1:45 PM - 3:15 PM
[SCG57-P01] A time series of the 7.3 ka caldera-forming eruption at Kikai caldera, southern Kyushu, Japan
Keywords:caldera, magmatic system, Kikai, eruption volume, climate change
The caldera-forming eruption that occurred at 7.3 ka at the Kikai caldera in southern Kyushu (the Kikai-Akahoya eruption) was the largest eruption on Earth in the Holocene period, and some records in polar ice cores suggest that the eruption increased sulfur concentrations in the atmosphere. Reconstructing the entire picture of this eruption, including surface phenomena and magmatic systems, is important for elucidating the mechanism of large-scale silicic pyroclastic eruptions that repeatedly occur on Earth and the widespread impact of these eruptions. The impact here includes modification of the surface environment by volcanic ash deposition and climate change caused by volatiles released into the atmosphere. In this presentation, we will discuss recent results and challenges in reconstructing the time series of the Kikai-Akahoya eruption, including precursor activities.
The general framework of the Kikai-Akahoya eruption consists of Plinian eruptions (Stage 1, equivalent to VEI 6) followed by the eruption of large pyroclastic flows and caldera formation (Stage 2, equivalent to VEI 7), which are basically common with the same characteristics as other large-scale silicic pyroclastic eruptions. On the other hand, drilling has revealed the existence of a volcanic activity that erupted rhyolitic magma (>0.5 km3) just before 7.3 ka, increasing the importance of elucidating the evolution of the magmatic system including precursor activities, in eruption reconstruction.
A major challenge in understanding surface phenomena and their impact is constraining eruption volumes at every eruptive stage and the mass eruption rate and duration of the eruptions that explain the eruption volumes. Recent detailed studies of stage 1 Plinian eruptions have provided new insights into their evolution and eruptive volume. In particular, the eruption volume of the preceding Plinian eruptions is of great importance because it is one of the keys to reconstructing surface phenomena at the climactic phase of the eruption, as the physical model suggests that the eruption volume is closely related to the criteria of caldera formation. In addition, field investigations of proximal deposits have revealed several pieces of geological evidence, such as erosional surfaces, that suggest the existence of a time gap of several days or weeks or longer between Stages 1 and 2, and that the eruption progressed discontinuously from Plinian eruptions to the eruption of large-scale pyroclastic flows. For Stage 2, which is the climactic phase and produced tephra of more than 100 km3, the physical parameters of the large-scale plume associated with the pyroclastic flows (the Koya pyroclastic flow) that reached the mainland of Kyushu, and the style and temporal evolution of the caldera formation have not been fully constrained. In addition to the eruption style and evolution of Stage 2, which accounts for most of the eruptive volume of the Kikai-Akahoya eruption, the chemical properties of magma, especially the concentration of volatile components such as sulfur, are essential for estimating the total amount of volatile components released into the atmosphere and, ultimately, for assessing the extent to which the eruption caused climate change. Although reconstructing the time series of the Kikai-Akahoya eruption has progressed, issues remain to be solved in order to reconstruct the entire picture of this large-scale eruption
The general framework of the Kikai-Akahoya eruption consists of Plinian eruptions (Stage 1, equivalent to VEI 6) followed by the eruption of large pyroclastic flows and caldera formation (Stage 2, equivalent to VEI 7), which are basically common with the same characteristics as other large-scale silicic pyroclastic eruptions. On the other hand, drilling has revealed the existence of a volcanic activity that erupted rhyolitic magma (>0.5 km3) just before 7.3 ka, increasing the importance of elucidating the evolution of the magmatic system including precursor activities, in eruption reconstruction.
A major challenge in understanding surface phenomena and their impact is constraining eruption volumes at every eruptive stage and the mass eruption rate and duration of the eruptions that explain the eruption volumes. Recent detailed studies of stage 1 Plinian eruptions have provided new insights into their evolution and eruptive volume. In particular, the eruption volume of the preceding Plinian eruptions is of great importance because it is one of the keys to reconstructing surface phenomena at the climactic phase of the eruption, as the physical model suggests that the eruption volume is closely related to the criteria of caldera formation. In addition, field investigations of proximal deposits have revealed several pieces of geological evidence, such as erosional surfaces, that suggest the existence of a time gap of several days or weeks or longer between Stages 1 and 2, and that the eruption progressed discontinuously from Plinian eruptions to the eruption of large-scale pyroclastic flows. For Stage 2, which is the climactic phase and produced tephra of more than 100 km3, the physical parameters of the large-scale plume associated with the pyroclastic flows (the Koya pyroclastic flow) that reached the mainland of Kyushu, and the style and temporal evolution of the caldera formation have not been fully constrained. In addition to the eruption style and evolution of Stage 2, which accounts for most of the eruptive volume of the Kikai-Akahoya eruption, the chemical properties of magma, especially the concentration of volatile components such as sulfur, are essential for estimating the total amount of volatile components released into the atmosphere and, ultimately, for assessing the extent to which the eruption caused climate change. Although reconstructing the time series of the Kikai-Akahoya eruption has progressed, issues remain to be solved in order to reconstruct the entire picture of this large-scale eruption