3 0 0 0 OA 伊豆半島,天城カワゴ平火山の噴火と縄文時代後~晩期の古環境
- 著者
- 嶋田 繁
- 出版者
- Japan Association for Quaternary Research
- 雑誌
- 第四紀研究 (ISSN:04182642)
- 巻号頁・発行日
- vol.39, no.2, pp.151-164, 2000-04-01 (Released:2009-08-21)
- 参考文献数
- 51
- 被引用文献数
- 17 13
天城カワゴ平火山は,伊豆半島の天城火山北西部に位置する単成火山である.噴出物層序から,同火山の噴火活動は噴火様式の異なる4ステージ(ステージI:火砕サージ噴火→ステージII:プリニー式噴火→ステージIII:火砕流噴火→ステージIV:溶岩流出)に区分できる.このうち,ステージIIで噴出した大量の降下軽石(KGP)はおもに東風により西へ吹送されたが,一部は火口の北または北東~南東方向にも認められ,全体の分布域は伊豆~東海地方のほか南関東,中部山岳地域,近畿地方などの広域に及ぶ.その分布には偏西風の影響がほとんど認められないことから,噴火は夏季に生じた可能性が示唆される.またステージIIIでは,火砕流が天城火山山麓の谷を流下したが,これと同時に灰かぐらが発生し,火砕流の流下域やその西側に細粒火山灰層が厚く堆積した.カワゴ平火山の噴火年代については,14C年代値,周辺地域のテフラ層序,KGPと考古遺物との層位関係から,3,060~3,190yrs BPと推定した.この噴火と同時期(縄文時代後~晩期)の伊豆半島や駿河湾岸周辺の沖積低地では,潟湖の埋積が進み,沼沢地・湿地が発達していた.特に静清平野では,KGPを挾む泥炭層が低地のほか,扇状地末端などの微高地上にも分布することから,KGP降下期の同平野では広域に沼沢地・湿地が形成されていたと推定される.
2 0 0 0 OA 八丈島, 西山火山南東麓における最近2,500年間の噴出物の層序と噴火年代
- 著者
- 杉原 重夫 嶋田 繁
- 出版者
- Tokyo Geographical Society
- 雑誌
- 地学雑誌 (ISSN:0022135X)
- 巻号頁・発行日
- vol.107, no.5, pp.695-712, 1998-10-25 (Released:2009-11-12)
- 参考文献数
- 45
- 被引用文献数
- 2 3
Hachijo Island, located on the volcanic front of the Izu-Ogasawara (Bonin) arc, consists of two stratovolcanoes : Nishiyama and Higashiyama. Nishiyama (Alt. 854.3 m) is a scarcely dissected cone called “Hachijo-fuji”. Nishiyama began its volcanic activities about 10, 000 years ago. Many lateral volcanoes exist around Nishiyama, and one of them, Kandoyama (Alt. 194.9 m) is a tuff cone which was formed by a phreatomagmatic eruption. This study discusses the stratigraphy of deposits, and the eruptive types and the eruption ages of Nishiyama after the formation of Kandoyama. The results are summarized as follows.1) This study names four air-fall pyroclastic materials “Ny1-4”, which erupted from Nishiyama after the pyroclastic surge eruption of Kandoyama (ca. 2, 500 y.BP). All of them are scoriaceous.2) Ny2 erupted from a lateral volcano on the west of Kandoyama. This scoria cone is composed of agglutinate.3) Ny3 erupted from the summit crater of Nishiyama ca. 1, 100 y.BP. Ny3 was widely distributed on the southeast flank of Nishiyama, but it did not reach the foot of Higashiyama. Prior to the eruption of Ny3, a large amount of lava flowed down the southeast flank of Nishiyama.4) Ny4 erupted from a crater row along the southeast side of Nishiyama. From the description found in archives, it is clear that the eruption of Ny4 corresponds to A.D. 1605 (Keicho 10th) eruption. Ny4 deposited around the vents as agglutinate, and air-fall materials were widely distributed on the southeast side of Nishiyama. However, the total volume of Ny4 was the same or less than that of the Miyakejima 1983 eruption and the Izu-Oshima 1986 eruption.5) There are descriptions in the archives of at least four eruptions and about ten occurrences of earthquakes and tsunami in and around Hachijo Island. The fissures and tsunami deposits which followed historic earthquakes were found at the Yaene site located on the south side of Nishiyama. The pyroclastic materials from Nishiyama date these past earthquakes.