# Simultaneous creation of a large vapor plume and pumice raft by the 2021 Fukutoku-Oka-no-Ba shallow submarine eruption

Kristen E. Fauria, Martin Jutzeler, Tushar Mittal, Ashok Kumar Gupta, Liam J. Kelly, John Rausch, Ralf Bennartz, Brent Delbridge, Lise Retailleau

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## To cite this version:

Kristen E. Fauria, Martin Jutzeler, Tushar Mittal, Ashok Kumar Gupta, Liam J. Kelly, et al., Simultaneous creation of a large vapor plume and pumice raft by the 2021 Fukutoku-Oka-no-Ba shallow submarine eruption. Earth and Planetary Science Letters, 2023, 609, 10.1016/j.epsl.2023.118076. insu-04155739

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# Simultaneous creation of a large vapor plume and pumice raft by the 2021 Fukutoku-Oka-no-Ba shallow submarine eruption

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Kristen E. Fauria<sup>a,*</sup>, Martin Jutzeler<sup>b</sup>, Tushar Mittal<sup>c,d</sup>, Ashok Kumar Gupta<sup>a,e</sup>,
Liam J. Kelly<sup>a</sup>, John Rausch<sup>a</sup>, Ralf Bennartz<sup>a</sup>, Brent Delbridge<sup>f</sup>, Lise Retailleau<sup>g,h</sup>

## ARTICLE INFO

**Article history:**

Received 21 July 2022

Received in revised form 12 February 2023

Accepted 18 February 2023

Available online 10 March 2023

Editor: C.M. Petrone

**Dataset link:** https://doi.org/10.5281/zenodo.7689653

**Keywords:**

volcanoclastic

satellite remote sensing

explosive eruption

Himawari-8

## ABSTRACT

The 12 August 2021 eruption of Fukutoku-Oka-no-Ba, a shallow submarine volcano in the Izu-Bonin arc of Japan, is one of few documented submarine eruptions to make a large aerial plume and floating pumice raft. Relative to past eruptions, this event was well-covered by multiple high resolution satellite remote sensors, raising the possibility of resolving important questions about the timing of raft and plume formation, and their relationship. Here we use satellite remote sensing to assess the eruption timeline, style, rates, and products. We use the Himawari-8 satellite to assess the plume volume flux and height through time. In addition, we use a combination of ultra-high resolution satellite imagery to assess the timing and mechanisms of raft formation. We find that the 16 km eruption plume was ice-rich and conclude that the 0.1 km³ raft and eruption plume were co-genetic. Finally, we suggest that pumice clasts were delivered to the raft by tephra jets, ballistics, and near-vent fallout from the plume. Together our observations reveal that the fallout of pumice lapilli from a water-rich eruption column generated a large pumice raft.

© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

Submarine volcanoes matter, in part, because they make up the majority of volcanic activity on Earth including a significant fraction of arc volcanism (Crisp, 1984). Submarine volcanic eruptions can also directly impact the atmosphere in cases when they breach the ocean surface. This was clearly illustrated by the 15 January 2022 eruption of Hunga Tonga-Hunga Ha'apai, which created damaging ashfall, tsunamis, high atmospheric plumes, and atmospheric Lamb waves (e.g., Wright et al., 2022; Matoza et al., 2022; Gupta et al., 2022). Another key aspect of submarine eruptions is their ability to create rafts of floating pumice, a highly porous volcanic material that can be hazardous to ships and coastal infrastructure (e.g., Bryan et al., 2004; Ohno et al., 2022). The August 2021 Fukutoku-Oka-no-Ba pumice raft clogged ports and damaged ves-

sels in Japan (Okinawa Prefecture, 2022). Pumice rafts, which can span 100s of km², are also important because they transport volcanic material 100s to 1000s of kms over months to years and carry biota (Bryan et al., 2004). Although pumice raft dispersal has been successfully tracked and modeled in a few cases (Bryan et al., 2004; Jutzeler et al., 2014, 2020), we lack observations of active pumice raft formation and a basic understanding of how pumice rafts are assembled. Open questions include: What are the processes that deliver pumice clasts to the sea surface, how do these processes differ compared to those in subaerial settings, and how are rafts related in space and time to other eruption features such as aerial plumes?

