Kikai caldera

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Location and extent of the 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 is located in the middle of the Ōsumi Islands belonging to the Japanese prefecture of Kagoshima .

Geography and geology

The Kikai Caldera is part of the Pacific Ring of Fire . It is located in the transition area from mainland Japan ( Kyūshū ) to the northern arched section of the Ryūkyū Islands . The caldera lies above the subduction zone of the Philippine Sea Plate, which descends in the northern Ryūkyū Trench under the East China Sea or the eastern continental margin of the Eurasian Plate at a speed of 49 millimeters / year in a north-westerly direction. GPS measurements show a very low motion vector of around 5 millimeters / year to the south-east in the area of ​​the caldera. The East China Sea shows the cross to the mainland Beppu-Shimabara Graben of northern Okinawa trough a beginning in this area, still in the Rift stage located backarc spreading . Further north, Kyūshū is joined by the Ata , Aira and Kakuto Caldera , which are also of highly explosive origin. Kuchinoerabu-jima , located further to the south-west, has composite stratovolcanoes , and the adjacent Amami Islands also have larger stratovolcanoes.

Origin and effects

The elliptical caldera (north-south direction 17 kilometers, east-west direction 20 kilometers) is the result of an ultraplinian volcanic eruption around 4550 BC. With the strength 7 on the volcanic explosion index . The topography of the sea floor suggests a double collapse structure. More than 150 cubic kilometers of rock were thrown out. Since the end of the last ice age it was one of the six most violent eruptions in the Holocene ( Atlantic ). The pyroclastic flow of the eruption ("Takeshima pyroclastite") reached the southern coast of Kyushu, 100 km away, and the Koignimbritic ash ("Akahoya-Tephra") flew as far as Hokkaidō . The eruption also created a huge tsunami . The environmental destruction was enormous, for example the flora in the south of Kyūshū suffered permanent damage. The effects on the people of the earliest Jōmon time at that time are also likely to have been catastrophic.

volcanology

The Kikai caldera was formed on an active stratovolcano that sits on the ocean floor of the Pacific .

Before this event, three more hot cloud eruptions had occurred at the Kikai volcano:

  • "Funakura pumice" on Iojima and "Funakura pyroclastite" on Takeshima , period 7700 to 4550 BC Chr.
  • Get "Nagase-Pyroklastit" only on Takeshima, about 75,000 years BP
  • “Koabiyama pyroclastite” on both islands, older than 75,000 years BP.

Between the youngest two pyroclastites, there was a history of about 7700 to 7300 BC on both islands. The "Kikai-Komoriko-Tephra" deposited.

The volcanic activity has been going on since the super-eruption in the 44th century BC. By no means extinguished. Smaller (VEI 2), predominantly tephra eruptions occurred quite often on Iō-dake ( 硫黄 岳 ), a 704 meter (or 717 meter) high volcanic peak on the island of Iō-jima (in English sulfur island ), which is also Tokara -Iō-jima ( ト カ ラ 硫黄島 ) or Satsuma-Iō-jima ( 薩摩 硫黄島 ) is called. A total of six packages of tephra were deposited at the foot of the Iō-dake, which can be separated from each other by soil horizons. The first two packages fall between 4550 and 1940 BC. The top four in the period 260 BC. Until 1030 AD

On the slope of the Iō-dake two pumice units can also be excreted, which are separated by a pyroclastic surge deposit . The lower pumice layer dates from 820 AD, the surge deposit from 1010 AD and the upper pumice layer from 1420 AD.

The Iō-dake is also known for its fumarole activity, which has been ongoing since 1000 AD .

The considerably lower cone of the Inamura-dake is made up of two tephra parcels with a separating paleo floor, which were built during the period 1940 to 260 BC. Were promoted.

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

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

Chemical composition

The ejecta of the main eruption consists of a pyroxene- containing dacite . The tephra layers of the Iō-dake are Dacite to rhyolite , whereas the Inamura-dake is made up of basaltic ash. The new island Showa-Iō-jima shows a Dacite to rhyolite composition. Before the pyroclastite eruptions began, bimodal volcanism with basalts and rhyolite lavas prevailed at the Kikai volcano.

