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タイトル: 36. Volcano-Stratigraphic Study of Oshima Volcano, Izu
その他のタイトル: 36. 伊豆大島火山の火山層位学的研究
著者: NAKAMURA, Kazuaki
著者(別言語): 中村, 一明
発行日: 1965年3月15日
出版者: 東京大学地震研究所
掲載誌情報: 東京大学地震研究所彙報. 第42冊第4号, 1965.3.15, pp. 649-728
抄録: Oshima Volcano is a basaltic, insular stratovolcano with a summit caldera within which lies an active cone Mihara-yama. Izu- Oshima island is situated on the northern part of a volcanic chain extending from central Honshu to Marianas. Oshima Volcano consists an essential part of Izu-Oshima island which is the largest of the seven Izu-islands, being 92km2 in area and 22.5km3 in volume above sea-level. Volcanic products of the volcano were studied stratigraphically. The method and results of the study are summarized and discussed in this paper. A group of erupted material, including lava-flows, scoria- andash-falls, which successively accumulated without any weatheringbreak is called a member in this paper. A member is the basic stratigraphic unit which is proved to be the product of a singleeruptive cycle. Within a member, there is a definite succession oferupted material, scoria-falls→lava-flows→alternation of ash-falls in ascending order. The time needed for the deposition of amember is considered to be 10 years or less from an examinationof the latest two records of major eruption(1777~1792 and 1684~1690). The time needed for the deposition of the basal scoria of amember is far shorter than that for the deposition of the alternating ash-falls. This is evidenced partly by the examination of historical documents and partly by the simple elliptic shape of the distribution of the basal scoria. Eruptive activity represented bya member may be a continuous process in which the magma in the central conduit rises with explosive eruptions of scoria, overflows and then gradually falls with an ejection of ash formany years. Oshima Volcano is composed of more than a hundred members. The youngest twelve members are lumped into the Younger Oshima Group, with which this paper is chiefly concerned. Dates of the deposition of the twelve members are inferred bythe correlation with historical documents and excavated fragmentalpottery remains and by radiocarbon measurements. As a result, the Younger Oshima Group is found to consist of the deposits during the last 1500 years or so, and the deposition of each member, in other words .major eruption, to have taken place periodically with an interval of about 135±50 years. The volume of individual members is calculated to range from 0.1 to 0.7km3. The volume of the Younger Oshima Group is estimated to be 3.6km3, of which only 0.7km3 is now accumulated on the slope of the main cone. The rest is the caldera fill (ca. 2.2km3) and the deposits outside the island. The explosion index is calculated to be, about 60 for the Younger Oshima Group. Thermal energy transported by the Younger Oshima Group attains to the order of 1×1026 erg and the rate of energy release is 8.7~6.0×1024 erg/100 years. More than forty parasitic volcanoes are distributed within two narrow fracture zones. They are monogenetic, that is, the entire erupted material of each is incorporated within a single member. The summit caldera was formed after the deposition of the twelveth member(S2) from the surface and prior to that of the tenth member(N4), or some fourteen centuries ago. The volume of the twelveth and the eleventh member(S2 and S1), which are considered to have deposited just before the caldera formation, is estimated to be 0.4km3 i. e. one-eighth of the volume of the decapitated summit(3.1km3). These are mainly steam explosion breccias and ash-falls with accretionary lapilli. This will indicate that the summit was engulfed associated with intense explosions. The actual course of the engulfment is inferred to be like the eruption of Kilauea in 1924, after a consideration of the similarity of the sequence of events for the both activities. Recent eruptions of Oshima Volcano after the last major activity(1777~1792, the product of which is the uppermost member Y1), are very different in nature from those represented by members. Recent eruptions are more effusive(explosion index 10±), smaller in the volume of erupted material(less than 0.03km3) and have affected only the interior of the caldera or even of the crater of Mihara-yama. The rate of thermal energy release is also lower(2.7×1024 erg/100 years). Moreover, the period after the last major activity is that of repose judging from the deposits on the slope of the main cone.
URI: http://hdl.handle.net/2261/24367
ISSN: 00408972


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