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Magma ascent processes during the 1977 eruption of Usu volcano inferred from petrological and experimental studies

^^Shuhei Hotta^1^, Satoshi Okumura1, Kazuhisa Matsumoto1, Tsuyoshi Miyamoto2, Akihiko Tomiya3 and Kiyoaki Niida4

  • Affiliations: 1 Division of Earth and Planetary Materials Science, Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. 2 Center for Northeast Asian Studies, Tohoku University, Sendai 980-8578, Japan. 3 Geological Survey of Japan, National Institute of Advanced Science and Technology, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan. 4 Geo-LAB Mt. Apoi Samani, Hokkaido, Japan. 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 202

  • Programme No: 3.2.25

  • Theme 3 > Session 2

Abstract

Magma discharge rates during repeated explosive eruptions often show variation. The discharge rate is expected to be controlled by magma ascent processes and the condition of the magma reservoir; however, their evolution during repeated explosive eruptions is poorly understood. In this study, we petrologically investigated the magma ascent processes during the 1977 eruption of Usu volcano, which included four sub-plinian eruptions and showed variation in the discharge rate. The water contents of the melt inclusions in the first sub-Plinian eruption indicate a pressure of 100--125 MPa with the assumption of water saturation. The minimum water content gradually decreased from the second to the fourth sub-Plinian eruptions. This means that the magma stagnated at shallow parts of the conduit before the eruption and the depth became shallower. The crystal size distributions of plagioclase microlite show large variations in small size (<1 µm); these small crystals are inferred to crystallize at low pressure (<40 MPa), confirmed by decompression experiments. Based on these observations, we infer the following scenario: the magma in the first sub-Plinian eruption ascended from the magma reservoir at a depth of ~4km; the magma stagnated at a shallow conduit after the first sub-Plinian eruption and then the stagnated magma ascended to the surface during the second to the fourth sub-Plinian eruptions with the gradual ascent of the magma head in the conduit. The quantitative relationship between the discharge rate and magma plumbing system will be investigated in future studies.