Yakushima and the Kikai Caldera
Yakushima experienced massive eruptions of the submerged Kikai Caldera, 95,000 and 7,300 years ago. These catastrophic events, which occurred about 40 kilometers northwest of the island, were large enough to wipe out the populations of Yakushima, Tanegashima, and southern Kagoshima, and reshape the ecosystem of Yakushima.
Japan’s Largest Eruption in 10,000 Years
The eruption of 95,000 years ago ejected a pyroclastic flow consisting mainly of volcanic ash and pumice, covering Yakushima with a layer of ash about one meter deep. Then, around 7,300 years ago, the Kikai Caldera caused Japan’s largest eruption in the last 10,000 years. Known as the Akahoya Eruption, it began with small events that dispersed pumice deposits, followed by one massive eruption. It ejected about 150 cubic kilometers of rhyolitic magma—three times more than that of the 1815 eruption of Mt. Tambora in Indonesia. A giant plume of volcanic smoke rose up into the stratosphere, producing a pyroclastic flow that spread in a radius of about 100 kilometers. The flow traveled across the sea at a speed higher than 100 kilometers per hour and dropped hot pumice and ash on Tanegashima and Yakushima, as well as on the southern half of what is now Kagoshima Prefecture in Kyushu. Winds also carried ash over the greater part of the Japanese archipelago, reaching more than 1,000 kilometers.
Volcanic material has been found not only in flat areas of Yakushima but also around the summit of its highest peak, Mt. Miyanoura (1,936 m), indicating that the pyroclastic flow rose to more than 2,000 meters above sea level. The flow wiped out the people and culture of the Jomon period (10,000–300 BCE) in southern Kyushu and had a significant effect on its flora and fauna. The lush forests that grew on Yakushima were destroyed by the flow, and the land remained barren for around 500 years.
The flow devastated fauna that lived at ground level, while those living underground had a greater chance of survival. Many species that survived exhibit traits unique to Yakushima. Remnants of burnt or carbonized trees have not been found in the flow, although they have been found in southern Kagoshima—which is approximately the same distance away, suggesting that the forests may have been burned in the same way. The flow did not reach southern Yakushima, however, and it is believed that some flora and fauna were spared and able to survive in the south, and then later spread across the whole island.
Kikai Caldera Today
Volcanic material from the Kikai Caldera in Yakushima is covered by weed and moss growth, but is observable in places such as at Shiratani Unsuikyo Ravine, and in the town the foot of Mt. Miyanoura. The characteristic orange ash is also visible at some construction sites where the top soil layers have been removed, and also near the Yakusugi Museum along the Anbo Forest Path from Anbo to Yakusugi Land.
The Kikai Caldera is still a hot spot. A hydrothermal plume continues to rise from the lava dome that formed after the Akahoya Eruption, and researchers suggest the plume may be a sign of magma buildup.
