11:00 AM - 1:00 PM
[SVC32-P13] Plinian phase of the 7.3ka Kikai Caldera eruption
Keywords:Kikai Caldera, Plinian eruption, Caldera formation
The Kikai Caldera is located off the southern coast of Kyushu, and now mostly submerged. The 7.3ka eruption of this caldera is the largest and most notable caldera-forming eruption in Japan during the Holocene, and some interesting phenomena were known, such as the Koya ignimbrite as a low aspect ratio ignimbrite or the generation of tsunami. During this eruption, the plinian phase (the Koya pumice fallout) and the intra-plinian flow phase (the Funakura pyroclastic flow) preceded the climax phase, caldera collapse and the deposition of the Koya ignimbrite. There is few research about these preceding phases, while it is very important to unveil characteristics and eruption parameters of the preceding phases quantitatively in order to understand the mechanism and process of this eruption.
In this study, we focused on the plinian phase, and attempted to re-evaluate eruption parameters such as eruption volume and understand the process of the eruption precisely, based on field surveys of the pyroclastic fall deposits. As a result, the plinian fall deposits are roughly divided into two units, and there is a lithic-rich layer between these units, which are supposed to mean vent erosion. With a 1-dimensional steady plume model, the magma discharge rate during the phase is estimated to be from 7.1×107-1.7×108kg/s (initial stage) to 4.7-8.2×108kg/s (late stage). Moreover, we re-estimated the total tephra volume to be 5-11km3, which is quite smaller than previous estimations. Using this value, we evaluated the possibility of caldera formation with a theoretical model and found that the amount of magma released by the plinian phase alone is not sufficient to cause coherent caldera formation. Considering the fact that the onset of caldera formation preceded the eruption of the Koya ignimbrite and its co-ignimbrite ash, the Akahoya ash, which accounted for most of the eruption volume of the 7.3 ka eruption, it is possible that magma discharge by the intra-plinian phase had a significant impact on caldera formation. The possibility of incoherent caldera formation is also considered.
In this study, we focused on the plinian phase, and attempted to re-evaluate eruption parameters such as eruption volume and understand the process of the eruption precisely, based on field surveys of the pyroclastic fall deposits. As a result, the plinian fall deposits are roughly divided into two units, and there is a lithic-rich layer between these units, which are supposed to mean vent erosion. With a 1-dimensional steady plume model, the magma discharge rate during the phase is estimated to be from 7.1×107-1.7×108kg/s (initial stage) to 4.7-8.2×108kg/s (late stage). Moreover, we re-estimated the total tephra volume to be 5-11km3, which is quite smaller than previous estimations. Using this value, we evaluated the possibility of caldera formation with a theoretical model and found that the amount of magma released by the plinian phase alone is not sufficient to cause coherent caldera formation. Considering the fact that the onset of caldera formation preceded the eruption of the Koya ignimbrite and its co-ignimbrite ash, the Akahoya ash, which accounted for most of the eruption volume of the 7.3 ka eruption, it is possible that magma discharge by the intra-plinian phase had a significant impact on caldera formation. The possibility of incoherent caldera formation is also considered.