[1poster18-20] R3 [Regular Session]Eruption, volcanic evolution and volcanic products

[R3-P-2] The 7.6 ka eruption of Mashu volcano, eastern Hokkaido, Japan: Explosive caldera-forming processes and generation of low aspect ratio ignimbrite

*TAKESHI HASEGAWA¹, Shohei Shibata¹, Tetsuo Kobayashi², Nobutatsu Mochizuki³, Mitsuhiro Nakagawa⁴, Hiroshi Kishimoto⁴ (1. Graduate School of Science and Engineering, Ibaraki University, 2. Research and Education Center for Natural Hazards, Kagoshima University, 3. Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University, 4. Faculty of Science, Hokkaido University)

Keywords:Mashu volcano, caldera, low aspect ratio ignimbrite, paleomagnetical investigations, chemical compositions

We present a stratigraphy of the 7.6-ka eruption at Mashu volcano and the formation process of its summit caldera based on geological, petrological and paleomagnetic investigations. The eruption products consist of an initial phreatomagmatic unit (Ma-j) and the overlying three pumice-fall layers (Ma-i, -h, and -g), which are in turn overlain by pyroclastic-flow deposits (Ma-f) as previously described (Katsui et al., 1975). We divide Ma-f into 4 subunits: Ma-f1/2, Ma-fAc, Ma-f3a and Ma-f3b in descending order. Ma-f3b is a valley-ponding, pumice-flow deposit with limited distribution. Ma-f3a comprises fines-depleted facies (FDI) and normal matrix-supported facies (NI), the two changing across topography. The FDI is characterized by a gray, clast-supported and lithic-breccia-rich layer with materials incorporated from the substrate. Impact sag structures from large (> 50 cm) dacite ballistic blocks were recognized at the base of the Ma-f3a within 10 km from source. Ma-fAc is a minor eruption unit consisting of accretionary lapilli. Ma-f1/2 is a most voluminous (8.8 km³), widely distributed and weakly stratified ignimbrite. Both Ma-f3a and Ma-f1/2 can be classified as low aspect ratio ignimbrite. Dacite lithic fragments are ubiquitously observed throughout the sequence and are not considered to be juvenile; they have distinctly different chemical compositions from the pumice fragments in the early pumice-fall (Ma-g~Ma-i) and pyroclastic-flow (Ma-f3b) deposits. The caldera-forming eruption of the Mashu volcano was initiated by Plinian fall (Ma-j ~ -g), and then, a small-volume ignimbrite (Ma-f3b) was deposited by a valley-confined pyroclastic flow from partial column collapse. After that, a violent pyroclastic flow was generated during a strong explosion of a dacite lava edifice on the summit of Mashu volcano. Ma-f3a flow was extremely fast. Ma-f1/2 flow was related to sustained flow due to low settling velocity and high discharge volume. These are supported by field observations and numerical simulation that shows the ability of the flow to surmount high topographic obstacles and spread widely. The caldera-forming process of Mashu volcano was driven not only by subsidence of roof block but also by violent explosions.
Katsui, Y., Ando, S. and Inaba, K. (1975) J. Fac. Sci., Hokkaido Univ., 16, 533-552.