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[SVC35-P11] Excavation of an overlooked active volcano I: Magmatic eruption history of Shiga volcano
Keywords:Shiga volcano, magmatic eruption, eruption history
This study aimed to determine the magmatic eruptive history of the previously overlooked active volcano Shiga (central Japan), and assess its long-term potential for magmatic eruptions. The magmatic eruption history of the volcano has been outlined below (Murakami and Ishizaki, 2020). Shiga volcano has two eruptive centers. In the early stages of activity, Asahiyama lava, Hachiyama lava, and Hachiyama pyroclastic deposits erupted from the eruption center where Mt. Hachiyama is currently located. The eruptive center then moved to the vicinity of present-day Mt. Shiga, and Shiga Lava 1, 2, and 3 erupted successively. After the magmatic eruptions, phreatic eruptions repeatedly occurred near the summit of Mt. Shiga, the most recent of which occurred approximately 500 years ago.
In this study, the ages of three lavas from Shiga volcano were estimated by geochronological analyses of the thermoluminescence method (TL) and comparison of paleomagnetic directions to the geomagnetic secular variation (PM): the Asahiyama lava was dated to 27.3 ± 1.7 kyr (TL) and the Shiga lava 2 and 3 were dated to 6380–6400 cal. yr BP (PM). The age obtained from the Asahiyama lava is considerably younger than the K-Ar age (0.24 ± 0.06 Ma), but coincides within error with the K-Ar age of the overlying Hachiyama lava (0.07 ± 0.05 Ma) found in Kaneko et al. (1991). The Asahiyama lava, the Hachiyama lava, and the Hachiyama pyroclastic deposits that erupted from Mt. Hachiyama have a high petrological affinity and were likely erupted sequentially in a single eruption approximately 27,000 years ago. The Shiga lava 1, 2, and 3 that erupted from Mt. Shiga were also highly likely to have erupted sequentially in a single eruption approximately 6,400 years ago, which can be determined based on their well-preserved surface microtopography and similar whole-rock chemical composition. The eruption volumes calculated using QGIS were 0.959 km3 for the Asahiyama lava, 0.129 km3 for the Hachiyama lava, 0.023 km3 for the Hachiyama pyroclastic deposits, 0.091 km3 for the Shiga lava 1, 0.090 km3 for the Shiga lava 2, and 0.044 km3 for the Shiga lava 3.
We developed a magma discharge step diagram based on the obtained eruption ages and eruptive volumes. The long-term magma eruption rate for Shiga volcano was estimated to be 0.05 km3/kyr (maximum) and 0.02 km3/kyr (minimum), which are almost equal to those of the adjacent Kusatsu–Shirane volcano (0.01 km3/kyr for the Shirane pyroclastic cone group and 0.05 km3/kyr for the Motoshirane pyroclastic cone group; Numata et al., 2023, JpGU Abstract). If the step diagram produced is of the time–prediction type, the onset of magmatic eruption from Shiga volcano occurred 2,000 years ago and the volcano still has the potential to re-erupt today. However, if the step diagram is of the volume–prediction type, it suggests that Shiga volcano has the potential at present to erupt approximately 0.07 km3 of magma.
Acknowledgments: This study was supported by the Integrated Program for Next Generation Volcano Research and Human Resource Development.
In this study, the ages of three lavas from Shiga volcano were estimated by geochronological analyses of the thermoluminescence method (TL) and comparison of paleomagnetic directions to the geomagnetic secular variation (PM): the Asahiyama lava was dated to 27.3 ± 1.7 kyr (TL) and the Shiga lava 2 and 3 were dated to 6380–6400 cal. yr BP (PM). The age obtained from the Asahiyama lava is considerably younger than the K-Ar age (0.24 ± 0.06 Ma), but coincides within error with the K-Ar age of the overlying Hachiyama lava (0.07 ± 0.05 Ma) found in Kaneko et al. (1991). The Asahiyama lava, the Hachiyama lava, and the Hachiyama pyroclastic deposits that erupted from Mt. Hachiyama have a high petrological affinity and were likely erupted sequentially in a single eruption approximately 27,000 years ago. The Shiga lava 1, 2, and 3 that erupted from Mt. Shiga were also highly likely to have erupted sequentially in a single eruption approximately 6,400 years ago, which can be determined based on their well-preserved surface microtopography and similar whole-rock chemical composition. The eruption volumes calculated using QGIS were 0.959 km3 for the Asahiyama lava, 0.129 km3 for the Hachiyama lava, 0.023 km3 for the Hachiyama pyroclastic deposits, 0.091 km3 for the Shiga lava 1, 0.090 km3 for the Shiga lava 2, and 0.044 km3 for the Shiga lava 3.
We developed a magma discharge step diagram based on the obtained eruption ages and eruptive volumes. The long-term magma eruption rate for Shiga volcano was estimated to be 0.05 km3/kyr (maximum) and 0.02 km3/kyr (minimum), which are almost equal to those of the adjacent Kusatsu–Shirane volcano (0.01 km3/kyr for the Shirane pyroclastic cone group and 0.05 km3/kyr for the Motoshirane pyroclastic cone group; Numata et al., 2023, JpGU Abstract). If the step diagram produced is of the time–prediction type, the onset of magmatic eruption from Shiga volcano occurred 2,000 years ago and the volcano still has the potential to re-erupt today. However, if the step diagram is of the volume–prediction type, it suggests that Shiga volcano has the potential at present to erupt approximately 0.07 km3 of magma.
Acknowledgments: This study was supported by the Integrated Program for Next Generation Volcano Research and Human Resource Development.