Seawater-magma interactions sustained the high column during the 2021 phreatomagmatic eruption of Fukutoku-Oka-no-Ba

The study investigates how seawater–magma interactions in a very shallow marine setting control explosivity, column height, and deposit distribution during the 13 August 2021 eruption of Fukutoku-Oka-no-Ba (FOB). Phreatomagmatic explosivity depends on the availability and mixing efficiency of external water, eruption depth, and magma discharge rate. Shallow-water environments typically promote more explosive behavior than deep-water or dry-subaerial conditions due to efficient incorporation of external water. The research aims to quantify eruption timing, plume dynamics, erupted volume, and the role of seawater in sustaining a high, water-rich column, thereby improving hazard understanding for shallow submarine volcanoes.

Background work highlights that phreatomagmatic eruptions, including Surtseyan-style activity, are driven by magma–water interactions and can generate PDCs, strong pressure waves, and tephra jets. Prior observations show shallow-water eruptions are typically more explosive than deep-water events and that plume height relates to thermal flux and discharge rate. Previous cases (e.g., Anak Krakatau 2018; various Surtseyan eruptions) inform expectations of plume heights rarely reaching the tropopause due to low thermal flux, while some submarine silicic eruptions disperse large pumice rafts. The study builds on models of eruption column formation from magma–surface-water mixing and petrologic constraints on volatile budgets.

- Remote sensing and infrasound: Optical/IR Himawari-8 satellite imagery was used to construct a high-resolution timeseries of plume evolution, onset timing, and umbrella cloud dimensions. Infrasound was recorded at Chichijima (approximately 330 km north of FOB) to detect explosive signals and track eruptive phases. - Thermal observations: Satellite thermal anomaly searches were conducted; an aerial infrared survey by the Japan Coast Guard (JCG) around 15:00–15:30 JST on 13 August captured ballistically ejected hot pyroclasts, although satellite thermal anomalies were not detected, consistent with efficient seawater vaporization consuming magmatic heat. - Deposit and geomorphic observations: Formation and erosion of a tuff cone and emergence of islets were mapped from satellite and JCG aerial imagery. Cone height (~15 m), crater diameter (~1 km), and qualitative facies (massive, poorly sorted loose pyroclastics) were described. Pumice raft initiation (~08:00 JST 13 Aug), advection, and areal extent (~300 km² by 10:00 JST 15 Aug) were mapped; spreading speeds were inferred from time-stamped imagery. - Volume estimation: Tuff cone volume (0.04–0.07 km³ bulk) was derived from pre/post-eruption bathymetry and island dimensions. Pumice-raft volume was estimated using raft area and assumed thickness/densities (bulk density ~700–800 kg/m³ for pumice, 1000–1500 kg/m³ for tuff deposits) and used to estimate total erupted DRE volume (~0.04–0.1 km³), acknowledging large uncertainties in thickness and density assumptions. - Petrology and geochemistry: Whole-rock and glass (groundmass, melt inclusions in plagioclase) compositions were measured (XRF, EPMA). 2021 products are trachyte (61.7–64.0 wt% SiO₂; 9.6–11.1 wt% Na₂O+K₂O). Groundmass glasses span ~56–68 wt% SiO₂; silicic melt inclusions mostly 65–67 wt% SiO₂. Sulfur (reported as SO₃/SO₂ equivalents) was measured for groundmass and melt inclusions to estimate degassed sulfur via the difference between inclusion and groundmass concentrations. - Satellite SO₂ retrieval: Atmospheric SO₂ was quantified with Sentinel-5P TROPOMI using DOAS-based retrievals and post-processing (SACS) to convert slant to vertical columns, accounting for plume height, aerosols, and surface effects. Total SO₂ mass for the ~15 h activity was estimated at 2.1 × 10⁶ kg. - Erupted mass/volume from SO₂ balance: Combining petrologic sulfur concentrations and satellite SO₂ mass yielded an erupted magma DRE volume of ~0.11 km³, consistent with deposit- and raft-based estimates. - Plume modeling: A 1-D eruption column model (Plumeria) incorporating external water phase change and magmatic water was used to estimate magma discharge rates required to sustain 16 km plume heights under observed meteorological conditions. The modeling considered the fraction of erupted magma not contributing thermal energy to the plume (e.g., coarse pumice decoupling). The vent exit diameter was set to 100 m; water mass fraction at the exit varied (0.5–0.95). - Phasing: Himawari-8 and infrasound constrained four phases: an initial sustained plume (Phase 1 subdivided into 1a and 1b), an unsteady pulsing phase (Phase 2), intermittent weak to sparse strong explosions (Phase 3), and a weak degassing/plume phase (Phase 4).