Here we report on the August 2021 shallow submarine eruption of Fukutoku-Oka-no-Ba (FOB), Japan, which created a 16 km water-rich atmospheric plume, 0.1 km³ raft of floating pumice, and new 1.1 km diameter tuff ring islands. We use this eruption as a case study to fill critical gaps in our understanding of raft formation and evolution and therefore constrain conceptual and quantitative models of a key class of submarine eruptions. Specifically, we

<footnote><sup>a</sup> Department of Earth and Environmental Sciences, Vanderbilt University; Nashville, TN, USA</footnote>

<footnote><sup>b</sup> School of Natural Sciences and Centre for Ore Deposit and Earth Sciences (CODES), University of Tasmania, Hobart, Tasmania, Australia</footnote>

<footnote><sup>c</sup> Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA</footnote>

<footnote><sup>d</sup> Department of Geosciences, Pennsylvania State University, PA, USA</footnote>

<footnote><sup>e</sup> Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, USA</footnote>

<footnote><sup>f</sup> EES-17 (Geophysics), Los Alamos National Laboratory, Los Alamos, NM, USA</footnote>

<footnote><sup>g</sup> Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, France</footnote>

<footnote><sup>h</sup> Observatoire volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, 14 RN3 - Km 27, F-97418 La Plaine des Cafres, La Réunion, France</footnote>

<footnote>* Corresponding author.
E-mail address: kristen.fauria@vanderbilt.edu (K.E. Fauria).</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** August 2021 eruption timeline. a) Summary of satellite availability and observations of eruption features. For the Islands, Plume at vent, and Raft at vent rows, the solid lines indicate where these features were observed and the dashed lines indicate when they were inferred. On average, 200-300 strokes per 20 minutes were observed during the first day of the eruption and the timing of these high lightning stroke rates correlates well with phase 1 of plume activity. These lighting strokes were located within 40 km of the vent. b) Plume height through time ($H_{min}$) calculated from Himawari-8 brightness temperature ($BT_{min}$) and compared to Infrared Atmospheric Sounding Interferometer (IASI) data. $BT_{avg}$ is also plotted to provide a sense of plume intensity. We overlay the eruptions phases 1-4 from Maeno et al., 2022. c) True color image of the volcanic plume at 4:30 UTC on August 13, 2021. d) 11.2 µm brightness temperature of the volcanic plume. The red box shows the domain over which average brightness temperature ($BT_{avg}$) was measured in panel d. e) $SO_2$ measurements within the plume in Dobson units (DU) and from TROPOMI (Supplementary Text S3). f) The 11.2-12.4 µm brightness temperature difference of the plume.</figcaption></figure>

combine data from more than 12 satellites to assess the eruption stages, styles, rates, and products of this eruption. Excellent observations by the Japan Coast Guard (Submarine Volcano Database, 2021) validate and add to parts of this analysis. Our contribution is complementary to the study by Maeno et al. (2022), which includes geochemical analysis of the FOB products and presents the FOB eruption timeline using, in part, infrasound data and plume diameter from Himawari-8 data. In addition, FOB was widely discussed and debated on social media as the eruption took place (Supplementary Text S6).

From the geostationary Himawari-8 data, we determine that the eruption sustained a water-ice dominated plume that spread near the tropopause. Further, we use ultra-high resolution (40 cm/pixel) DigitalGlobe imagery (Neigh et al., 2013) to provide the first observation of the syn-eruptive formation of a large pumice raft. By

assembling a multi-satellite view of the eruption, we resolve processes such as submarine eruption onset and pumice raft evolution at levels that have not been possible to date. Our study illustrates the importance of high-resolution satellite remote sensing datasets to observe and understand volcanic processes at remote volcanoes such as FOB.

## 1.1. Geologic background

Fukutoku-Oka-no-Ba is an active, shallow submarine arc volcano located 1300 km south of Tokyo in the Ogasawara Islands where volcanism is associated with subduction of the Pacific Plate beneath the Philippine Sea Plate [Fig. 1B]. Over the last century, at least seven eruptions have been documented at FOB, the most recent being in 2010 (Japan Meteorological Agency, 2013). Con-
firmed eruptions include those that produced large pumice rafts in 1904, 1914, 1986 and 1992 and temporary islands that were subsequently eroded in 1904, 1914, and 1986 (Furukawa, 1992; Urai, 2014). All these recent eruptions, and the eruption currently studied, were of intermediate composition (trachyandesite) (Furukawa, 1992; Japan Meteorological Agency, 2013; Yoshida et al., 2022a). Notably, the 1904 and 1914 eruptions generated weak aerial plumes that, in the case of the 1914 eruption, rose up to 3 km (Kuno, 1962; Wakimizu, 1908). Discolored water activity is also common at the vent site and could indicate continued hydrothermal fluid discharge and/or small eruptions (Japan Meteorological Agency, 2013). As recently as 2010, bathymetric surveys showed that FOB was a conical seamount with a 40 meters below sea level (mbsl) flat oval top (1.7 km × 1.2 km) (Ito et al., 2011; Otani et al., 2006).

# 2. Materials and methods

## 2.1. Plume characteristics

We relied on the Japan Meteorological Agency's geostationary satellite Himawari-8 to assess the height, aerial extent, and water content of the FOB plume through time.

