著者

田沢 堅太郎

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.36, no.4, pp.419-430, 1991-12-25

被引用文献数

1

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Studies on coastal deposits and geomorphology of Oshima island have revealed some uplifts and subsidences associated with the volcanic activity of Oshima volcano. Uplifts and subsidences of the island are inferred by subtracting the sea levels at the time from the heights of the past sea surface recorded on coastal deposits. Two series of large-scale eruptions starting at the ages of O_<41> (7,780±160 yB.P.) and S_2 (1,360±40 yB.P.) have similar characteristics as follows. (1) At the ages of O_<42> (8,200-7,800 yE.P.) and O_1(1,500-1,400 yE.P.) just before the two series of large-scale eruptions, lagoons were formed along the west coast of Oshima island, suggesting the subsidence of the island. (2) At the beginning in each series, a large-scale steam explosion occurred at the summit crater to generate a debris flow. (3) In each series, the volume of the eruptive materials rapidly increased at the beginnings and gradually decreased after those times. (4) Volcanic activities were explosive at the beginnings in the series and gradually changed to effusive. (5) Oshima island was remarkably and rapidly uplifted at the beginnings of the two series of the large-scale eruptions and was gradually subsided. (6) In the stage of uplifling of the island, the scale of eruptions had been enlarging, and in that of subsiding, the scale of eruptions had been getting smaller.

著者

田沢 堅太郎

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.29, no.1, pp.1-15, 1984-04-01

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Izu-Oshima volcano consists mostly of subaerially deposited volcanic products. On the sea cliff, however, the water-laid deposits are intercalated in the O_<42> (about 7, 500 yB.P.) and O_1 (about 1, 500 yB.P.) members of the Older Oshima Group. Layers of coastal gravels are observed in the Y_5-Y_1 (A.D. 1338-the present) members of the Younger Oshima Group near the present coast. These layers show imbricate structure indicating landward flow. They are interpreted as wind-transported from nearby coastal beach. These nearshore deposits were presumably resulted from the variation of the sea level in height. In the O_1 age, lagoons were formed along the coast. In the S_2 age (about A.D. 550), remarkable emergence of the shoreline occurred. After that, the island has kept subsiding gradually.