Kikai Caldera

Kikai Caldera is a very large and mostly underwater caldera, stretching up to 19 kilometers or 12 miles in diameter. It is located in the Ōsumi Islands, part of Kagoshima Prefecture in Japan. Though mostly submerged, this massive caldera is an important geological feature.

The Kikai Caldera is linked to significant volcanic activity in the region. It is not to be confused with the nearby island Kikaijima or the town of Kikai in Kagoshima. This underwater caldera plays a key role in understanding the volcanic history and structure of the area.

Geology

The Kikai Caldera is the southernmost caldera in the Kagoshima graben, a volcano-tectonic graben extending under the northeast East China Sea to the island of Kyushu in southern Japan. Its volcanic activity is linked to the movement of the Philippine Sea plate beneath the Eurasia plate.

The Kikai Caldera Complex has two oval-shaped calderas measuring 20 km (12 mi) by 17 km (11 mi) in diameter. Yahazu-dake, located on the northwest part of Satsuma Io-jima, and Takeshima on the caldera rim, are older volcanoes. The earliest known caldera formation happened at least 140,000 years ago from an eruption that created Koabiyama pyroclastic flows. The caldera formed from at least three huge ignimbrite eruptions.

Kikai-Koabiyama eruption

The Kikai-Koabiyama pyroclastic flows are made of rhyolite and cover much of Takeshima and the northwest side of the caldera rim on Satsuma Iwo-Jima. These flows vary in thickness, being thicker on Takeshima (20–100 m) and thinner on Iwo Jima (a few to 30 m). This eruption happened about 140,000 years ago, as determined by K-Ar dating.

Kikai-Tozurahara eruption

The Kikai-Tozurahara tephra layer is a widespread rhyolitic layer from a large VEI-7 eruption of the Kikai caldera during the Late Pleistocene. It stretches from south Kyushu to eastern Honshu and the Pacific Ocean, possibly reaching the Shandong Peninsula. The eruption happened around 94,500 years ago, based on studies from the Lake Suigetsu sediment sequence.

Kikai-Akahoya eruption

Main article: Akahoya eruption

The Kikai Caldera was the source of the Kikai-Akahoya eruption, one of the largest eruptions in the past 10,000 years. Between 7,200 and 7,300 years ago, pyroclastic flows from this eruption reached southern Kyūshū up to 100 km (62 mi) away, and ash fell as far as Hokkaido. The eruption had a Volcanic Explosivity Index of 7, making it one of the most powerful eruptions in recent history. It affected the Jōmon culture in southern Kyūshū, though not as severely as once thought.

2024 studies

In 2024, Japanese scientists studied the underwater Kikai Caldera. They looked at how much volcanic material was thrown out during an eruption that happened 7,300 years ago. This was the largest eruption in the last 11,700 years. The scientists found that the eruption sent material into the air, along the ocean floor, and along the water's edge.

The scientists also studied the ocean floor around the caldera and collected samples of mud from the sea. They learned that the volcanic formations on the ocean floor and nearby islands are connected. They discovered that a huge cloud of ash covered a very large area. The eruption created a double caldera because the center of the volcano was underwater.