HR: 0800h
AN: V21E-0674    [Abstracts]
TI: Degassing Processes Of Usu 1977 Plinian Eruption: Implications From Its Pumice Textures
AU: * Nakamura, K
EM: kazuki@cneas.tohoku.ac.jp
AF: Inst. Mineral. Petrol. Econ. Geol., Tohoku Univ., Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan
AU: Suzuki, Y
EM: suzukiyk@ganko.tohoku.ac.jp
AF: Inst. Mineral. Petrol. Econ. Geol., Tohoku Univ., Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan
AU: Taniguchi, H
EM: taniguti@cneas.tohoku.ac.jp
AF: CNEAS, Tohoku Univ., Kawauchi, Aoba-ku, Sendai, 980-8576 Japan
AB: We investigate the vesicle and microlite textures of pumice produced by Usu 1977 Plinian eruption, and characterize their variation in the eruptive sequence in order to understand its degassing processes. The Usu 1977 Plinian eruption was characterized by 4 Sub-Plinian eruptions. Niida et al. (1982) describes the eruption column behavior of Big I, which is the first eruption of 4, in detail. According to it, the Big I eruption started at 9:10, August 7, 1977, which was preceded by earthquake swarms about 32 hours before, and its eruption column grew up step by step, then became weak rapidly, after that the small eruption with co-ignimbrite ash occurred in about 2 hours. The eruption produced various types of pumice in color, vesicularity, bubble number density, and microlite texture. Based on the color, we classify them into 3 types: white pumice, bright-gray one, and dark-gray one. The whiter pumice is characterized by the higher vesicularity and bubble number density, and the lower crystallinity of microlite than grayer one. Recently, it has been experimentally and numerically revealed that the smaller the decompression rate, the smaller the bubble number density, and the crystallization of microlite of skeletal plagioclase should also be induced by the devolatilization of magma. So we conclude that the whiter pumice was produced by the magma which was decompressed more rapidly, and its volatile component degassed less efficiently than grayer one. The abundance of white pumice component in the Big I deposit gradually increases toward the climax of the eruption, and then rapidly decreases toward the end of it at both approximate and distal localities from the vent. By the way, it is empirically found that the eruption column height is proportional to the 1/4 power of the magma discharge rate. The magma discharge rate should be proportional to magma ascent rate if the conduit diameter is constant. So we suppose that the systematic variations of abundance of white pumice component in the Big I deposit account for the increase of the magma ascent rate.
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005