The relationship between the unidentified ash fall layer just under Kuju D ash layer and 54 ka eruption at Kuju volcano in Japan

Abstract

The largest eruption at Kuju volcano occurred at 54 ka. The previous studies have already reported the distribution and properties of the ejecta at this eruption, i.e. Kuju-D Ash Fall (Kj-D) layer, Handa Pyroclastic Flow (Kj-Hd) layer, and Kj-P1 Pumice Fall (Kj-P1) layer in ascending order. Moreover, Kj-D which is lowest layer of 54 ka ejecta is divided into Lower (L) part and Upper (U) part. The undefined ash fall layer was identified just under Kj-D L, and was named Yukouji Ash Fall (Kj-Y) layer. Because Kj-Y is situated just under Kj-D L without sandwiching soil layer, Kj-Y is considered as a series of ejecta at 54 ka eruption. This study aims to make the relationship between Kj-Y and 54 ka eruption clear based on field survey and microscopic observation. Kj-Y is composed of silt to clay sized particles, which is a characteristic of phreatic eruption ejecta. On the other hand, Kj-Y contains colorless and clear volcanic glasses. Therefore, we consider that Kj-Y eruption was phreato-magmatic eruption. Because Kj-Y is situated just under Kj-D L without sandwiching soil layer, it is considered that there is no time gap between these layer’s deposition. As Kj-Y and Kj-D L have similar mineral assemblages, it is concluded that these tephra layers were derived from same magma. From these evidences, we conclude that Kj-Y eruption was precursory activity of 54 ka eruption. The isopach map of Kj-Y illustrates that the eruption center of Kj-Y was in the eastern region of current Kuju volcano. On the other hand, it has assumed that Kj-D erupted at the central and eastern region, and that the Kj-Hd and Kj-P1 erupted at the central region. Therefore, it is suggested that the eruption center of 54 ka eruption migrated from eastern region to central region.