 76 km³ in just the pyroclastic density currents!

Volume

Adding all together, the bulk volume of the Kikai eruption amounts to 335 km³ at minimum, and potentially as high as 490 km³. The likely value is of course midway, around 400 km³. The DRE (magma) volume becomes 160+-30 km³. This is approximately the same as the volume of the inner (50 km³) and outer (90 km³) caldera combined, suggesting both formed in this eruption.

This volume makes it the largest known eruption world-wide during the holocene. The volume is twice that of the Kuri Lake eruption, and thrice that of the Crater Lake eruption in California, both of which by coincidence happened within 1000 to 2000 years of the Kikai-Akahoya eruption.

There is another caldera-forming eruption in Japan which happened at almost the same time. This is the Mashu eruption in Hokkaido which occurred between 7700 and 7300 BP, with a bulk volume of 20 km³. This was probably just so that the people of northern Japan would not feel left out.

Caldera formation

But how did this happen? What makes the roof fail? The roof of a magma chamber is supported by the magma pressure, including the buoyancy of the gas bubbles in the magma. The first eruption leads to magma withdrawal and a reduction of the pressure. The pressure reduction depends on the size of the caldera: a larger caldera needs more magma withdrawal for the same pressure reduction. (After all, pressure is force per area.) Whether this leaves the roof unstable depends on how much the pressure reduces and on the thickness of the roof.

The figure shows the various models for this, with a number of known caldera events shown. (The factor x is the gas fraction of the magma.) It shows that for a 10-km caldera, a magma volume of 10 km³ would need to be withdrawn before collapse can occur. But Kikai is much larger than this and would have required around 40 km³ magma withdrawal, corresponding to an eruption volume of 100 km³. As far as we know, the plinian phase did not reach this. So why did the chamber collapse?

There are a number of suggestions. The volume of the plinian phase may have been underestimated. That seems less likely given that the ejecta of the plinian phase did not travel beyond southern Kyushu whilst the rhyolitic magma indicates that the eruption was predominantly explosive. The caldera fault may have been weakened because of the older caldera-forming eruption – however, most calderas go through several such events so this is not unique. The roof may have been weak: the caldera is located in a graben that runs between Kikai and southern Kyushu, and in a graben the extension can thin and weaken the crust over time. Finally, the proximity to sea level may have allowed water to interact with the magma chamber. It remains an open question.

Impacts

The eruption changed Japan. People migrated and had to change their ways of living, with their crops deeply buried. Much of Kyushu remained depopulated for centuries after. The pottery culture in the region disappeared. When a new pottery culture settled, it was a different style of pottery which came from western Japan or Korea.

The climate response remains unclear. A Siberian tree ring record indicates a 5-year long phase with very poor tree growth, which started very suddenly, but it is around 60 years older than the best ice core date. The marine sediment around Okinawa shows a drop of sea surface temperatures of 5C, which lasted for several centuries. The timing is not inconsistent with Kikai but the duration would be far longer than expected for a volcanic winter.

Before the eruoption Kyushu had been forested. This suddenly changed after the Kikai eruption and a barren grassland developed. It took more than 500 years for the forest to recover. Was this still because of the thick ash, or was this the result of a long cooling of the climate?

Resurgence

In Japan, large eruptions are commonly followed by the formation of small post-caldera volcanoes. At Kikai, Iodake may take on this role. But there is a much larger dome at the centre of the Kikai caldera which postdates the eruption, and that is unusual in Japan. The dome reaches close to sea level in the west of the caldera, at the Asase reef.

Resurgent domes are typically caused by remaining magma which has lower density than the crust above, and rises up, pushing up the ground above. But the Kikai dome is different. The dome has little or no sediment or pyroclastics on top. Pillow lavas are seen, and the composition of the dome is rhyolitic with similarities to Showa-iojima, slightly different from the main Kikai eruption. It is therefore considered a lava dome. Kikai is rapidly rebuilding its volcano, but ion keeping with its hidden nature, it is doing so out of sight. The dome is 10 km long, 600 meter tall and has a volume of 32 km³. It could be among the largest such structures in the world. The next largest in Japan is the dome in the Aso caldera, (also in Kyushu), Naka-dake with a volume of 12 km³.

Recently, seismology has unearthed a low-velocity zone below the Kikai caldera. It is interpreted as a region containing rhyolitic magma with a melt fraction of around 5%. The chamber lies lies directly beneath the lava dome, at a depth of 2.5-6 km and has a width of 13 km at the deep end and 4 km at the shallow end. The total volume is 220 km³, of which some 10 km³ is molten. The dome is expected to have grown from this magma chamber.

This magma chamber is at the same depth as the original collapsed mama chamber. It could not have survived the Akahoya eruption, and in any case the lava composition is slightly different from that eruption. The magma chamber must therefore have refilled with fresh magma. The melt injection amounts to around 0.01 km³/yr.

Japan

Japan is a nation of contradictions. It has the reputation of being among the most civilised and safest countries on Earth. And it also has a history of brutal wars. Nature here too shows these two faces. It is a beautiful country, from the tropical southern islands to the winter wonderlands of Hoikkado. But the world here has exploded in fury in the past – and may do so again.

Only the white cloud above Iodake indicates what lies beneath. The fumaroles and sulphur smells are all the remains of the largest eruption since the ice age. Satsuma-iojima is a worthy memorial to the Tale of the Heike and its samurai monk, Shunkan.

Albert, July 2026

Reading

Haruta, Y. et al., 2026, Reconstruction of the plinian phase of the 7.3 ka “Akahoya” caldera-forming eruption at the Kikai caldera, Japan. Journal of Volcanology and Geothermal Research, Volume 472,  108541, https://doi.org/10.1016/j.jvolgeores.2026.108541

Shimzu, S., et al. 2024, Submarine pyroclastic deposits from 7.3 ka caldera-forming Kikai-Akahoya eruption. Journal of Volcanology and Geothermal Research, Volume 448, 108017. https://doi.org/10.1016/j.jvolgeores.2024.108017

Tatsumi, Y. et al., 2018, Giant rhyolite lava dome formation after 7.3 ka supereruption at Kikai caldera, SW Japan. Scientific Reports, Volume 8, 2753. https://www.nature.com/articles/s41598-018-21066-w.pdf

Nagaya, A.,  et al. 2026, Melt re-injection into large magma reservoir after giant caldera eruption at Kikai Caldera Volcano. Commun Earth Environ, Volume 7, 237. https://doi.org/10.1038/s43247-026-03347-9