### 2.1.1. Plume height

We used the brightness temperature of the volcanic cloud top to estimate plume top height through time. This temperature method (Oppenheimer, 1998; Prata and Grant, 2001) assumes that the cloud top is in thermal equilibrium with its surroundings and relates the top of the volcanic cloud temperature to a vertical temperature profile to determine height. Specifically, we used the Himawari-8 (2 km/pixel resolution, 10 minute frequency) data at a wavelength of 11.2 microns. We evaluated the minimum brightness temperature ($BT_{min}$) immediately above the vent by taking the lowest brightness temperature pixel within the domain 141.013-141.821°E and 24.3190-24.5291°N. Because the highest plume top will be the coldest, this method evaluates the altitude of the highest part of the plume and thus may characterize the altitude of the plume top that overshoots the neutral buoyancy region (e.g., Morton et al., 1956). Clouds passing over the FOB vent can also cause meteorological contamination of the BT estimates. However, we visually confirm that there is only minor meteorological contamination on 14 August (and not earlier). On 15 August we visually confirmed the presence of meteorological clouds and a lack of new volcanic activity in the Himawari-8 data.

After acquiring $BT_{min}$ values, we converted these values to plume height ($H_{min}$) using atmospheric temperature profiles from ERA5 reanalysis data (Hersbach et al., 2020; Fig. S3). To verify our plume height estimates, we compared our estimated plume heights against $SO_2$ plume height data from the Infrared Atmospheric Sounding Interferometer (IASI)-Metop C [blue dots in Fig. 1b], total attenuated backscatter profile at 532 nm from space-borne lidar (CALIPSO, Fig. 2), and a profile of aerosol-to-molecular extinction ratio from Stratospheric Aerosol and Gas Experiment (SAGE) III on ISS (Atkinson, 2021) for the available overpass. We find close agreement between the estimates.

In addition, we averaged brightness temperature ($BT_{avg}$) within a rectangular domain over the FOB vent (140-141.4°E and 24.3-24.5°N, red box in Fig. 1d). This domain captured the plume well during the first 14 hours of the eruption, however as the plume became smaller and unsteady the plume did not always fill the entire averaging box. As a result, the $BT_{avg}$ values become contaminated with clear sky pixels after the first 14 hours of the eruption. We therefore do not use $BT_{avg}$ to estimate plume height because $BT_{avg}$ could result in an underestimate of the actual plume. That said, we plot $BT_{avg}$ because this illustrates the time when the

plume does not fill the majority of the averaging domain and thus becomes less vigorous [Fig. 1d].

### 2.1.2. Plume composition

To determine whether the plume was optically thin or thick and assess the presence of water within the plume, we relied on a reverse absorption method (e.g., Prata, 1989a, 1989b; Prata and Grant, 2001). That is, we assessed the sign and magnitude of brightness temperature difference between thermal infrared bands at 11.2 and 12.4 µm (hereafter, $BTD_{11.2-12.4 \mu m}$) (e.g., Prata and Grant, 2001). Near zero $BTD_{11.2-12.4 \mu m}$ values show that the cloud was optically thick (an assumption that is critical to our method for assessing plume height), while positive values show that the plume is optically thin. Negative $BTD_{11.2-12.4 \mu m}$ values can, in some circumstances, indicate the presence of ash. However, the ability to interpret negative $BTD_{11.2-12.4 \mu m}$ values is limited for optically thick clouds in humid environments, both of which are relevant for FOB and therefore we do not interpret negative $BTD_{11.2-12.4 \mu m}$ to indicate ash (e.g., Rose et al., 1995).

To infer whether liquid water or ice is present in the plume, we combine $BT$ and $BTD_{11.2-12.4 \mu m}$ observations with true color imagery. Specifically, bright white true color imagery, cold brightness temperature values (<225 K), and near zero (within ~1 K) values of $BTD_{11.2-12.4 \mu m}$ [white color in Fig. 1f] together indicate thick ice clouds [Fig. 1].

### 2.1.3. Background atmospheric conditions

We determined the mean tropopause height where the temperature decreases with height at a rate of no more than 2 K km⁻¹ (World Meteorological Organization, 1957). We used hourly ERA5 data to estimate the mean tropopause height to be 15.5 ± 0.5 km during 12-18 August 2021 and over FOB (24.28°N, 141.48°E) [Fig. S3].

We also used hourly ERA5 reanalysis data to calculate the Brunt Väisälä frequency for the same period covering 12 to 18 August 2021. The Brunt Väisälä frequency is needed in our calculation of plume volume flux and describes the frequency at which an air parcel oscillates in a stably stratified atmosphere $N^2 = \frac{g}{\theta} \frac{d\theta}{dz}$, where $\theta$ [K] is the potential temperature, g [m s⁻²] is gravitational acceleration, and z [m] is the height. Over FOB (24.28°N, 141.48°E), we find that the mean Brunt Väisälä frequency, N, near the tropopause height was 0.02 s⁻¹.

