著者

湯浅 真人

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.40, no.4, pp.277-284, 1995-09-20 (Released:2017-03-20)

参考文献数

29

被引用文献数

1

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Many caldera volcanoes occur in the northern part of the Izu-Ogasawara Arc. These submarine calderas produced a large amount of pumice and show high gravity anomalies in the central parts along with small amplitude of magnetic anomalies on the volcanic edifices. They have no equivalent in the usual classification of on-land calderas as far as known. Elsewhere I proposed that the submarine "calderas" associated with pumice in the arc were pumice cones which were difficult to form under subaerial conditions but easy under subaqueous conditions. The Myojin Knoll is one of the typical pumice cones with calderas in the arc. It is located between Aogashima Island and Myojinsho Reef. The size of the knoll is about 18 km in diameter at the base and 950 m in height. The knoll has caldera structure on its summit. The nearly circular caldera rim is 5-7 km in diameter. The caldera floor is about 1400 m deep and 5-6 km across. A central cone rises from the floor. Relief of the caldera wall is about 700-900 m with the inner slope being at about 20-30°. The submersible study by Shinkai 2000 reported here shows that the top of knoll is constructed mainly of stratified pumice deposits underlain by rhyolitic lava. These rocks have characteristic chemical compositions with high SiO2 (more than 71 wt%) and low K2O (less than 0.86 wt%), and show similar range to the rocks from Aogashima Rift volcanoes to the west of the knoll rather than those from adjacent Myojinsho Reef Volcano. The eruption under the deep water conditions caused the deposition of pumice near the crater. The repetition of such eruptions formed the submarine pumice volcano. Acid volcanism under subaqueous conditions in the northern part of the arc formed the volcanoes with pumice cones and calderas.

著者

湯浅 真人 石原 丈実 岸本 清行 村上 文敏 新井田 清信

出版者

一般社団法人 日本地質学会

雑誌

日本地質学会学術大会講演要旨 (ISSN:13483935)

巻号頁・発行日

vol.1994, 1994

著者

岡村 聡 坂本 泉 金 容義 石塚 治 湯浅 真人 冨士原 敏也 藤岡 換太郎 倉本 能行 前田 仁一郎

出版者

一般社団法人日本鉱物科学会

雑誌

日本鉱物科学会年会講演要旨集

巻号頁・発行日

vol.2007, pp.7, 2007

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孀婦岩構造線は,伊豆・小笠原弧を北部と南部に二分する大構造線であり,北北東―南南西方向の走向を示し,その東側斜面に沿って比高最大1500mの急崖が発達し,地殻の深部断面を観察することができる.<Br> 孀婦岩構造線の南東に位置する沢海山は,鮮新世の活動年代を示す火山フロント帯火山であり,島弧玄武岩の化学組成を示す.<Br> 孀婦岩構造線沿いに観察される地殻断面(孀婦地塊)は,後期中新世を示す塊状の溶岩・貫入岩とハイアロクラスタイト及び,それらを供給するフィーダーダイク・溶岩が観察される.孀婦地塊を基盤とする背弧側には中新世~鮮新世に活動した小海丘群が存在する.孀婦地塊と小海丘群の火成岩類は,いずれも背弧海盆玄武岩の特徴を示す点で共通するが,後者はIndian Ocean MORBタイプアセノスフェアの寄与が大きかったことを示唆する.