| Kikai Caldera | |
|---|---|
 | |
| Highest point | |
| Peak | Mount Iō (Iōjima), Iōjima, Ōsumi Islands, Japan |
| Elevation | 704 m (2,310 ft) |
| Coordinates | 30°47′N130°19′E / 30.79°N 130.31°E /30.79; 130.31[1] |
| Dimensions | |
| Length | 17 km (11 mi) NS |
| Width | 20 km (12 mi) EW |
| Naming | |
| Native name | 鬼界カルデラ (Japanese) |
| Geography | |
 | |
| Country | Japan |
| State | Kagoshima Prefecture |
| Region | Ōsumi Islands |
| District | Kagoshima District |
| Subdivisions | |
| Municipality | Mishima |
| Geology | |
| Rock age | 6,300 to 95,000 years ago |
Kikai Caldera (鬼界カルデラ,Kikai karudera) (alternativelyKikaiga-shima,Kikai Caldera Complex) is a massive, mostly submergedcaldera up to 19 kilometres (12 mi) in diameter in theŌsumi Islands ofKagoshima Prefecture, Japan.[2]
The Kikai Caldera Complex has twin ovoid caldera 20 km (12 mi) by 17 km (11 mi) in diameter.[2] Yahazu-dake (north west part ofSatsuma Io-jima) andTakeshima, located on the caldera rim, are pre-caldera volcanoes.[2] The pre-caldera stage of volcanic activity involvedrhyolite,basalt, andandesite phases.[2][3] The earliest definitive caldera formation has been dated back to at least 140,000 years ago, resulting from the eruption of Koabiyamapyroclastic flows.[2][4][3] The formation of caldera has been associated with at least three catastrophicignimbrite eruptions.[2][4][3] Additionally, there are two older deposits (Koseda pyroclastic flows and Anbo tephra) of large caldera-forming eruptions in the vicinity, although their attribution to the Kikai caldera remains controversial.[5][6][7]
The Kikai-Koabiyama (K-Kob) pyroclastic flows are rhyolitic and are distributed across most of Takeshima and the plateau-like area on the northwest side of the caldera rim of Satsuma Iwo-Jima. They consist of numerous thin flow units and fill the basins in the basement, exhibiting significant variation in thickness. In Takeshima, the pyroclastic flows are thick, ranging from 20–100 m (66–328 ft), whereas in Iwo Jima, they are relatively thin, measuring a few to 30 m (98 ft).[2][3]
The eruption of the K-Kob pyroclastic flows has been dated usingK-Ar dating to be140,000±20,000 years before present.[2] While no distal tephra from this eruption has been reported, a tephra layer with potential geochemical and age correlation has been discovered inLake Suigetsu.[8]
Kikai-Tozurahara (K-Tz)tephra is a widespread rhyolitic tephra layer ofLate Pleistocene age, attributed to a largeVEI-7 eruption from the Kikai caldera.[9] This layer is confirmed to have a wide distribution, extending from southKyushu to easternHonshu and reaching thePacific Ocean,[10] and possibly including theShandong Peninsula.[11] The proximal equivalents of K-Tz are the Nagasepyroclastic flow and the Nishinoomotepyroclastic surges.[9][12] The combined bulk volume of both distal and proximal deposits is estimated to exceed 150 km3 (36 mi3).[10]
Inmarine isotope stratigraphy (MIS), K-Tz is located between MIS 5.2 and 5.3, providing a loosely constrained preliminary eruption age of approximately 95,000 years before present.[10] More reliable age constraints were imposed by the high-resolution chronology derived from the Lake Suigetsu sediment sequence, which yielded an age of94,500±4,800 years before present for this eruption.[13]
The caldera was the source of theKikai-Akahoya eruption, one of the largest eruptions during theHolocene (10,000 years ago to present) that produced the Kikai-Akahoya (K-Ah)tephra.[14] Between 7,200 and 7,300 years ago,[14][15][16]pyroclastic flows producing Koyaignimbrite from that eruption reached the coast of southernKyūshū up to 100 km (62 mi) away, and ash fell as far asHokkaido. The eruption produced about 133–183 km3 (32–44 cu mi) DRE, most of it tephra.[17][18] giving it aVolcanic Explosivity Index of 7,[18] so making it one of the most explosive in the last 10,000 years, ranking alongside the eruptions ofSantorini,Paektu,Crater Lake,Kurile Lake,Samalas andTambora.[19]
The eruption had a major impact on theJōmon culture in southernKyūshū although the impact was not as great as some commentary had suggested with Nishinozono sub-type pottery tradition, that had started prior to the eruption, maintained in Kyūshū.[20]
Japanese scientists conducted an extensive study of the volcanic activity of the Kikai underwater caldera. They had estimated the volumes of ejected volcanic material, which range from 332 to 457 cubic kilometers, and proved that it was the largest eruption in the last 11,700 years that occurred here 7,300 years ago. They were able to recreate the sequence of a large-scale volcanic event and identified three directions of flow of eruption products: in the atmosphere, along the seabed and along the water's edge.
Details of the marine expedition include conducting seismological studies and collecting sediment samples around the Kikai caldera. Scientists have confirmed that volcanic formations on the ocean floor and nearby islands have a common position. Analysis of the distribution of these deposits around the eruption site helps to understand how the pyroclastic flow and water interacted. The eruption occurred with a strong ejection of debris and ash, which corresponds to the usual phase of the Plinian type, during which there was a series of prolonged emissions under high pressure of fragmented lava and pumice in the form of a gas-ash mixture. It was a volumetric pyroclastic flow as a final stage, which partially spread along the seabed and released into the atmosphere in the form of an eruptive column (ash, fragments of pumice, small crystals and tephra). The tephra cloud covered an area of more than 2.8 million km2. The volume of ash material amounted to more than 370 km3 in terms of hard rock. The Plinian phase ended with the destruction of the eruptive column. A huge column of hot tephra fell a few hundred meters from the eruption’s center, causing the formation of a pyroclastic flow.
Since the center of the volcano was under water, the Akahoya eruption had the character of a steam explosion (or a series of explosions) due to the instantaneous release of steam upon contact of hot magma with water. As a result, a double caldera was formed.
Scientists had conducted a detailed study of the spread of volcanic material over an area of about 4,500 square kilometers around the center of the eruption and mapped the thickness of the underwater pyroclastic sediment. In their opinion, 133 to 183 cubic kilometers of pumice and ash settled on the studied area.
After analyzing the textures and nature of the fragments of the underwater volcanic strata, the authors concluded that it was formed from a suspended stream, which can cover long distances even up the slope, as it turned out. Having built a model of the Kikai-Akahoya eruption, researchers have found that in addition to the underwater pyroclastic flow and the powerful release of the tephra cloud into the atmosphere, there was also a third stream of thin volcanic material that spread along the surface of the water to the nearest islands.[21]
Kikai is still an active volcano.Io-dake (Mount Iō), Inamura-dake (south coast ofSatsuma-Io-jima), Tokara-Iwo-Jima (north east coast of Satsuma-Io-jima) andShōwa Iōjima (Shin-Io-jima) are post-caldera volcanoes within it.[2] Minor eruptions occur frequently on Mount Iō, one of the post-calderasubaerial volcanic peaks on Iōjima. Iōjima is one of three volcanic islands, two of which lie on the caldera rim. On June 4, 2013, weak tremors were recorded. Shortly after, eruptions began and continued off-and-on for several hours.[18] Io-dake is monitored for earthquake, gas and steam plume activity so that between the 2020 and 2023 eruptions it is known to have had continuous low grade activity.[18]
