著者

中田 節也 田中 雅人

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.36, no.1, pp.113-121, 1991

被引用文献数

4

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Unzen Volcano, Kyushu, Japan, has grown since about 0.5 Ma in a graben where N-S tensile stress is dominant. It contains three volcanic cones with individual life times of 10⁵y. Historical eruptions occurred at intervals of several hundred years. The products with the total volume of 103 km³ are composed mainly of thick lava flows/domes of hornblende biotite dacite and their collapsed deposits. Seismological data indicated relatively low-density materials at 8 to 18 km depth under the youngest volcanic cone, although there is no direct evidence showing a large magma chamber. A focal solution for an earthquake suggested that a vertical, open crack formed in an EW direction at about 10 km depth, implying magma intrusion. These facts give an idea that multiple, small, vertically standing magma plates (lenticular batches) oriented in an EW direction have existed beneath Unzen Volcano like dike swarm. The Unzen volcanic rocks show the evidence for mixing of felsic and mafic magmas just before or during eruption. The phenocryst assemblages estimated for mixing-endmember magmas were not uniform among lavas. The bulk compositions of rocks do not show a simple trend of mixing between two endmembers. It is likely that lavas of Unzen Volcano erupted from separate magma batches in that felsic magmas overlay high-alumina basaltic magmas. The felsic magmas may have been produced both by partial melting of crustal materials and by fractional crystallization of the mafic magmas. As a mass of magma in a small, separate lenticular batch can cool easily in the uppermost crust, the life time of each magma batch may have been as short as several hundred years. Probably, any large, shallow magma chamber has not formed beneath Unzen Volcano. However, magmas rising from the upper mantle may have been tapped successively in a large, flat chamber between the upper and lower crusts, where they evolved into high-alumina basaltic magma through fractional crystallization, mixing, and contamination processes.