3:00 PM - 3:15 PM
[SVC36-06] Report on pumiceous debris from the Izu-Bonin arc back-arc rift zone discovered in the waters around Izu-Torishima, Izu-Torishima and the Nansei Islands after October 2023
Keywords:drifting pumice, Izu-Bonin arc, Monitoring
In this presentation, we will show the results of identifying the source of pumice newly discovered since October 2023 in the waters near Izu-Torishima, on Izu-Torishima itself, and in the and the rocks found on Izu-Torishima were analyzed to determine their source. After four earthquakes of magnitude 6 or greater occurred around the Sofu Seamount, located 20 km west of Sofugan, on 2 – 8 October 2023, a tsunami occurred on 9 October without any significant earthquakes. It was presumed that the tsunami was related to the volcanic activity of the Sofu seamount (Mizutani and Melgar, 2023; Sandanbata et al., 2024). In fact, comparison of the seafloor topography before and after the earthquake revealed that a caldera-like topography 1.6 km wide and 400 m deep had formed on the top of Kuchinoerabu-jima, and it is estimated that this topography formed on 9 October (Minami and Tani, 2024). On 27 October 2023, the Keifu Maru of the Japan Meteorological Agency collected pumice floating on the sea surface about 50 km south-southwest of Kuchinoerabu-jima pumice floating on the sea surface was collected, and it was revealed that the pumice was produced by volcanic activity on the back arc rift zone of the Izu-Mariana Arc, based on its chemical composition. Similar pumice was collected from the surface of the sea near Hatsunezaki, west of Izu-Torishima (collected on 11 November) and from an albatross nest (collected on 25 November) by one of the present authors, Mr. Tomita. Meanwhile, similar pumice was also found washed up on the shores of the Nansei Islands by Mr. Maruya, one of the presenters. Similar pumice also seems to have been widely washed up on the Nansei Islands in Okinawa Prefecture, and it was also confirmed on Iriomote Island and Ishigaki Island in September 2024.
The major element and trace element compositions of these pumice were analyzed by XRF and ICP-MS at the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology. The results showed that the major element composition of the whole rock ranged from 62 to 74 wt.% SiO2 and 5.8 to 6.6 wt.% K2O+Na2O, indicating a chemical composition of dacite to rhyolite. On the other hand, the trace element compositions of the whole rocks were similar to those of pumice from the back-arc rift zone of the Izu-Bonin Arc, with Ba/La ratios of around 19 and La/Sm ratios of around 1.6–1.9. Therefore, it is thought that the pumice collected from these different locations all originated from the back-arc rift zone. Since pumice with the same chemical composition as that found in this study has not been identified in either location in recent years, it is highly likely that these pumice clasts were brought to the area by the tsunami and related landform change event that occurred in the area around the Sofu sea mount in October 2023.
The results of this study show that pumice was ejected from the back-arc rift zone of the Izu-Bonin Arc for some reason and drifted to the Nansei Islands. In this case, there were no obvious signs of volcanic activity on the surface of the sea, but it was confirmed that the pumice had drifted ashore in the Nansei Islands in Okinawa Prefecture. In other words, it can be said that monitoring new pumice on the shores of Okinawa Prefecture may allow us to detect submarine eruptions on the Izu-Bonin Arc that do not show obvious volcanic activity on the surface of the sea.
The major element and trace element compositions of these pumice were analyzed by XRF and ICP-MS at the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology. The results showed that the major element composition of the whole rock ranged from 62 to 74 wt.% SiO2 and 5.8 to 6.6 wt.% K2O+Na2O, indicating a chemical composition of dacite to rhyolite. On the other hand, the trace element compositions of the whole rocks were similar to those of pumice from the back-arc rift zone of the Izu-Bonin Arc, with Ba/La ratios of around 19 and La/Sm ratios of around 1.6–1.9. Therefore, it is thought that the pumice collected from these different locations all originated from the back-arc rift zone. Since pumice with the same chemical composition as that found in this study has not been identified in either location in recent years, it is highly likely that these pumice clasts were brought to the area by the tsunami and related landform change event that occurred in the area around the Sofu sea mount in October 2023.
The results of this study show that pumice was ejected from the back-arc rift zone of the Izu-Bonin Arc for some reason and drifted to the Nansei Islands. In this case, there were no obvious signs of volcanic activity on the surface of the sea, but it was confirmed that the pumice had drifted ashore in the Nansei Islands in Okinawa Prefecture. In other words, it can be said that monitoring new pumice on the shores of Okinawa Prefecture may allow us to detect submarine eruptions on the Izu-Bonin Arc that do not show obvious volcanic activity on the surface of the sea.