### 2.1.4. Initial volume flux

To determine the initial volume flux of material in the plume we used two methods: 1) we fit an intrusive density current model to measurements of plume top area (Suzuki and Koyaguchi, 2009; Costa et al., 2013); and 2) we fit a model for downwind gravity current spread to measurements of crosswind plume width (Pouget et al., 2013). Fig. 3 shows how the plume appears to be spreading in an anvil shape at its neutral buoyancy level with a minor amount of up wind propagation, lending support to the applicability of the intrusive density current model. That said, the highly elongated nature of the plume in the downwind direction [Fig. 1c-f] motivates us to also estimate volume flux using a model that takes wind driven propagation into account.

For the first method, we assessed the area of the plume top in the first two hours of the eruption by thresholding the Himawari-8 11.2 micron brightness temperature data, beginning at 13 Aug 2021 21:20 UTC. Specifically, we chose 210 K as a threshold value (visual analysis showed that most plume tops were between 190 and 210 K) and classified any pixels colder than 210 K as volcanic cloud top. After thresholding the images, we binarized the images to calculate plume top area. We consider the error associated with estimation of the areal extent of the plume from Himawari-8 pixels to be ~10% (Prata et al., 2020; Takeuchi, 2016). The plume top
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** A profile of CALIPSO space-born LiDAR data at 532 nm on 13 Aug 2021 at 17:40 UTC shows the FOB plume height to be 15-17 km, consistent with plume height estimates from Himawari-8. The CALIPSO's total attenuated backscatter coefficient provides information on the vertical structures of clouds and aerosols.</figcaption></figure>

was elongated in the westward direction due to wind. However, we assumed that the total area of the cloud top remained unchanged by wind and calculated the circular equivalent radius of the plume top, $r = \sqrt{\frac{A}{\pi}}$, where A is the measured plume top area. We then fit a model of plume top radius as a function of time to solve for volume flux, $r = \left(\frac{3\lambda QN}{2\pi}\right)^{1/3} t^{2/3}$, where $\lambda$ is a constant that is approximately 0.2, Q is the volume flux, N is the Brunt Väisälä frequency that describes the ambient stratification, and t is time (Suzuki and Koyaguchi, 2009; Costa et al., 2013; Prata et al., 2020).

Our second method for calculating plume volume flux is described in the Supplementary Material and relies on wind speed estimates of 15-18 m s⁻¹ near the tropopause from ERA5 reanalysis data [Fig. S5].

## 2.2. Lightning

To assess the occurrence of lighting near the FOB vent, we used data from the World Wide Lightning Location Network (WWLLN) (Lay, 2004; http://wwlln.net). Specifically, we considered strokes within 40 km of FOB because the number of strokes decreased dramatically after this distance. We plot the number of lightning strokes in 20-minute increments between eruption onset and 15 August 2021 in Fig. 1.

## 2.3. Raft and erupted volume characteristics

We used visual imagery from 11 satellites (Table S1) including ultra-high resolution imagery from Digital Globe (Neigh et al.,

2013) and Planet (Planet Team, 2017) to identify and assess features in the pumice raft and on the new island.

### 2.3.1. Erupted volume calculations

We calculated the initial raft volume to be 250 km² based on first estimating raft area from satellite images on August 14–16 and then by multiplying raft area by raft thickness (e.g., Jutzeler et al., 2014, 2020). This calculation does not include very thin rafts that are not well detected by reflectance methods and thin rafts may increase the affected area by ca. 50 km². We used in situ observations of past rafts and a correlation between raft thickness and ocean swell damping (Jutzeler et al., 2020) to estimate an average raft thickness of 40 cm after the first two days of drifting. Raft area did not change much over the next two weeks of drifting, indicating relatively stable raft thickness and weak lateral expansion, although abrasion may have removed a substantial part of the pumice volume. Because the raft thicknesses were not measured in situ we estimate this thickness to have uncertainties in the range of a factor of two.

Photographs from the Japan Coast Guard and satellite imagery showed that the newly formed islands were initially made of two partially submerged crescentic tuff rings (400 and 200 m in width, and 870 and 800 m in length, respectively). We estimated an average island elevation of 15 m based on Japan Coast Guard images and later confirmed by Simurda et al. (2022) [Figs. S11-S12]. We multiply island width by length by height to estimate total subaerial volume.

We used pre-eruption bathymetry to estimate the minimum volume of the underwater deposits associated with the new island. In 2010 FOB was a flat top, conical volcano (ca. 1.2 × 1.7 km) that was 29–40 m below sea level. Because the total com-
<figure><img src="image_7.png" /><figcaption>Fig. 3. Initial spreading of the FOB plume near neutral buoyancy level. A) A model for the spread of a neutrally buoyant gravity current fit to Himawari-8 observations of plume top radius starting at 13 Aug 2021 21:20 UTC. We find that the model fits the data well and radius scales with time (in seconds) to the 2/3 power (R ~ 139.52t^2/3). B) Japan Coast Guard Image of the August 2021 FOB eruption column and which shows spread of the plume near its level of neutral buoyancy near the tropopause (15.5 km) (white arrow). This image was taken from an altitude of ~6,000 m and from 90 km north of FOB on 13 Aug 2021 between 15:00 and 15:30 local time. Image credit: Japan Coast Guard 3rd Regional Coast Guard Headquarters.</figcaption></figure>

