Thickness and Grain-size Distribution of the Osumi Pumice Fall Deposit from the Aira Caldera

Abstract

The Osumi pumice fall deposit is the product of a large-scale Plinian eruption which started a series of gigantic pyroclastic eruptions culminating in the formation of the Aira caldera. The deposit consists of gray-white pumice fragments, isolated phenocrysts and a small amount of lithic fragments derived from the basement. Generally it is homogeneous with a moderate reverse grading, with finer layers near the bottom. Maps of isopachs and isopleths of average maximum diameters of pumice and lithic fragments and of median diameter all indicate that the vent was located at or near the present site of Sakurajima volcano, a post-caldera cone located on the southern rim of the caldera. The dispersal axis runs N120°E from the vent with a fairly large apex angle of the envelope. Extrapolation after WALKER’S method (1980, 1981) on log-thickness vs log-area plot gives an estimated volume of 98 km^3, second only to the Shikotsu-1 pumice fall among the known Plinian deposits.