(alternatively Kikaiga-shima, Kikai Caldera Complex) is a massive, mostly submerged

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

up to in diameter in the

Ōsumi Islands The is an archipelago in the Ryukyu Islands, Ryukyu Archipelago, and are the northernmost group of the Ryukyu Islands, The chain extends from the southern tip of Kyushu to Yakushima. Administratively, the group belongs within Kagoshima Prefecture ...

Kagoshima Prefecture is a Prefectures of Japan, prefecture of Japan located on the island of Kyushu and the Ryukyu Islands. Kagoshima Prefecture has a population of 1,527,019 (1 February 2025) and has a geographic area of 9,187 Square kilometre, km2 (3,547 Square m ...

, Japan.

Geology

The Kikai Caldera Complex has twin ovoid caldera by in diameter. Yahazu-dake (north west part of Satsuma Io-jima) and Takeshima, located on the caldera rim, are pre-caldera volcanoes. The pre-caldera stage of volcanic activity involved

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

, and

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

phases. The earliest definitive caldera formation has been dated back to at least 140,000 years ago, resulting from the eruption of Koabiyama

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

s.Nishino, Y., Takarada, S. and Geshi, N. (2019
Volcanic activities and eruption sequence of the large-scale eruption at Aira, Kikai, Ata, Kutcharo, Towada, Mashu and Ikeda Volcanoes.
Open-File Report of the Geological Survey of Japan, AIST, no.680, p. 1-126. The formation of caldera has been associated with at least three catastrophic

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

eruptions. 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.

Kikai-Koabiyama eruption

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 , whereas in Iwo Jima, they are relatively thin, measuring a few to . The eruption of the K-Kob pyroclastic flows has been dated using K-Ar dating to be years before present. While no distal tephra from this eruption has been reported, a tephra layer with potential geochemical and age correlation has been discovered in Lake Suigetsu.

Kikai-Tozurahara eruption

Kikai-Tozurahara (K-Tz)

tephra Tephra is fragmental material produced by a Volcano, volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, ...

is a widespread rhyolitic tephra layer of

Late Pleistocene The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as the Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division ...

age, attributed to a large VEI-7 eruption from the Kikai caldera. This layer is confirmed to have a wide distribution, extending from south

Kyushu is the third-largest island of Japan's Japanese archipelago, four main islands and the most southerly of the four largest islands (i.e. excluding Okinawa Island, Okinawa and the other Ryukyu Islands, Ryukyu (''Nansei'') Ryukyu Islands, Islands ...

to eastern

Honshu , historically known as , is the largest of the four main islands of Japan. It lies between the Pacific Ocean (east) and the Sea of Japan (west). It is the list of islands by area, seventh-largest island in the world, and the list of islands by ...

and reaching the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

, and possibly including the Shandong Peninsula. The proximal equivalents of K-Tz are the Nagase

and the Nishinoomote pyroclastic surges. The combined bulk volume of both distal and proximal deposits is estimated to exceed . In marine 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. More reliable age constraints were imposed by the high-resolution chronology derived from the Lake Suigetsu sediment sequence, which yielded an age of years before present for this eruption.

Kikai-Akahoya eruption

The caldera was the source of the Kikai-Akahoya eruption, one of the largest eruptions during the

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

(10,000 years ago to present) that produced the Kikai-Akahoya (K-Ah)

. Between 7,200 and 7,300 years ago,

s producing Koya

from that eruption reached the coast of southern

Kyūshū is the third-largest island of Japan's four main islands and the most southerly of the four largest islands (i.e. excluding Okinawa and the other Ryukyu (''Nansei'') Islands). In the past, it has been known as , and . The historical regio ...

up to away, and ash fell as far as

Hokkaido is the list of islands of Japan by area, second-largest island of Japan and comprises the largest and northernmost prefectures of Japan, prefecture, making up its own list of regions of Japan, region. The Tsugaru Strait separates Hokkaidō fr ...

. The eruption produced about DRE, most of it tephra. giving it a

Volcanic Explosivity Index The volcanic explosivity index (VEI) is a scale used to measure the size of explosive volcanic eruptions. It was devised by Christopher G. Newhall of the United States Geological Survey and Stephen Self in 1982. Volume of products, eruption c ...

of 7, so making it one of the most explosive in the last 10,000 years, ranking alongside the eruptions of

Santorini Santorini (, ), officially Thira (, ) or Thera, is a Greek island in the southern Aegean Sea, about southeast from the mainland. It is the largest island of a small, circular archipelago formed by the Santorini caldera. It is the southern ...