bined diameter of the two crescentic islands (1.1 km) was similar to the pre-eruption area of the volcanic summit, we concluded that the 2021 eruption products covered most of the 2010 summit. We used a truncated cone geometry with a 1.2 × 1.7 km² base, 1.1 × 1.1 km² top, and 40 m thickness to estimate a minimum volume of submerged deposits. This volume calculation does not include the unknown volume of material that was sedimented downslope of the volcano. Further, the volume of the products from the small 2010-2021 eruptions were unaccounted for.

## 2.4. Mass flux estimates

We estimate eruptive mass flux of the initial high plume eruption phase using three different methods. First, we convert plume top height to mass flux based on the empirical relationship

$$M = 140H^{4.15}, \qquad (1)$$

where M is mass flux [kg/s] and H is plume height [km] (Mastin, 2014). Second, we convert our estimated volumetric flow rate to mass flux based on calibration of a 1D plume volume flux scaling (Morton et al., 1956) with a 3D plume model (Suzuki and Koyaguchi, 2009; Costa et al., 2013)

$$Q = C \sqrt{k} \frac{M^{3/4}}{N^{5/4}} \qquad (2)$$

$$C \approx \begin{cases} 0.43 \times 10^3 \text{ m}^3 \text{ kg}^{-3/4} \text{ s}^{-3/2} & \text{for tropical eruptions} \\ 0.87 \times 10^3 \text{ m}^3 \text{ kg}^{-3/4} \text{ s}^{-3/2} & \text{for midlatitude and polar eruptions} \end{cases} \qquad (3)$$

Above Q is the volume flux, C is a constant calibrated by models from Suzuki and Koyaguchi (2009), and k is the entrainment coefficient that we assume to be 0.1. Last, we estimate mass flux by dividing the total erupted mass, m, by the duration of the eruption, t, such that

$$M = \frac{m}{t}. \qquad (4)$$

# 3. Results

## 3.1. Eruption processes and products

The eruption at FOB began on 12 August 2021 at 21:00 UTC (13 August 2021 6:00 JST, estimated to the nearest 10 minutes)

and can be identified in geostationary Himawari-8 imagery as the first occurrence of a bright white aerial plume. During the initial hours (first 110 minutes) of the eruption the plume spread west with an initial volumetric flux of $1.5 \times 10^9 \text{ m}^3 \text{ s}^{-1}$ (based on analysis of a spreading gravity current; Fig. 3), or $1-2.5 \times 10^9 \text{ m}^3 \text{ s}^{-1}$ (based on analysis of downwind gravity current spread, Supplementary Material). All volume flux estimates are similar to the volume flux of the December 2018 Anak Krakatau eruption which also created a water-rich plume that reached the tropopause (Prata et al., 2020) but two orders of magnitude smaller than the 15 Jan 2022 Hunga Tonga-Hunga Ha'apai eruption (Gupta et al., 2022). The plume top reached a height of 16.7 km (slightly above the mean tropopause height) and sustained a steady and vigorous plume at this altitude for 14 hours (phase 1, Fig. 1b and 3b). We confirm our Himawari-8 based plume top height estimate by comparing it against a CALIPSO space born lidar profile of the plume, which suggests that cloud layers are present between 15 and 17 km [Fig. 2]. The 14 hour time period over which a vigorous plume was sustained (Movie S1) corresponds to phase 1 of eruptive activity defined by Maeno et al. (2022) and is consistent with our assessment of BT<sub>avg</sub>. Over the next 37 hours (phases 2 and 3 from Maeno et al., 2022), the plume became unsteady and waning with multiple smaller pulses (Movie S1). We note that these smaller pulses are still able to reach altitudes of ~16 km, but that the BT<sub>avg</sub> plot reveals the waning nature of the eruption [Fig. 1b]. The last pulse visible in Himawari-8 data was on 14 August 2021 at 23:20 UTC [Fig. 1b], suggesting an end to the main eruption. Minor seismicity is observed coincident with the eruption [Supplementary text S1, Figs S1-S2].

### 3.1.1. Aerial plume composition

The composition of the top of the volcanic plume can be assessed using its absorption signature, or more specifically, brightness temperature (BT) and brightness temperature difference (BTD) using Himawari-8. The plume top was bright white in true color imagery, its brightness temperature was ~200-250 K, and its BTD was 0-5 K, indicating the presence of significant ice-particles in the plume [Fig. 1]. We note that below 225 K water cannot exist in a supercooled liquid form and thus the observed BT values show that water below 225 K in the plume was in the ice phase (e.g., Moore and Molinero, 2011).