- Onset and phasing: The eruption began at 05:55 JST, 13 August 2021, and evolved through four phases. Phase 1 featured a sustained, water-rich column reaching ~16 km (tropopause level), with the most vigorous activity after ~14:00 JST on 13 August. - Plume morphology: Lateral umbrella clouds at ~16 km had 15–20 km radius with slight overshoot; plume color was white, indicating water-rich vapor. No near-source satellite thermal anomaly was detected, consistent with heat consumption by seawater vaporization. - Vent depth and deposits: Eruption initiated on seafloor shallower than ~70 m b.s.l. A tuff cone with ~1 km crater formed (max height ~15 m) and was rapidly eroded into two small islets that vanished by early 2022. Cone deposits were massive and poorly sorted, indicating multiple near-vent depositional processes including partial column collapses and PDCs observed around 15:00–15:30 JST, 13 August. - Pumice raft: A brown pumice raft started forming around 08:00 JST, spread primarily with winds/ocean currents, and reached ~300 km² by 10:00 JST on 15 August. Growth was not from fallout beneath the high plume (due to strong winds) but from near-vent processes and/or subaerial PDC sedimentation. The raft likely represents 50–90% of total erupted volume. - Chemistry and sulfur: 2021 products are trachytic; groundmass glasses 56–68 wt% SiO₂; melt inclusions mainly 65–67 wt% SiO₂. Measured SO₂ concentrations for silicic groundmass and melt inclusions indicate substantial degassing (desegregated SO₂ ~73%). Satellite TROPOMI observed total atmospheric SO₂ mass of 2.1 × 10⁶ kg over ~15 h. The combined petrologic–satellite SO₂ budget yields an erupted magma volume of ~0.11 km³ DRE, consistent with deposit-based estimates. - Discharge rate and plume modeling: One-dimensional plume modeling indicates that sustaining a ~16 km column requires a magma discharge rate M₀ ≈ 3–6 × 10⁵ kg/s when 5–50% of erupted magma contributes thermal energy to the plume (with significant coarse clast decoupling). For an approximately nine-hour sustained plume interval, erupted mass is estimated at ~1–2 × 10¹⁰ kg (0.04–0.08 km³ DRE) contributing to the sustained column; much of the ejecta accumulated proximally (tuff cone and raft), reducing distal fallout. - Mechanism sustaining high column: Efficient seawater entrainment and rapid vaporization at the jet–water interface led to a buoyant, vapor-rich plume, while coarse clasts decoupled and accumulated near the vent. This partitioning allowed a high, water-rich column despite limited distal ash fallout. - Style evolution: Phase 1 behavior differs from typical Surtseyan (discrete, low discharge) and subplinian (hot ash-rich dispersal) eruptions due to strong seawater coupling and proximal deposition. As the cone emerged and discharge declined, activity transitioned to more typical Surtseyan-style explosions in Phases 2–3.

Findings show that shallow-water magma–seawater interactions can sustain high eruption columns by partitioning heat to rapid steam production while decoupling coarse pyroclasts that deposit near the vent. This explains the coexistence of a 16 km, water-rich sustained column with minimal distal ash deposition and the formation of a substantial tuff cone and extensive pumice raft. The required magma discharge rates (3–6 × 10⁵ kg/s) and observed plume heights resemble subplinian columns in altitude but differ fundamentally in plume composition and particle loading due to external-water involvement. Style transitions from a sustained, water-rich column (Phase 1) to intermittent Surtseyan activity (Phases 2–3) were likely governed by vent shallowing (cone build-up) and reduced discharge rates, modulating the availability and efficiency of seawater interaction. The study underscores that plume height–mass eruption rate relationships derived for subaerial eruptions may not directly translate to shallow submarine settings; partitioning of mass and heat across the water–air boundary critically controls outcomes. Consequently, VEI assessments based on plume height may mischaracterize submarine events, requiring adapted frameworks.

The 2021 FOB eruption demonstrates that efficient seawater–magma interactions in very shallow water can sustain a high, water-rich eruption column while directing most pyroclasts to proximal accumulation as a tuff cone and pumice raft. Integrated satellite imagery, infrasound, petrologic SO₂ budgets, and plume modeling constrain a total erupted volume on the order of ~0.1 km³ DRE and magma discharge rates of ~3–6 × 10⁵ kg/s to maintain a ~16 km column. The results highlight that shallow submarine plume dynamics are governed by heat and mass partitioning at the seawater–air interface, challenging direct application of subaerial plume scaling and VEI to such eruptions. Future work should refine near-vent two-phase jet physics, quantify partitioning of coarse versus fine material, and couple observations with numerical models to improve hazard assessments for shallow submarine volcanism.

- Erupted volume estimates have large uncertainties due to poorly constrained raft thickness, deposit densities, and limited direct bathymetric re-surveys. The tuff cone and raft volume calculations rely on imagery and assumed parameters. - Satellite thermal sensors did not detect a near-source anomaly during peak activity, limiting direct constraints on heat flux; aerial IR snapshots captured only brief intervals. - Petrologic sulfur budgets carry uncertainties: potential melt inclusion leakage, selection bias toward glassy groundmass, analytical errors, and assumptions about pre-eruptive volatile contents and post-entrapment processes. These can bias degassed sulfur and erupted mass estimates. - Satellite SO₂ retrievals depend on plume height, aerosol loading, and radiative transfer assumptions; conversion from slant to vertical columns and mass has associated uncertainties. - Plume modeling uses simplified 1-D formulations and assumed vent geometry (e.g., 100 m exit diameter) and water fractions; results are sensitive to the fraction of magma contributing thermal energy to the plume due to clast decoupling. - Limited in situ sampling and absence of comprehensive proximal/distal deposit measurements hinder validation of dispersal and partitioning inferences.