[1poster18-20] R3[レギュラー］噴火・火山発達史と噴出物

[R3-P-2] 北海道東部，摩周火山の7.6 ka噴火：爆発的カルデラ形成過程とLow aspect ratio ignimbriteの発生

*長谷川 健¹、柴田 翔平¹、小林 哲夫²、望月 伸竜³、中川 光弘⁴、岸本 博志⁴ (1. 茨城大学大学院理工学研究科、2. 鹿児島大学地震火山地域防災センター、3. 熊本大学大学院先端科学研究部、4. 北海道大学理学研究院)

キーワード：摩周火山、カルデラ、拡散型大規模火砕流、古地磁気分析、化学組成

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.