東伊豆沖海底火山群-2-および伊豆諸島近海海底火山

概要

論文の詳細を見る
1979年12月の淡青丸KT-79-18航海の際,東伊豆沖海底火山群,大島南方の大室出シ堆,新島東方の新島ウラノ瀬などの海底火山の岩石をドレッジにより採集した.東伊豆沖海底火山群は,岩石の鉱物・化学的特徴により北半部,中部,南部に細分することができる.北半部の岩石はすべて高アルカリソレアイト系列の玄武岩・安山岩溶岩であり,伊豆半島中部東部に分布する東伊豆単成火山群がそのまま海底に延長して孤立した海底火山として分布しているものと考えて差支えない.中部では,低アルカリ(低Na2O)ソレアイト系列の玄武岩が5点のドレッジから発見された.そのうち伊豆大島に近い2点は,伊豆大島火山の玄武岩類に似た鉱物・化学組成をもち,新鮮である.西側の3点の岩石はいずれも風化変質作用を受けており,小角礫の集合として産する.これらは伊豆大島火山や東伊豆単成火山群よりも古い海底火山に属するものと判断される.南部のドレッジ2点からは東伊豆単成火山群南西部のグループの岩石に似た玄武岩が得られた。大室出シは,ガラス質多孔質の新鮮な流紋岩溶岩流から成る平坦な頂部をもつ海底火山と考えられる.山体の中央部に深さ100m,長さ1.5km,幅0.5kmの凹陥地(大室海穴)があるが,その壁からは流紋岩溶岩が採集された.新島ウラノ瀬の南東麓からは,流紋岩溶岩と変質した玄武岩礫,石英閃緑岩礫などが採集された.後者はこの地域を構成する基盤岩類と考えられる.Basalts, andesites and rhyolites were dredged during cruise KT-79-18 of the R/V. Tansei-maru of the Ocean Research Institute, University of Tokyo, in December 1979. The dredged areas included the Higashi-Izu-oki submarine volcanoes and the flat-topped Omurodashi to the south of Izu-Oshima island and a peak called Niijima-urano-se located east of Niijima island. The Higashi-Izu-oki submarine volcanoes may be subdivided into the following three groups based on their petrography and chemistry: 1) the northern half of the area are all made up of rocks very much similar to those of the Higashi-Izu monogenetic volcanoes on the Izu Peninsula, composed mostly of high-alkali(high-Na2O) tholeiitic basalts and andesites. The andesites are supposed to have been derived through assimilation of granitic rocks by the basalt magma. 2) In the middle part of the area, five dredged samples are all low-alkali(low-Na2O) tholeiite series rocks arranged in an E-W to NE-SW direction. The easternmost two of these five, i.e. rocks occurring closest to Izu-Oshima island(low-alkali tholeiites) are very much similar to the rocks of the Izu-Oshima volcano and are believed to be a submarine extension of the same magma regime. The western three samples are more or less altered olivine-augite-plagioclase phyric basalts and are supposed to have been derived from submarine volcanoes older than both the Higashi-Izu monogenetic volcano group and Izu-Oshima island. 3) The southern part produced two dredge hauls with fresh high-Na2O basalts similar to the southwestern group of the Higashi-Izu monogenetic volcano group on land. Thus the Higashi-Izu submarine volcanoes appear to be composed of an eastern submarine extension of the Higashi-Izu monogenetic volcano group and the northwestern extension of the Izu-Oshima volcano and a few older submarine volcanoes which are different from the above two. The flat-topped Omurodashi submarine volcano south of Izu-Oshima island appears to consist of glassy and porous rhyolite lava flows. In the center of the edifice, a crater-like depression 1.5km×0.5km wide and 100m deep was found. Rhyolite lavas are exposed on the wall of this depression. Dredged samples from the southeastern foot of an elongated submarine volcano called Niijima-urano-se, east of Niijima island were composed of fresh rhyolites, altered basalts and some quartz diorite blocks. The basalts and quartz diorites may be the basement rocks of the region. The rhylites from Omurodashi and Niijima-urano-se are distinguished from those of Niijima and Kozushima islands in that they are higher in Na2O.
東京大学地震研究所,Earthquake Research Institute, University of Tokyo,外務省経済局海洋課,東京大学地震研究所,東京大学海洋研究所,工業技術院地質調査所の論文

東伊豆沖海底火山群-2-および伊豆諸島近海海底火山

スポンサーリンク

概要

著者

関連論文

スポンサーリンク