樽前降下軽石堆積物 Ta-b 層の粒度組成

抄録

Four major pumice-fall deposits, Ta-a, b, c, and d in descending order, are distributed widely on the eastern side of Mt. Tarumai which is an active volcano situated on the southern rim of the Shikotsu caldera in southwestern Hokkaido. The Ta-b deposit which was produced at the 1667 eruption, is divided into 10 fall units, the biggest of which is Ta-b₈ (Soya, 1971), (Figs.1, 2, and 3). Systematic observation, sampling, and mechanical analysis were carried out on the Ta-b deposit as shown in Figs. 1-10, and the following results were obtained. 1) A method of cross section area is employed here for the volume estimation of deposit (Figs.11-14, and equations [2], [3] and [4]). This method is useful for the volume estimation because extrapolation to the distance far from the source using the near source data is valid. The volume calculated by this method is ca.2km³ for Ta-b total and ca.1km³ for Ta-b₈ fall unit. 2) The total mass of Ta-b₈ fall unit is calculated as 0.93×10⁹ ton from observed apparent density and cross section area on each arc. 3) Median diameter (Md) and sorting coefficient (So) of the Ta-b₈ fall unit decrease as distance from the source increases (Figs.6, 7, and 10 ; Table 1). A remarkable difference in size composition between both sides of the distribution axis suggests either the structure of eruption column was not homogeneous during the eruption, or the wind-direction was different at higher and lower levels (Fig.9). 4) The average of maximum 10 grain size of pumice which is measured in the field, is nearly comparable to the size of 1% of cumulative curves (Fig.8). 5) Isograin curves of the average of maximum grain size reveal the source point of an air-fall deposit. 6) The size composition of total deposit of Ta-b₈ fall unit was obtained, which conforms to Rosin’s law (Figs.15 and 16).