The World Wide Lightning Location Network reports approximately 10,500 lightning strokes within 40 km of FOB between
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** All images except (E) are (c) 2021, DigitalGlobe, Inc., a Maxar company; NextView License and show the aerial plume and/or pumice raft on 13 Aug 2021 01:22 UTC (four hours after eruption onset). (A) A bright white aerial plume and a pumice raft that has spread up to 5 km from the vent. Radial water waves are present (yellow arrows). (B) Fine-scale laminations within the 13 Aug raft (grey) are present and aligned with the radial water waves (yellow arrows) and ocean swell (green arrows). (C) Red arrows show the sharp contrast between an oval feature that we interpret to be a raft and slightly older and more dilute raft material. (D) The contact between the white aerial plume and the grey oval feature. Contrails on the oval feature pointing towards the plume could indicate cooling bombs (blue arrows). (E) Outlines of the raft over the first week of the eruption and extracted from multiple satellite sources. Raft is to the northwest. (F) A single ~100 m dark, and thus ash rich, region within the bright white plume.</figcaption></figure>

12-15 August 2021 (Lay, 2004). The highest rate of lighting strokes (~16 strokes/min) were largely coincident with phase 1 of the eruption [Fig. 1]. This ~16 strokes/min rate is higher than peak lighting rates at two recent eruptions (also with data from the WWLN) that had taller plumes but no interaction with external water (2014 Kelud, 26 km plume, ~6 strokes/min rate, Hargie et al., 2019; and 2015 Calbuco, 23 km plume, ~10 strokes/min, Van Eaton et al., 2016). The high lighting stroke rate at FOB further supports the conclusion that there was significant ice within the plume and suggests that external water involvement strengthened the electrical charging relative to a comparable dry eruptions (Prata et al., 2020; Van Eaton et al., 2020). In addition, ash may have been present in the plume but shielded from satellite observations because of abundant water and ice, as was the case for the 2016-2017 eruption of Bogoslof, Alaska (Schneider et al., 2020).

Sentinel 5 Tropomi data shows that SO₂ was present in the plume with maximum SO₂ values reaching 10-20 dobson units [Fig. 1e]. The duration of the SO₂ signature was short (less than four days), consistent with the emplacement of water facilitating rapid conversion of SO₂ to sulfate aerosols (Prata et al., 2020).

## 3.2. Relationship between aerial plume and raft formation

A 7 km diameter pumice raft is visible 4 hours after eruption onset in a Digital Globe ultra-high resolution (0.4 m) image [Fig. 4; 13 Aug 2021 01:22 UTC]. From this image we see concentric outwards growth of the raft surrounding the white and vapor-rich aerial plume. In the imagery that pre-dates the plume by four hours, there is no clear signature of a pumice raft or discolored water. Given this observation and the highly spatially spread-out nature of the raft in the image four hours after plume initiation,
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** All imagery from Planet (Planet Team, 2017) and white scale bars are 1 km. (A) The raft and vent approximately 27 hours after eruption onset (14 Aug 2021 0:50 UTC). The two crescentic islands are present (blue arrow) and discolored water (orange arrows) can be distinguished from the raft. (B) A portion of the raft on 15 Aug 2021 0:50 UTC. The ocean swell is visible within the lightest (and thinnest) sections of the raft (blue arrows). Ribbon-like patterns are present within the raft (white arrows) and likely caused by inter-raft collisions. (C) The raft and islands on 15 Aug 2021. Note the edge of discolored water (orange arrows). (D) Raft material leaving the vent area on 15 Aug 2021. Dark and thick raft exits from the northern gap between the two islands. White vapor at the crater center shows that wind direction is to the NW. Fragments of the raft are oriented parallel to the wind direction (black arrows), ocean wave direction (red arrows), and radial wave direction (yellow arrows). (E) Variations in color and thickness are present within the raft as are complex curves and swirls (white arrows), similar to pressure ridges in sea ice.</figcaption></figure>

we posit that the plume and raft began to form simultaneously, and thus are co-genetic.

We examined all available visual imagery, including Himawari-8, MODIS, and Sentinel-3 data, to evaluate when raft material was present at the vent. Because pumice rafts are quickly transported from their source by ocean currents and wind (Jutzeler et al., 2014, 2020), we used the presence of raft material at the vent as an indicator for raft formation. Our ability to resolve the raft's size and location through time was sometimes hindered by: 1) the aerial vapor plume which completely covered the vent area during its most vigorous stage; 2) lack of visual imagery at night; 3) occasional cloud coverage; and 4) the frequency of repeat coverage of medium and high resolution satellites. Despite these challenges, we established that raft material was at the vent (in all images in which the vent area is visible, ~40% of total Himawari-8 imagery) during the initial ~51 hours and when large aerial plumes (as defined as being visible in the Himawari-8 data) were created [Fig. 1].

