Volcanic Geology of Kozu-shima, Japan

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  • 神津島の火山地質
  • コウズシマ ノ カザン チシツ

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Abstract

Kozu-shima, which belongs to the northern subzone of the Fuji Volcanic Zone, is one of the Seven Izu Islands and is situated about 160km south of Tokyo. In this report, the author intends to clarify the volcanic geology of the island. The main results are as follows: 1) The rhyolitic volcanic activity (soda-rhyolite) in the latest Pleistocene to the Holocene age in the island can be divided into three stages. The activity of the first stage occurred under shallow submarine enviroment and formed several islets. The second stage is characterized by extrusions of a large amount of pyroclastic rocks and several lava domes. Some activities might have occurred on land. The channels between the islets made in the first stage were filled with the pyroclastic rocks of this stage. The third stage activity occurred on land and made five lava domes. 2) The eruptive centers of biotite rhyolite are arranged in a NW direction. 3) Many fractures and lineaments are obsered in the island. The fractures can be divided into three kinds as follows: a. normal fault: NS (strike), ca 60°W (dip) b. fault: ca NW, ca 90°c. fault: ca NE, ca 90°4) Alignment of domes of biotite rhyolite agrees with the NW-trending fault in direction. The relation between the volcanic alignment and the fractures is discussed at the last part.

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