, Paektu, Crater Lake, Kurile Lake, Samalas and Tambora. The eruption had a major impact on the Jōmon culture in southern

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.

2024 studies

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.

Eruptive history since Kikai-Akahoya eruption

Kikai is still an active volcano. Io-dake (Mount Iō), Inamura-dake (south coast of Satsuma-Io-jima), Tokara-Iwo-Jima (north east coast of Satsuma-Io-jima) and Shōwa Iōjima (Shin-Io-jima) are post-caldera volcanoes within it. Minor eruptions occur frequently on Mount Iō, one of the post-caldera subaerial 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. 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. 250px, Mount Iō (Iōjima), Mount Iō, a

lava dome, May, 2015, viewed from the east Eruptions occurred: *old Iwo-dake stage (stage OIo-I-II) **phreatomagmatic eruptions and pumice fallout (stage OIo-I), followed by rhyolitic lava with continuous

, resulting in a volcanic edifice (stage OIo-II) **3250 BCE ± 75 years (uncalibrated) Old Iwo-dake ***OIo1a,b tephras **2450 BCE ± 840 years ( tephrochronology) Old Iwo-dake ***OIo2a,b tephras * Inamura-dake stage (stage In-I-IV) **basaltic lava flows and scoria-cone building (stage In-I-II), then phreatomagmatic eruptions (stage In-III), and then andesitic lava (stage In-IV) **1830 BCE ± 75 years (uncalibrated) Inamura-dake ***In-I tephra **1090 BCE ± 100 years (uncalibrated) Inamura-dake *young Iwo-dake stage (stage YIo-I-IV) **continuing with a different magma source including rhyolitic lava and intermittent pumice **280 BCE ± 75 years (uncalibrated) Iwo-dake **390 ± 100 years (uncalibrated) Iwo-dake **750 (tephrochronology) Iwo-dake **830 ± 40 years (uncalibrated) Iwo-dake ***K-Iw-P1 tephra **1010 ± 40 years (uncalibrated) Iwo-dake *** K-Sk-u-3 tephra **1030 ± 40 years (uncalibrated) Iwo-dake ***K-Sk-u-4 tephra **1340 ± 30 years (uncalibrated) Iwo-dake ***K-Iw-P2 tephra **1430 ± 75 years (uncalibrated) Iwo-dake ** 13 Feb 1914 Tokara-Iwo-Jima **Sep-Nov 1934 ***Submarine eruption with

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of extremely vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicula ...

**7 Dec 1934 -Mar 1935 2 km east of Tokara-Iwo-Jima ***New island Shōwa Iōjima (Shin-Io-jima) with

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

finally stabilised 19 January 1935 **1997-2003 Iwo-dake ***Formation and enlargement of new pit crater inside the summit crater **Apr-Nov 1998 Iwo-dake ***Ash **May-Aug 1999 Iwo-dake ***Ash **Jan, Mar, Oct-Dec 2000 Iwo-dake ***Ash **Feb, Apr-Dec 2001 Iwo-dake ***Ash **May-Jul 2002 Iwo-dake ***Ash **Feb, Apr-Oct 2003 Iwo-dake ***Ash **May-Apr, Jun, Aug-Oct 2004 Iwo-dake ***Ash **3-5 May, 3-5 Jun 2013 Iwo-dake ***Minor eruptions **2 Nov 2019 Iwo-dake ***Minor eruption **29 Apr 2020 Iwo-dake ***Minor eruption **6 Oct 2020 Iwo-dake ***Minor eruption * 27 Mar 2023 – 22 Nov 2023 ***Minor eruptions

References

* *
VolcanoWorld – Kikai, Kyūshū, Japan

* ttp://geographic.org/photos/volcanoes/volcano_photos_153.html Kikai Caldera – Volcano Photos{{reflist

External links

Satsuma-iojima
- Geological Survey of Japan

- Geological Survey of Japan

- Earthquake Research Institute,The University of Tokyo
Global Volcanism Program
Smithsonian Institution Ōsumi Islands VEI-7 volcanoes Calderas of Japan Active volcanoes Submarine calderas Volcanoes of Kagoshima Prefecture Holocene calderas