10:45 AM - 11:00 AM
[SVC31-01] 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
Keywords:Volcanic eruption, Kikai Caldera, Caldera-forming eruption, Akahoya eruption, Plume modeling
In many catastrophic caldera-forming eruptions, Plinian eruptions occur in the initial stage of the eruption and are thought to initiate subsequent caldera collapse and the formation of large-scale pyroclastic density currents. Therefore, a comprehensive understanding of the eruption conditions during the pre-caldera-forming phases (precursory phases) is crucial for elucidating the mechanism of caldera-forming eruptions. This study aims to reconstruct the eruption sequences and quantitatively estimate eruption parameters of the precursory phases of catastrophic caldera-forming eruptions using a combined approach of geological methods and plume modeling. We specifically focus on the 7.3 ka caldera-forming eruption (Akahoya eruption) at the Kikai Caldera, off the southern coast of Kyushu. This eruption generated typical deposits indicative of caldera-forming eruptions: precursory Plinian deposits, large-scale pyroclastic density current deposits, and co-ignimbrite fall deposits. Because of the well-preserved nature of the precursory Plinian deposits, it serves as an ideal case study for our analyses.
Geological surveys were conducted across the Satsunan Islands, Satsuma Peninsula, and Osumi Peninsula to reveal the stratigraphy of the precursory phase deposits. Isopach maps and maximum clast size isopleth maps were generated for each subunit of the pyroclastic fall deposits (Unit A). Eruptive volumes were estimated by fitting the thickness distribution data using several empirical functions. To evaluate the effects of the thickness data in the proximal and distal areas on the fitting, which have considerable uncertainty, probability distributions of the eruption volumes were determined through Monte Carlo simulations. Additionally, we estimated mass discharge rates that could explain the maximum clast distributions based on a numerical model. This model is composed of a steady one-dimensional plume model accounting for bending by wind, and trajectory calculations considering clast shapes.
Geological analyses revealed that the precursory phase deposits of the Akahoya eruption are divided into three units by minor erosional features. The lower unit comprises an ash fall layer distributed only in the proximal area. The middle unit comprises a pumice fall layer distributed across the southern part of the Osumi Peninsula, overlaid by a thin pyroclastic density current deposit. The upper unit exhibits distinct facies between the proximal and distal areas. In the proximal area, several pumice fall layers and an overlying pyroclastic density current deposit were identified. Conversely, in the southern Osumi Peninsula, up to 11 pumice fall layers and ash-concentrated layers were observed. Among these tephra layers, the uppermost pumice fall layer displays reverse grading and has a more widespread distribution than the other layers. The eruptive volumes and mass discharge rates for each unit are estimated as follows; Lower unit: 0.0021−0.012 km3, Middle unit: 0.18−1.4 km3, 3.5×107−1.2×108 kg/s, and Upper unit: 4.1−17 km3, 5.4×108−1.8×109 kg/s. The total volume of the pyroclastic fall deposits of the precursory phase is estimated to be 4.6−17 km3, which is significantly smaller than the volumes reported in previous studies.
Based on these findings, the pre-caldera-forming phase of the Akahoya eruption is elucidated through three distinct eruptive events interrupted by minor pauses. Event 1 constituted a small-scale, explosive magmatic eruption. Event 2 was a Plinian eruption featuring a stable convective plume and transitioned into the eruption of small-scale pyroclastic density currents. Event 3 represented another Plinian eruption marked by sustained partial column collapses, characterized by an increase in eruption intensity over time. Thus, during the precursory phase, there was a significant increase in eruption scale and intensity as the eruption progressed. Furthermore, the erupted magma volume during the precursory phase of the Akahoya eruption was notably smaller compared to other caldera-forming eruptions with similar-sized calderas. The mechanism underlying this characteristic will be discussed.
Geological surveys were conducted across the Satsunan Islands, Satsuma Peninsula, and Osumi Peninsula to reveal the stratigraphy of the precursory phase deposits. Isopach maps and maximum clast size isopleth maps were generated for each subunit of the pyroclastic fall deposits (Unit A). Eruptive volumes were estimated by fitting the thickness distribution data using several empirical functions. To evaluate the effects of the thickness data in the proximal and distal areas on the fitting, which have considerable uncertainty, probability distributions of the eruption volumes were determined through Monte Carlo simulations. Additionally, we estimated mass discharge rates that could explain the maximum clast distributions based on a numerical model. This model is composed of a steady one-dimensional plume model accounting for bending by wind, and trajectory calculations considering clast shapes.
Geological analyses revealed that the precursory phase deposits of the Akahoya eruption are divided into three units by minor erosional features. The lower unit comprises an ash fall layer distributed only in the proximal area. The middle unit comprises a pumice fall layer distributed across the southern part of the Osumi Peninsula, overlaid by a thin pyroclastic density current deposit. The upper unit exhibits distinct facies between the proximal and distal areas. In the proximal area, several pumice fall layers and an overlying pyroclastic density current deposit were identified. Conversely, in the southern Osumi Peninsula, up to 11 pumice fall layers and ash-concentrated layers were observed. Among these tephra layers, the uppermost pumice fall layer displays reverse grading and has a more widespread distribution than the other layers. The eruptive volumes and mass discharge rates for each unit are estimated as follows; Lower unit: 0.0021−0.012 km3, Middle unit: 0.18−1.4 km3, 3.5×107−1.2×108 kg/s, and Upper unit: 4.1−17 km3, 5.4×108−1.8×109 kg/s. The total volume of the pyroclastic fall deposits of the precursory phase is estimated to be 4.6−17 km3, which is significantly smaller than the volumes reported in previous studies.
Based on these findings, the pre-caldera-forming phase of the Akahoya eruption is elucidated through three distinct eruptive events interrupted by minor pauses. Event 1 constituted a small-scale, explosive magmatic eruption. Event 2 was a Plinian eruption featuring a stable convective plume and transitioned into the eruption of small-scale pyroclastic density currents. Event 3 represented another Plinian eruption marked by sustained partial column collapses, characterized by an increase in eruption intensity over time. Thus, during the precursory phase, there was a significant increase in eruption scale and intensity as the eruption progressed. Furthermore, the erupted magma volume during the precursory phase of the Akahoya eruption was notably smaller compared to other caldera-forming eruptions with similar-sized calderas. The mechanism underlying this characteristic will be discussed.