Kikai-Caldera

The Kikai caldera (Japanese鬼界 カルデラ, Kikai - karudera ) is a large underwater caldera with an average diameter of 19 kilometers. It lies in the midst of the Japanese prefecture of Kagoshima belonging Osumi Islands.

Geography and Geology

The Kikai caldera forms part of the Pacific Ring of Fire. It is located in the transition region from mainland Japan ( Kyushu ) to the northern island arc portion of the Ryukyu Islands. The caldera is above the subduction zone of the Philippinenseeplatte that at a speed of 49 mm / yr in a northwesterly direction dives in the northern Ryukyu Trench beneath the East China Sea and the eastern continental margin of the Eurasian plate. GPS measurements show for the area of the caldera a fairly small motion vector of about 5 mm / year to south-east. The East China Sea is to the cross to the mainland Beppu - Shimabara Graben of the northern Okinawa trough a beginning in this area, still in the rift stage located backarc spreading. Further north, close to Kyushu with the Ata, the Aira caldera and the Kakuto more giant calderas, which are also highly explosive origin. The further south west Kuchinoerabu -jima has Kompositstratovulkane and also the subsequent Amami Islands in turn have larger stratovolcanoes.

Emergence and impact

The elliptical caldera ( 17 km north-south, east-west direction 20 km ) is the result of a volcanic eruption ultraplinianischen to 4550 BC with more than 7 on the Vulkanexplosivitätsindex. The topography of the sea floor indicates a double collapse structure. There are more than 150 cubic kilometers of rock masses were ejected. Thus, since the end of the last ice age it was one of the six most violent eruptions in the Holocene ( Atlantic period ). The pyroclastic flow eruption ( Takeshima Pyroklastit ) reached 100 km from the southern coast of Kyushu and the koignimbritische ash ( tephra - Akahoya ) flew up after Hokkaidō. The eruption also produced a massive tsunami. The environmental destruction was enormous, so suffered, for example, the plant life in the south of Kyushu permanent damage. The impact on the people of that Earliest Jōmon - time should also have been catastrophic.

Volcanology

The Kikai caldera was created in an active stratovolcano which rests the seabed of the Pacific.

Even before this event at Kikai volcano three more Glutwolkenausbrüche had occurred:

• Funakura pumice on Iō -jima and Funakura - Pyroklastit on Takeshima period 7700-4550 BC
• Nagase Pyroklastit obtained only on Takeshima, about 75,000 years BP
• Koabiyama - Pyroklastit on both islands, older than 75,000 years BP.

Between the two most recent Pyroklastiten BC was deposited on both islands around 7700-7300 the Kikai - Komoriko - tephra.

The volcanic activities are by no means extinct BC since the Super Outbreak in the 44th century. Smaller ( VEI 2), mainly tephra eruptions occurred quite frequently at Iō -dake (硫黄 岳), a 704 meters (or even 717 meters) high volcanic peaks on the island Iō -jima ( to German sulfur island), which also Tokara - Iō -jima (トカラ 硫黄 島) or Satsuma Iō -jima (萨摩 硫黄岛) is called. A total of six Tephrapakete were deposited at the foot of Iō -dake, which can be separated off from one another by soil horizons. The first two packages fall within the period 4550-1940 BC, the top four BC to 1030 AD in the period 260 BC.

On the slope of Iō -dake two Bimseinheiten may furthermore be excreted, which are separated by a pyroclastic surge deposit. The lower Bimslage dates from the year 820 AD, the surge deposits from the year 1010 AD and the upper Bimslage from 1420 AD.

The Iō -dake is also known for its n since the year 1000 BC continually ongoing Fumarolentätigkeit.

The much lower cone of Inamura -dake is made ​​up of two Tephrapaketen with abtrennendem paleosol, which were funded during the period 1940-260 BC.

In 1934/35 a new island emerged from the sea with Showa Iō -jima. It measures 500 x 300 meters and its lava dome rises only 26 meters from the sea.

The last eruption took place in 2002, 2004 and 2013.

Chemical composition

The ejecta of the main eruption consist of a pyroxene - bearing dacite. The tephra layers of Iō -dake are dazitisch to rhyolithisch, whereas the Inamura -dake is composed of basaltic ash. The new island Showa -jima Iō dacitic to rhyolitic composition shows. Before inserting the Pyroklastit eruptions prevailed at Kikai volcano bimodal volcanism with basalts and rhyolitic lavas.

Cited were rock analyzes from Inamura -dake, Iō -dake (average of three analyzes ) and Showa Iō -jima ( four analyzes ):

All rocks are subalkalisch and belong to the main volcanic series. In the analysis of Inamura -dake is a basaltic andesite with low potassium levels (low - K series, Eng. Low- K) and with tholeiitic affinities. Iō -dake and Showa Iō -jima are dazitisch with mean potassium values ​​(mean K- series, medium -K) and kalkalkalischen affinities.

Dating

The Akahoya - tephra has been dated by the radiocarbon dating to 6500 years BP and 4550 calendar years BC. In the Global Volcanism Program are given as 4350 BC eruption date. However, archaeological methods gave a slightly higher age of 7300 years BP and 5350 years BC Warvenchronologisch were tephra deposits in the brackish Lake Suigetsu determined with 7324 Warvenjahren north of Kyoto from Fukusawa (1995).; this consequently would correspond to 5329 BC.