白山火山の形成史

抄録

Hakusan Volcano, situated about 50 km south of Kanazawa, central Honshu, is composed of two stratified volcanic cones of different ages, Ko-Hakusan (Old Hakusan) and Shin-Hakusan (New Hakusan), and its volcanic history is divided into the following periods : Ko-Hakusan Period Ko-Hakusan was formed probably in early Pleistocene through repeated eruptions of lava and pyroclastic materials mainly of pyroxene hornblende andesite. The total volume of materials erupted is estimated to be 15 km³. Pyroclastic flow eruptions took place in early stages of the period. The volcanic body has been deeply dissected, and the central portion has been largely removed by erosion. Shin-Hakusan Period After a period of dormancy, volcanism recurred in late Pleistocene, and Shin-Hakusan was formed on the south slope of Ko-Hakusan. Two stages are distinguished in this period. The bulk of the cone was formed during Gozen stage. It was followed by Midoriga-ike stage, during which the center of activity moved a little to the north, and a group of small craters were formed by explosive eruptions including those of historic ages. The materials erupted in this period, being about 1 km³ by volume, are composed mainly of pyroxene hornblende andesite. The basement, on which Hakusan was formed, had been arched possibly by a deep-seated intrusion of magma, and the center of volcanism was located at the top of the swell thus formed. The surface of the basement near the center is more than 1000m higher than in the neighbouring areas of Hakusan. A group of strike-slip faults pass through the Hakusan area from northwest to southeast, and the basement rocks were sinistrally dislocated and densely fractured along them. It is conceivable that the magma conduit of Hakusan was formed in this zone of fracturing.