Bomb-rich basaltic pyroclastic flow deposit from Nakadake, Aso Volcano, southwestern Japan

摘要

A bomb-rich 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 1.9 km~2, and the bulk volume is estimated at 9.4 x 10~6 m~3. 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 (1.0-1.5 cm thick) and denser interiors, are more abundant in the lower unit than in the upper unit. The presence of cauliflower bombs suggests that the Izumikawa pyroclastic flow was generated by an explosion at the source lava lake or conduit, which was filled with mixture of solidified and molten lavas. We propose the term "bomb-and-ash flow deposit" to describe this type of deposit. The age of the Izumikawa pyroclastic flow deposit was estimated at ca. 19 cal ka, based on a ~(14)C age obtained from charred wood fragments in the deposit. Although all recent activity of Nakadake has been ash eruptions, strombolian eruptions and phreatomagmatic explosions, the Izumikawa pyroclastic flow deposit underscores the hazard of bomb-rich pyroclastic flows that rush down the flanks of the volcano.