男体火山活動末期における斜め噴火と異種のマグマの連続噴出

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Nantai volcano, about 120 km north of Tokyo, manifested the catastrophic eruption of scoria and pumice in prehistoric time in the last stage of activity. The scoria eruption is divided into the following 4 phases; I. Vertical eruption of scoria after a long period of dormancy. II. Collapse of northern wall of the summit crater, which generated the avalanche. III-sorted rock fragments were deposited on the northern foot of the mountain. III. phase of scoria flow eruption. The collapse of the crater wall caused an obliquely directed ejection of scoria, which generated a scoria flow. The flow traveled north-north-eastward traversing the eastern slope of Mt. Omanago (Fig. 3). The thickness of the deposit is uniformly small (about 5m) irrespective of the topographic relief of the basement (Fig. 4). The deposit is ill-sorted and in spite of the small thickness, it is strongly welded even at the flow front (Fig. 9). The eruption was followed by an outpour of scoria flow, the deposit of which was also strongly welded and veneered the slope of the northern half of Nantai. IV. Oblique eruption of scoria similar to that of Vesuvius in 1779 and 1906. III-sorted and poorly vesiculated scoria was projected northward and deposited on the north foot of neiboring Omanago volcano, on the opposite side of Nantai. The eruption of pumice fall took place immediately after the scoria eruption (Table 1). The bubbling of magma recovered violence and then subsided gradually again causing successive outpour of pumice flow and lava flow. The petrographic feature and chemical variation trend of the magma of this stage differ from that of the magma of the preceding scoria eruption (Figs. 7, 8 and Tables 2, 3), and it appears that the two magmas had filled not a single zoned magma chamber but two separate chambers before the eruption started.
特定非営利活動法人日本火山学会の論文
1980-07-01

男体火山活動末期における斜め噴火と異種のマグマの連続噴出

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