樽前火山の形成 : とくに Ta-a, Ta-b 期の活動について

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The Tarumae volcano (1024 m.a.s.l.) which is situated at the eastern margin of South-west Hokkaido is the youngest of the three volcanoes formed after caldera depression of the Shikotsu volcano (Figure 1). The volcanic history of the Tarumae volcano can be divided into five stages as shown in Table 1. The volcano is composed only of pyroclastic materials except a lava dome formed in 1909. The plain immediately to the east of the volcano is covered by thick air-fall deposits consisting of Ta-a, Ta-b, Ta-c and Ta-d members in descending order. Each member consists of multiple fall units representing one cycle of eruption without dormant time. In the Ta-a member 8 fall units are discriminated and in the Ta-b, 10. The isopach of Ta-a and Ta-b members are shown in Figures 3 and 4, respectively. Petrographycally, the air-fall deposits are augite-hypersthene andesitic. Pumice flows correlative to Ta-a and Ta-b discharged from the volcano deposited in all directions except west and formed relatively gentle slope within the range of 7 km from the vent. Detailed stratigraphic study revealed that pumice flows took place twice during fall activities in each Ta-a and Ta-b cycle. The fact that the pumice flow deposit overlies rather thick fall unit suggests that the flow results from relatively large eruption. Pumice flow deposit in each time is not a single flow unit but multiple flow units. Individual flow unit is relatively thin and usually not welded. The volume of pumice fall deposite (Ta-a 1.9 km^3, Ta-b 3.4 km^3) are larger than that of pumice flow deposits (1.8 km^3 in total). These figures suggest that the eruptive activities of the Tarumae volcano in Ta-a and Ta-b stages were very explosive and the vertical eruption column reached to a considerable elevation and that large proportion of ash and pumice in eruption column deposited as pumice falls and minor proportion as pumice flows. From the evidences as above mentioned the Tarumae pumice flow deposits are considered to be formed by same mechanism of St. Vincent glowing avalanche proposed by HAY (1959) or by the base surge proposed by MOORE (1967). The Tarumae volcano was formed only by repeated activities of pumice fall and pumice flow without any lave except latest extrusion of central lava dome. This type of volcano may be found in small andesitic volcanoes so far thought to be usual "stratovlocano".
特定非営利活動法人日本火山学会の論文
1971-04-01

樽前火山の形成 : とくに Ta-a, Ta-b 期の活動について

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