Skip to content

Reconstruction of the pre-caldera-forming phase during the 7.3 ka eruption at the Kikai Caldera (Akahoya eruption) based on geological analyses and plume modeling

Yusuke Haruta , Fukashi Maeno, Yujiro Suzuki

Abstract

In many catastrophic caldera-forming eruptions (CFEs), Plinian eruptions often precede the climactic phases characterized by caldera collapse and the formation of large-scale pyroclastic density currents (PDCs). It is considered that such a preceding eruption triggers caldera collapse due to the decompression of the magma reservoir. Therefore, estimating the eruption conditions during the pre-caldera-forming Plinian phases is crucial for understanding the mechanism of CFEs. In this study, we focus on the 7.3 ka CFE (Akahoya eruption) at the Kikai Caldera, Japan, and discuss the eruption sequence and condition of the pre-caldera-forming phase through geological survey and plume modeling. The pre-caldera-forming deposits of the Akahoya eruption are divided into three units. The lower unit comprises a small ash fall layer. The middle unit comprises a Plinian fall deposit and a PDC deposit. The upper unit includes multiple Plinian fall deposits, PDC deposits, and co-PDC ash layers. The bulk volumes and mass discharge rates for each event are estimated as follows: Lower unit: 10-3−10-2 km3, Middle unit: 10-1−100 km3, 107−108 kg/s, and Upper unit: 100−101 km3, 108−109 kg/s. These results indicate that the pre-caldera-forming phase of the Akahoya eruption began as a small-moderate explosive eruption, transitioned into a Plinian eruption and small-scale PDCs, and culminated into a large-scale Plinian eruption with sustained partial column collapses and intraplinian PDCs. This presentation will also discuss the mechanism of the subsequent caldera collapse from the estimated parameters of the pre-caldera-forming phase.