We identified the last large aerial plume pulse from the Himawari-8 data on 14 Aug 2021 23:20 UTC. However, a video by the Japan Coast Guard on 15 Aug 2021 4:00 UTC shows an explosion feeding a small vapor plume (Submarine Volcano Database,

2021). Based on this observation, we make a distinction between large, high-altitude plumes that are captured by Himawari-8 (~2 km/pixel resolution) and small, short-lived and low-altitude plumes that can only be identified in higher resolution satellite imagery or by direct observations. Based on Japan Coast Guard video and satellite imagery that shows raft material near the vent until at least nightfall on Aug 15 [Fig. 5], we posit that the eruption continued, albeit at a lower intensity, through at least nightfall (15 Aug 2021 8:50 UTC). This interpretation is consistent with phase 4 of activity defined by Maeno et al. (2022). By the following dawn (15 Aug 2021 20:20 UTC), satellite imagery reveals that the raft had detached from the vent and drifted >12 km north of the vent [movie S2]. Taken together, our analyses suggest a correlation between eruptive flux into the aerial plume and raft formation. That is, the raft was created while the aerial vapor plume was produced.

### 3.2.1. *New islands*

The exact emergence time of the island remains unknown because the vent area was obstructed by the aerial plume until August 14, when two crescentic islands of 1.1 km diameter were visible [Fig. 5A]. Considering the initial shallow vent depth (Ito et
al., 2011), and the high initial intensity of the eruption, it is possible that the island formed during the first day of the eruption itself. We note that the entire eruption was centered within the islands' central crater.

### 3.3. Eruption volume

We estimate the total raft volume to be ~ 0.1 km³, based on an average observed area of 250 km² and a raft thickness estimate of 40 cm (Jutzeler et al., 2020). Although direct measurements of raft thicknesses are rare, multiple observations suggest that pumice rafts are typically within a range of a few tens of cm in the first weeks post eruption (Jutzeler et al., 2020). We consider 40 cm to be a reasonable estimate of raft thickness in the days following raft generation, because the raft area did not change substantially in the two weeks after the initial spread and because images of the raft from the R/V Keifu Maru on 22 Aug 2021 suggest that the raft edges were thin (e.g., < 0.5 m thick) (Japan Meteorological Agency, 2021). Assuming 60% packing of clasts and 70% clast porosity, the total pumice volume is 60 million m³ for a dense rock equivalent volume of 18 million m³.

Using the pre-eruption bathymetry (see methods), we estimate a submerged sediment volume of ca. 47 million m³, subaerial volume of ca. 6 million m³, and thus a total minimum new island volume of 53 million m³. Assuming a denser packing of 80% and clast porosity of 30% for the material that formed the island, the dense rock equivalent volume is approximately 29 million m³. This estimate does not include an unknown volume of material that may have sedimented downslope, for example in submarine density currents, and thus represents a minimum estimate. By summing the raft and island DRE volumes, we estimate a total minimum DRE volume of 0.05 km³, which is similar to the DRE volume estimate of 0.03–0.1 km³ from Maeno et al. (2022). However, these estimates may later be revised upward as submarine surveys have been recently conducted (Tani et al., 2022). Assuming a dense rock equivalent density of 2500 kg m⁻³, the total erupted mass (including the new island and pumice raft) is a minimum of ~ 1.2 × 10¹¹ kg.

### 3.4. High resolution analysis of the pumice raft and eruption

Four hours after the eruption onset, the first available Digital-Globe image shows a ~2 km diameter, oval-shaped, grainy feature (lower reflectance than seawater) at sea level with a sharp border that surrounds the vent area [Fig. 4C & 4D]. In part because this feature extends beyond the island, we interpret the feature to be recently erupted pumice clasts that have fallen out of the plume (fallout and ballistics). These clasts have been rafted outwards from the vent and have partly merged with a slightly older and more dilute portion of the pumice raft. We interpret the feature to be a pumice raft, rather than a pyroclastic surge traveling over water (e.g., Bogoslof Volcano, 28 May 2017, Waythomas et al., 2020), partly because of the absence of lobe and cleft structures typically observed for pyroclastic surges. Fig. 4D shows water vapor contrails that have been emitted from point sources (features on the scale of the image pixel resolution, ~0.5 m) within the newly formed raft and that are northwest oriented towards the eruption column. We interpret the point sources of the vapor contrails to be cooling pumiceous bombs, in part because the steaming intensity decreases outwards from the plume.

#### 3.4.1. Color variation within the raft

All images of the <1-week old pumice raft show bands and complex geometries highlighted by variations in raft color [Fig. 5]. Yoshida et al. (2022a) demonstrated that the FOB raft contained both black and grey clasts and suggested that raft color varied

because of the variation in individual clast colors. However, we interpret the raft's color and surface morphology to represent differences in raft thickness due to changes in albedo and reflectance properties. Although the specific causes of reflectance changes in rafts have not been studied, thick rafts are achieved through clast stacking (Jutzeler et al., 2020), which affects surface roughness and likely changes albedo. Thick rafts also raise the top pumice clasts entirely out of seawater (Simkin and Fiske, 1983), where clast drying may contribute to reflectance changes. Evaporation enhanced by the raft has been seen to create low atmospheric clouds above the pumice raft (Jutzeler et al., 2020).

