(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 volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber ...

up to in diameter in the

Ōsumi Islands The is an archipelago in the Nansei Islands, and are the northernmost group of the Satsunan Islands, which is in turn part of the Ryukyu Archipelago. The chain extends from the southern tip of Kyushu to Yakushima. Administratively, the group ...

of Kagoshima Prefecture, Japan.

Geology

Caldera formation has been dated from about 95,000 years ago and has involved

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

basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90% of a ...

, 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 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.

Kikai-Tozurahara eruption

This was about 95,000 years before the present and erupted Kikai-Tozurahara (K-Tz) tephra. Various dating techniques give ages between 70,000 to 100,000 years before present. This was distributed all over Japan but did not reach South Korea.

Akahoya eruption

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

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

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

tephra Tephra is fragmental material produced by a 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, they re ...

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

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 producing Koya

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 surro ...

from that eruption reached the coast of southern Kyūshū up to away, and ash fell as far as Hokkaidō. The eruption produced about 150 km³ of

,Kikai – Eruptive history
Global Volcanism Program. Smithsonian Institution. giving it a Volcanic Explosivity Index of 7 and making it one of the most explosive in the last 10,000 years, ranking alongside the eruptions of

Santorini Santorini ( el, Σαντορίνη, ), officially Thira (Greek: Θήρα ) and classical Greek Thera ( English pronunciation ), is an island in the southern Aegean Sea, about 200 km (120 mi) southeast from the Greek mainland. It is th ...

, Paektu,

Crater Lake Crater Lake (Klamath: ''Giiwas'') is a volcanic crater lake in south-central Oregon in the western United States. It is the main feature of Crater Lake National Park and is famous for its deep blue color and water clarity. The lake partly fills ...

, Kurile Lake, Samalas and Tambora. According to ice cores, initially it was thought that the Akahoya eruption may have occurred in 4350 BC, however this timing has later been adjusted to about 1000 years earlier. The eruption had a major impact on the Jōmon culture in southern Kyūshū 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.

Eruptive history since 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 In natural science, subaerial (literally "under the air"), has been used since 1833,Subaerial
in the Merriam ...

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. file:2015 Satsuma-Iojima Iodake.jpg, 250px, 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 250px, Tephra horizons in south-central Iceland. The thick and light coloured layer at the height of the volcanologist's hands is rhyolitic tephra from Hekla. Tephrochronology is a Geochronology, geochronological technique that uses discrete l ...

) 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 highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular v ...

**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 moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...

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

References

* *
VolcanoWorld – Kikai, Kyūshū, Japan
* ttp://geographic.org/photos/volcanoes/volcano_photos_153.html Kikai Caldera – Volcano Photos

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 Supervolcanoes Holocene calderas {{Kagoshima-geo-stub