Rock analyzes from Inamura-dake, Iō-dake (average of three analyzes) and Showa-Iō-jima (four analyzes) are cited:

Oxide
wt.%		 Inamura-dake Iō-dake Showa-iō-jima
SiO 2 55.04 66.99 64.51
TiO 2 0.80 0.31 0.24
Al 2 O 3 14.60 13.90 16.25
FeO dead 5.58 5.36 5.02
MnO 0.22 0.09 0.12
MgO 5.91 1.06 2.33
CaO 9.13 4.45 5.42
Na 2 O 1.76 3.49 3.41
K 2 O 0.41 1.82 1.66
P 2 O 5 0.43 0.31 0.19

All rocks are sub-alkaline and belong to the main volcanic series. The analysis of Inamura-dake is a basaltic andesite with low potassium values ​​(low-K series, English low-K ) and with tholeiitic affinities. Iō-dake and Showa-Iō-jima are Dacitic with medium potassium values ​​(medium-K series, medium-K ) and calcareous affinities.

Dating

The Akahoya Tephra was determined by means of radiocarbon dating to 6500 years BP or 4550 calendar years BC. Chr. Dated. In the Global Volcanism Program 4350 B.C. Given as the eruption date. Archaeological methods , however, showed a slightly higher age of 7300 years BP or 5350 years BC. Chr. Warvenchronologisch were in brackish tephra deposits Suigetsu Lake north of Kyoto determined by Fukusawa (1995) with 7324 Warvenjahren; this would therefore correspond to 5329 BC. Chr.

Individual evidence

  1. ↑ T. Matumoto: The four gigantic caldera volcanoes of Kyushu . In: Jap. Jour. Geol. Geograph. Volume 19 , 1943, pp. 57 .
  2. ^ J. Letouzey, M. Kimura: The Okinawa Trough: Genesis of a back-arc basin developing along a continental margin . In: Tectonophysics . tape 125 , 1986, pp. 209-230 .
  3. ↑ M. Hashimoto et al .: A block-fault model for deformation of the Japanese Islands derived from continuous GPS observation . In: Earth Planets Space . tape 52 , 2000, pp. 1095-1100 .
  4. ↑ H. Kitagawa, among others: AMS 14C dating of varved sediments from Lake Suigetsu, central Japan and atmospheric 14C change during the late Pleistocene . In: Radiocarbon . tape 37 , 1995, pp. 371-378 .
  5. ↑ Shinji Sugiyama: The Impact of the Kikai-Akahoya Explosive Eruption on Vegetation in Southern Kyushu, Japan, Clarified by Phytolith Studies . In: Quaternary Research . tape 41 , no. 4 , 2002, ISSN  0418-2642 , p. 311-316 , doi : 10.4116 / jaqua.41.311 .
  6. ↑ a b Mitsuhiro Kuwahata: Age and Cultural Influence of the Kikai-Akahoya Eruption as Seen from Archaeological Material in South Kyushu, Japan . In: Quaternary Research . tape 41 , no. 4 , 2002, ISSN  0418-2642 , p. 317-330 , doi : 10.4116 / jaqua.41.317 .
  7. ^ H. Machida, F. Arai: Atlas of tephra in and around Japan . Univ. of Tokyo Press, 1992, pp. 276 .
  8. ↑ a b c d Yoshihisa Kawanabe, Genji Saito: Volcanic activity of the Satsuma-Iwojima area during the past 6500 years . In: Earth Planets Space . tape 54 , 2002, pp. 295-301 .
  9. ↑ H. Shinohara, inter alia: Geochemistry of volcanic gases and hot springs of Satsuma-Iwojima, Japan: Following Matsuo . In: Geochem. J. Band 27 , 1993, pp. 271-285 .
  10. ^ H. Kuno: Part XI, Japan, Taiwan and Marianas: Catalog of the active volcanoes of the world including solfatara fields . Ed .: International Association of Volcanology. Rome, Italy 1962, p. 332 .
  11. ↑ Fukashi Maeno, Hiromitsu Taniguchi: Eruptive History of Satsuma Iwo-jima Island, Kikai Caldera, after a 6.5ka caldera-forming eruption . In: Bulletin of the Volcanological Society of Japan . tape 50 , no. 2 , 2005, ISSN  0453-4360 , p. 71-85 .
  12. ↑ H. Fukusawa: Non-glacial varved lake sediment as a natural timekeeper and detector on environmental changes . In: The Quaternary Research of Japan . tape 34 , 1995, pp. 135-149 .

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Coordinates: 30 ° 47 ′ 20.4 "  N , 130 ° 18 ′ 28.8"  E