5:15 PM - 6:45 PM
[SVC30-P09] Investigating variations in eruption patterns of Kusatsu-Shirane Volcano over the Holocene period through tephra analysis
Keywords:tephra stratigraphy, phreatic eruption, Kusatsu-Shirane Volcano
In recent years, small phreatic eruptions such as the 2014 eruption of Mt. Ontake and the 2018 eruption of Mt. Kusatsu-Shirane have caused human suffering. To mitigate such eruption disasters, it is important to conduct precise observations and clarify the details of the eruption history (sites, frequency, and magnitude of eruption), including small-scale eruptions. In addition, the formation, preservation, and development of hydrothermal systems, the source of phreatic eruptions, may be affected by magmatic activity (such as supply of high-temperature volatiles, disturbance of a stable geothermal environment by shallow penetration and movement of magma), so it is necessary to study the temporal and spatial relationships between phreatic and magmatic eruptions.
Kusatsu-Shirane Volcano, the target of this study, is an active volcano that has repeatedly recorded phreatic eruptions since prehistoric times. Recently, Ishizaki et al. (2021) and Kametani et al. (2020, 2021) clarified the stratigraphy of eruption products of the Shirane (SPCG) and Motoshirane (MPCG) pyroclastic cone groups, the main sites of Holocene eruptions. Numata et al. (2023) evaluated magmatic eruption potentials based on a cumulative volumetric step diagram. However, the history of small-scale eruptions and the eruption pattern of each pyroclastic cone remain unknown.
This study aimed to investigate the eruption history, including small-scale eruptions, through geological survey, radiocarbon dating of soils, and tephra analysis. Specifically, we aimed to understand the frequency, magnitude, and patterns of eruptions in the SPCG and MPCG of Kusatsu-Shirane Volcano. The SPCG has experienced relatively large magmatic and phreatic eruptions from 7,600 to 3,800 years ago, and phreatic eruptions have been repeated more than 10 times since then. In the MPCG, pyroclastic cones were formed in sequence from southwest to northeast by intermittent magmatic eruptions from about 11,000 to 1,400 years ago, and phreatic eruptions have occurred several times since about 4,800 years ago. Surveys of tephra layers indicate that the frequency and magnitude of phreatic eruptions are estimated to be greater from the SPCG than from the MPCG.
The particle composition ratios in the phreatic tephras and the ratio of X-ray peak intensities of quartz and cristobalite (Qtz/Crs ratio) obtained from X-ray diffraction analysis reveal distinct characteristics. Tephras from the MPCG primarily consist of mosaic quartz fragments with few altered cryptocrystalline fragments, high Qtz/Crs ratios, and many unaltered fragments derived from the volcanic edifice, suggesting that the hydrothermal system under the MPCG has not yet developed or is in the process of development. Conversely, tephras from the SPCG are mainly composed of altered cryptocrystalline fragments with low Qtz/Crs ratios and few unaltered fragments, indicating the existence of a mature hydrothermal system. These differences in hydrothermal system maturity may influence the frequency and magnitude of phreatic eruptions.
Acknowledgments: This study was supported by JSPS KAKENHI Grant Number 23K13536, the Earthquake and Volcano Hazards Observation and Research Program, and the Research Grant to Support Young Researchers (13th period), Paleo Labo Co., Ltd.
Kusatsu-Shirane Volcano, the target of this study, is an active volcano that has repeatedly recorded phreatic eruptions since prehistoric times. Recently, Ishizaki et al. (2021) and Kametani et al. (2020, 2021) clarified the stratigraphy of eruption products of the Shirane (SPCG) and Motoshirane (MPCG) pyroclastic cone groups, the main sites of Holocene eruptions. Numata et al. (2023) evaluated magmatic eruption potentials based on a cumulative volumetric step diagram. However, the history of small-scale eruptions and the eruption pattern of each pyroclastic cone remain unknown.
This study aimed to investigate the eruption history, including small-scale eruptions, through geological survey, radiocarbon dating of soils, and tephra analysis. Specifically, we aimed to understand the frequency, magnitude, and patterns of eruptions in the SPCG and MPCG of Kusatsu-Shirane Volcano. The SPCG has experienced relatively large magmatic and phreatic eruptions from 7,600 to 3,800 years ago, and phreatic eruptions have been repeated more than 10 times since then. In the MPCG, pyroclastic cones were formed in sequence from southwest to northeast by intermittent magmatic eruptions from about 11,000 to 1,400 years ago, and phreatic eruptions have occurred several times since about 4,800 years ago. Surveys of tephra layers indicate that the frequency and magnitude of phreatic eruptions are estimated to be greater from the SPCG than from the MPCG.
The particle composition ratios in the phreatic tephras and the ratio of X-ray peak intensities of quartz and cristobalite (Qtz/Crs ratio) obtained from X-ray diffraction analysis reveal distinct characteristics. Tephras from the MPCG primarily consist of mosaic quartz fragments with few altered cryptocrystalline fragments, high Qtz/Crs ratios, and many unaltered fragments derived from the volcanic edifice, suggesting that the hydrothermal system under the MPCG has not yet developed or is in the process of development. Conversely, tephras from the SPCG are mainly composed of altered cryptocrystalline fragments with low Qtz/Crs ratios and few unaltered fragments, indicating the existence of a mature hydrothermal system. These differences in hydrothermal system maturity may influence the frequency and magnitude of phreatic eruptions.
Acknowledgments: This study was supported by JSPS KAKENHI Grant Number 23K13536, the Earthquake and Volcano Hazards Observation and Research Program, and the Research Grant to Support Young Researchers (13th period), Paleo Labo Co., Ltd.