Post-caldera explosive activity inferred from improved 67–30 ka tephrostratigraphy at Aso Volcano, Japan

Abstract

An improved 67–30 ka tephrostratigraphy for Aso Volcano in southwestern Japan can be used to reconstruct the eruptive history during most of the post-caldera activity of the volcano. The thick tephra sequence preserved atop the Aso pyroclastic-flow deposits around the caldera mainly comprises mafic scoria-fall and ash-fall deposits, because basaltic to basaltic andesitic magmas have been dominant at Aso Volcano. Previous studies have identified 36 silicic pumice-fall deposits that act as valuable beds for correlation of the complicated tephra sequence. In addition to the twelve pumice-fall deposits previously identified, the present study has newly recognized 17 tephra units, denoted Nakakubo tephra 17 to 1 (NbT17–NbT1) in ascending order, in the stratigraphic sequence between 67 and 30 ka. Overall, explosive mafic eruptions were more common than explosive silicic eruptions. Explosive eruptions occurred frequently in the period 67–50 ka, but happened only infrequently in 50–30 ka. Between 50 and 31 ka, silicic and mafic tephra volumes were both relatively small, but multiple explosive silicic eruptions occurred at 31–30 ka, and the 30 ka catastrophic Kusasenrigahama eruption, which was the largest eruption in the post-caldera stage of Aso Volcano, discharged voluminous magma (1.1 km 3) as a pumice-fall deposit. Individual tephra discharge volumes were relatively small (< 0.57 km 3) during the periods when explosive eruptions were frequent, whereas large amounts of magma were discharged after the long quiescent period of about 20,000 years in the case of both silicic and mafic eruptions. The reconstructed tephrostratigraphy and chemical characteristics of tephras during the period between 67 and 30 ka suggest a magma plumbing system model in which mafic magmas were steadily intruded from the mantle to the lower crust, and silicic magmas generating explosive pumice eruptions were produced by crustal anatexis due to heating of the crust by mantle-derived mafic magmas beneath Aso Volcano in the post-caldera stage.