A Pyroclastic Flow Deposit Occurring at the Northeastern Foot of Nakadake, Aso Volcano (Japan) and its Stratigraphic Significance

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  • 阿蘇火山中岳北東麓に分布する火砕流堆積物とその層序的意義
  • アソカザン ナカダケ ホクトウロク ニ ブンプ スル カサイリュウ タイセキブツ ト ソノ ソウ ジョテキ イギ

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A basaltic pyroclastic flow deposit, the Izumikawa pyroclastic flow deposit, occurs at the northeastern foot of Nakadake Volcano, which is the only active central cone of Aso caldera, southwestern Japan. The pyroclastic flow deposit covers a fan-shaped area of about 0.9-1.9km2, and the bulk volume is estimated at 4.4-9.4×106m3. The deposit is poorly sorted, and consists of subangular faceted clasts and spherical cauliflower bombs set in a sandy non-cohesive matrix. The deposit forms two different facies: a black reversely graded lower unit and a reddish-gray reversely graded upper unit. The cauliflower bombs, which have slightly vesiculated crusts and denser interiors, are more abundant in the lower unit than in the upper unit. The presence of the cauliflower bombs suggests that the pyroclastic flow was generated by an explosion at the source lava lake or conduit, which was filled with mixture of solidified and molten lavas. The age of the deposit was estimated at ca. 19 cal ka, based on 14C ages obtained from charred wood fragments in the deposit. Recent tephrochronological studies reveal that Nakadake Volcano became active from ca. 22-21 cal ka and that violent scoria and ash eruptions of Nakadake were concentrated in two periods of ca. 22-21 cal ka and 18-16 cal ka. The age of the Izumikawa pyroclastic flow corresponds an intermediate period between the two violent eruption periods. A similar pyroclastic flow deposit and a basaltic lava flow were also identified. They cover immediately the Izumikawa pyroclastic flow deposit. These facts indicate that multiple violent eruptions producing pyroclastic flows and lava flows occurred in a short period at ca. 19 cal ka. Recent activity of Nakadake has been characterized by ash eruptions, strombolian eruptions and phreatomagmatic explosions. However, the presence of the Izumikawa pyroclastic flow deposit emphasizes the potential hazard induced by bomb-rich pyroclastic flows that may rush down the flanks of Nakadake Volcano.

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