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Seawater interaction and degassing process of magma during the 2022-2023 eruptions at Ioto volcano, Ogasawara, Japan: Implications for hazard assessment of shallow sea eruption

Takahiro Miwa 1, Masashi Nagai1, Setsuya Nakada1, Atsushi Yasuda2, Tomofumi Kozono1, Hideki Ueda1

  • Affiliations: 1National Research Institute for Earth Science and Disaster Resilience, Tsukuba, Japan; 2Earthquake Research Institute, The University of Tokyo, Tokyo, Japan

  • Presentation type: Talk

  • Presentation time: Tuesday 08:30 - 08:45, Room R290

  • Programme No: 6.2.1

  • Theme 6 > Session 2

Abstract

Submarine eruptions ubiquitously occur across diverse depths and locations on Earth, causing significant volcanic hazards. In shallow seas, continuous magma-water interaction is thought to control eruption intensity. While it is well known that open-system degassing of magma plays a key role in controlling eruptive intensity by limiting gas expansion in magmas. Understanding shallow sea eruptions thus requires integrating magma-water interaction with degassing processes to better assess eruption hazards. This study investigates the 2022-2023 shallow sea eruption off Ioto Island, Ogasawara, Japan, to illuminate these processes. The eruption, observed in shallow sea (10--20 m depth) with intermittent cock's tail jets, deposited pumice along the southern coast of the island and produced pumice raft that reached southwestern Japan. Petrological analyses reveal chilled margins of pumice, with vesicularity of 20--40 vol.% and a water content of ~0.21 wt.%, suggesting rapid cooling near the seafloor (<0.2 MPa). Thermodynamic modeling and water content of melt inclusions in phenocrysts indicate magma storage at 970 °C with water content of 1.3 wt.%. The water budget and vesicularity of magma indicate that open-system degassing occurred during the eruption. These findings suggest that the eruption was driven by ascent of magma which underwent open-system degassing, followed by seawater interaction near the seafloor, triggering rapid cooling and fragmentation. Boiled seawater and residual gas in magma likely fueled cock's tail jets. By integrating magma-water interaction and degassing processes, this study suggests monitoring of volcanic gas derived from the open-system degassing is important for hazard assessment of shallow sea eruption.