Phreatomagmatic eruptions, resulting from magma-water interaction, are highly hazardous volcanic events. The explosivity of these eruptions is significantly influenced by the amount of external water involved and the magma discharge rate. Shallow-water environments generally lead to more explosive eruptions due to efficient water incorporation. This study focuses on the 2021 eruption at Fukutoku-Oka-no-Ba (FOB) volcano, a sizeable phreatomagmatic event that occurred in a shallow-sea environment. Understanding the dynamics of this eruption is crucial for improving our comprehension of shallow submarine volcanism and its associated hazards. The research question centers around how the interaction between rapidly ascending magma and seawater contributed to the sustained high eruption column observed at FOB. The study aims to provide a detailed analysis of the eruption's temporal evolution, quantify the volume of erupted magma, and elucidate the processes controlling the eruption column's height and longevity. The importance of this study lies in its potential to refine hazard assessment models for shallow submarine volcanoes and improve our predictive capabilities for such events.

The existing literature extensively documents the various aspects of phreatomagmatic eruptions. Studies highlight the role of external water in enhancing explosivity (Houghton et al., 2015; White et al., 2003). The water depth and magma discharge rate are identified as key factors influencing eruptive style and explosivity (Koyaguchi & Woods, 1996; Kokolakis, 1983). Several studies have analyzed specific phreatomagmatic eruptions, such as the 1963-64 Westman Islands eruption (Thordarson et al., 1964) and the 2018 Anak Krakatau eruption (Carey et al., 2018). These studies have provided valuable insights into the processes involved in these eruptions. However, a comprehensive understanding of the dynamics of high-column, sustained phreatomagmatic eruptions, particularly those occurring in shallow marine environments, remains limited. The FOB eruption presents a unique opportunity to advance this understanding due to the availability of high-resolution data.

The study utilized a multi-faceted approach integrating various data sources and analytical techniques. High-resolution time-series data from the Himawari-8 satellite, providing optical and infrared images, were crucial for monitoring the eruption's temporal evolution and plume characteristics. Infrasound data from a monitoring station at Chikijima Island were used to track the acoustic signals associated with the eruption. These data allowed for a detailed reconstruction of the eruption's four phases, characterizing the intensity, duration, and plume dynamics of each phase. Geochemical analyses of pumice samples collected after the eruption, including major element compositions and SO₂ concentrations, provided constraints on the magma composition and the amount of erupted magma. The SO₂ emissions were also estimated from TROPOM satellite observations. To determine the magma discharge rate needed to sustain the observed eruption column, a one-dimensional eruption plume model, 'Plumeria', was employed. This model considered the effects of phase changes during seawater vaporization and the thermal contribution of pumice. The model output was used to constrain the conditions (magma discharge rate and water mass fraction) capable of producing a 16 km high plume. The volume of the erupted material was estimated using multiple methods, including calculations from the volume of the tuff cone formed during the eruption and the area of the pumice raft. The bathymetric changes at the source were also used to assess the volume of erupted material. These methods provide independent estimates of the erupted volume, allowing for cross-validation and error analysis. The combination of these different data sources and methodologies provides a robust and comprehensive analysis of the eruption's dynamics.

The 2021 FOB eruption consisted of four phases: Phase 1, a sustained plume phase lasting ~14 hours, with eruption columns reaching ~16 km; Phase 2, a high-pulsing unsteady phase; Phase 3, a period of intermittent weak explosions; and Phase 4, a phase of weak activity. The most intense phase, Phase 1b, featured eruption columns that repeatedly reached the tropopause height. No thermal anomaly was detected by satellites during the eruption's most explosive phase, possibly because most of the magma's heat was consumed in vaporizing seawater. The eruption formed a tuff cone (~0.04–0.07 km³) and a voluminous pumice raft (~300 km²). Geochemical analysis of pumice samples yielded a range of 61.7–64.0 wt% SiO₂ and 9.6–11.1 wt% Na₂O + K₂O, classifying them as trachyte. SO₂ concentrations in the pumice and satellite-observed SO₂ emissions allowed for the estimation of the erupted magma volume (~0.1 km³). Plume modeling using 'Plumeria' indicated that a magma discharge rate of 3–6 × 10⁵ kg/s was needed to sustain the 16 km-high plume in Phase 1, suggesting effective near-vent accumulation of pyroclasts due to interactions between fragmented magma and seawater. The eruption transitioned from a sustained high-column phase (Phase 1) to more characteristic Surtseyan phases (Phases 2 and 3). This change was likely influenced by the shallowing of the vent and the decrease in magma discharge rate. The erupted volume, estimated from multiple methods, was consistently around ~0.1 km³ DRE (Dense Rock Equivalent).

The findings indicate that the sustained high-column eruption at FOB was driven by a high magma discharge rate and efficient magma-seawater interaction in a shallow marine environment. The interaction between fragmented magma and seawater facilitated the decoupling of coarse pyroclasts from the eruption jet, allowing a buoyant vapor-rich plume to develop and rise to great heights. While the eruption exhibited characteristics of both Plinian and Surtseyan eruptions, its unique features, such as the sustained high column and the dominance of near-vent deposition, do not fit neatly into existing eruption style classifications. The large volume of the pumice raft, estimated to be 50-90% of the total erupted volume, highlights the significant role of seawater interaction in dispersing volcanic material. The transition to the later surtseyan phases is explained by changes in vent depth and magma discharge rate. The study’s findings necessitate a re-evaluation of existing eruption classification schemes and emphasizes the need for more nuanced models to capture the complex dynamics of shallow submarine eruptions.

This study provides a comprehensive analysis of the 2021 FOB eruption, revealing the critical role of seawater-magma interaction in driving the sustained high-column eruption. The high magma discharge rate and efficient near-vent pyroclast accumulation from jet-seawater interactions explain the observed plume height and voluminous pumice raft. The eruption’s unique characteristics highlight the limitations of existing eruption classification schemes and underscore the need for further research integrating observational data and numerical simulations to improve our understanding of submarine volcanism.

While the study offers a comprehensive analysis, limitations exist. The volume estimates have inherent uncertainties due to the challenges of accurately measuring submarine volcanic deposits and the pumice raft's dynamic nature. The one-dimensional plume model used simplifies a complex three-dimensional process. More detailed numerical models incorporating three-dimensional effects might provide a more precise understanding of the plume dynamics. Additionally, the lack of direct measurements of the pumice raft thickness introduces some uncertainty in the erupted volume estimations.