4:45 PM - 5:00 PM
[SVC36-12] Detailed volcanic morphology revealed by bathymetric surveys around Ioto (Iwo-jima) in the Izu-Bonin Arc
Keywords:seafloor morphology, caldera ring fault, subaqueous lava flow, pyroclastic density current, caldera resurgence, crustal deformation
Ioto (Iwo-jima) is a caldera volcano located on the volcanic front of the southernmost Izu-Ogasawara Arc. It is well known for its active crustal deformation, frequent seismic activity, and geothermal phenomena, including recurring phreatic eruptions. Since 2022, small-scale magmatic eruptions have been intermittently occurring off the southern coast. While understanding the volcanic eruption history is crucial for elucidating the mechanisms of post-caldera volcanic activity and disaster prevention, research on Ioto remains insufficient due to the fact that most of the volcanic edifice is submerged beneath the sea. The Japan Coast Guard conducted four survey missions in 2013, 2019, 2021, and 2023, acquiring detailed seafloor morphologic data for almost the entire area around Ioto, except for the shallow coastal waters. This report presents the seafloor morphologic features revealed by these surveys and the changes over time.
The survey results clearly show that the ring fault system along the caldera rim retains fresh fault scarp. Lava flows are widely distributed across the outer slopes of the caldera. Thick lava flows, covered by only a thin sediment and exhibiting distinct surface irregularities, are considered to have erupted relatively recently. Some of these flows are highly likely to be flank volcanoes. The northwestern and eastern outer slopes of the caldera appear smooth, suggesting that they may be composed of gravity flow deposits, such as subaqueous pyroclastic flows. To detect topographic changes, bathymetric value from past and recent seafloor surveys were compared. A comparison with the 1991 survey data, which covered the entire area, showed significant decreases in depth, mainly in coastal areas inside the ring fault system—including offshore Kita-no-Hana, the southeastern coast, Chidori-ga-Hama, and between Kangoku-iwa and Ioto—with a maximum reduction of 35 meters. If all this change is attributed to uplift, the estimated average uplift rate is approximately 0.6-1.2 meters per year. A comparison of overlapping bathymetric data from 2013 and 2023 for the northern caldera floor also yielded an average uplift rate of up to approximately 1.5 meters per year. According to interferometric SAR crustal deformation analysis (e.g., Ozawa, 2007; GSI ,2024), a ring-shaped uplift zone with significant deformation is present on land, centered around Mt. Motoyama. The uplifted seafloor areas are considered to be the offshore extension of this uplift zone. The amount of uplift decreases sharply offshore, with no significant depth changes observed outside the caldera. This suggests that the source of the crustal deformation is located at a very shallow depth within the ring-shaped uplift zone, which is consistent with the hypothesis proposed by Nagai et al. (2017) that a cone-sheet-like intrusive body is forming in the shallow subsurface of the Ioto caldera.
The survey results clearly show that the ring fault system along the caldera rim retains fresh fault scarp. Lava flows are widely distributed across the outer slopes of the caldera. Thick lava flows, covered by only a thin sediment and exhibiting distinct surface irregularities, are considered to have erupted relatively recently. Some of these flows are highly likely to be flank volcanoes. The northwestern and eastern outer slopes of the caldera appear smooth, suggesting that they may be composed of gravity flow deposits, such as subaqueous pyroclastic flows. To detect topographic changes, bathymetric value from past and recent seafloor surveys were compared. A comparison with the 1991 survey data, which covered the entire area, showed significant decreases in depth, mainly in coastal areas inside the ring fault system—including offshore Kita-no-Hana, the southeastern coast, Chidori-ga-Hama, and between Kangoku-iwa and Ioto—with a maximum reduction of 35 meters. If all this change is attributed to uplift, the estimated average uplift rate is approximately 0.6-1.2 meters per year. A comparison of overlapping bathymetric data from 2013 and 2023 for the northern caldera floor also yielded an average uplift rate of up to approximately 1.5 meters per year. According to interferometric SAR crustal deformation analysis (e.g., Ozawa, 2007; GSI ,2024), a ring-shaped uplift zone with significant deformation is present on land, centered around Mt. Motoyama. The uplifted seafloor areas are considered to be the offshore extension of this uplift zone. The amount of uplift decreases sharply offshore, with no significant depth changes observed outside the caldera. This suggests that the source of the crustal deformation is located at a very shallow depth within the ring-shaped uplift zone, which is consistent with the hypothesis proposed by Nagai et al. (2017) that a cone-sheet-like intrusive body is forming in the shallow subsurface of the Ioto caldera.