有珠火山1977年噴火におけるプリニアン噴出物
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In this paper we describe the Plinian products of the Usu 1977 eruption, and discuss the relation between the deposit and the eruption sequence. Special attention is focused on lithofacies, components of the deposit, and the characteristics of pumice in the Big I [Katsui et al. 1978: 385-408]. The deposits of the Big I consist of four fall units with sharp boundaries. According to an eyewitness account in [Niida et al. 1982: 97-118], the eruption column developed up step by step, and then rapidly decreased. The characters in the deposits are interpreted in connection the eyewitness account of the earlier gradual growth and the later rapid decrease. The Big I deposit has various types and colors of pumice in bubble-form. Some of the bubbles are in elongated-form with a fibrous texture, while others are spherical or ovoid-form. Colors of the pumice vary from white to gray. Although we can observe these different points, the bulk chemical compositions of all pumice are almost identical; meaning that the differences in the pumice were not caused by differences of bulk chemistry, and suggests that they were produced by the same magma. The porosity of the white pumice was found to be higher and the crystallinity in the groundmass is lower. This suggests that the white pumice was produced by magma that ascended more rapidly in the conduit. Based on the component analysis of the Big I deposit, it is found that the proportion of accessory rock, elongated pumice, and white pumice gradually increased toward the climax stage of the eruption. This may suggest that the increase of the eruption-rate induced increases of accessory rocks, and elongated and white pumices.論文
- 東北大学の論文
- 2004-03-19
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