Fig. 5 shows examples of thick and dark portions of the pumice raft grading into thinner and paler areas where ocean swell is observable. The raft's color correlates well with the amplitude of the local swell [Fig. 5B]. Correlations between raft thickness and swell amplitude were witnessed in the 2019 Tonga raft (Jutzeler et al., 2020), furthering supporting the interpretation that raft color correlates with raft thickness rather than clast composition or color. Thick rafts are known to dampen swell and waves (Jutzeler et al., 2020) similar to oil slicks (Shen et al., 2019) and sea ice (Kohout et al., 2020; Passerotti et al., 2021; Rogers et al., 2021), by reducing wind drag on the ocean surface. We see strong patchwork-like variations in swell dampening that correlate with raft color in the FOB raft [Fig. 5]. Because ca. 30 cm thick pumice rafts have been shown to strongly reduce waves, whereas thin < 10 cm thick rafts have weak to no influence on wave height (Jutzeler et al., 2020), we interpret the FOB raft to range between 10 and 50 cm (maximum observed raft thickness (Jutzeler et al., 2020)) in the first week of dispersal.

The concentric visual patterns present soon after raft formation, interpreted as signatures of thickness differences, are pervasive throughout the following weeks of rafting [Figs S6-S7]. As the raft moves away from the vent, these patterns have been deformed into complex curves and swirls within the main rafts potentially due to a combination of wind forcing, ocean eddies, and ocean waves. In addition to these swirls, other complex geometries related to collisions between segments of the raft are also common [Fig. 5E]. These structures demonstrate that pumice clasts do not behave as single isolated inertial particles once they are part of a raft but instead have significant particle-particle interaction as in a granular flow. Thus, pumice rafts may provide useful opportunities to test models for the dispersal and transport of sea ice and marine plastics with a different material.

#### 3.4.2. Fine-scale raft features

A number of fine-scale structures were present in the raft at proximity to the vent. The proximal raft tracked both radial water waves that emanated from the vent and ocean swells [Figs. 4 & 5]. The raft interaction with ~NS ocean swells created (~10 m) laminations [Fig. 4B] and the combined interactions between the raft, radial waves, and ocean swells created interference patterns [Fig. 5D]. Checkerboard patterns were present in the raft in subsequent days [Fig. S6]. We interpret many of these to be due to interaction between wind and ocean waves because the raft fragments were repeatedly oriented parallel to ocean swell and wind directions. None of these small-scale structures are captured by existing raft dispersal models (Bryan et al., 2004; Jutzeler et al., 2014, 2020), which describe dynamics at the 10s of km scale. However, these small-scale processes are critical for understanding the initial raft dynamics and for accurate prediction of raft motion near coastal areas where small-scale dynamics control raft motion near harbors or beaches.

#### 3.4.3. Large-scale raft transport

We assessed large-scale raft transport velocities for the first week of rafting using imagery from seven satellites [Fig. 4E, Ta-
<table><caption>**Table 1**
Mass flux estimates of the climactic (high plume producing) phase of the FOB eruption.</caption><thead><tr><th rowspan="2">Mass flux from plume top height (eq. (1))</th><th rowspan="2">Mass flux from volume flux (eqs. (2) & (3))</th><th colspan="2">Mass flux from total erupted mass (eq. (4))</th><th rowspan="2">Mass flux from plume height and plumeria (Maeno et al., 2022)</th></tr><tr><th>Assuming a 28 hour duration</th><th>Assuming a 51 hour duration</th></tr></thead><tbody><tr><td>1.4 × 10<sup>7</sup> kg s<sup>-1</sup></td><td>1.4 × 10<sup>6</sup> kg s<sup>-1</sup></td><td>1.2 × 10<sup>6</sup> kg s<sup>-1</sup></td><td>6.5 × 10<sup>5</sup> kg s<sup>-1</sup></td><td>3-6 × 10<sup>5</sup> kg s<sup>-1</sup></td></tr></tbody></table>

ble S1). During the first weeks of dispersal, the raft traveled 250 km NW at 12 to 42 km/day (average of 35 km/day), which is faster than the historical worldwide speeds of 6 to 24 km/day (17 km/day average) (Jutzeler et al., 2014). Yoshida et al. (2022a) suggested that the Kuroshio Counter-Current caused the 2021 pumice raft to drift faster than the 1986 FOB pumice raft.

In late October 2021, the raft washed up on the Nansei Islands of Japan, including Okinawa Island, ~1400 km west of FOB and later reached Taiwan, Thailand, the Philippines, and the Kanto region of Japan (Yoshida et al., 2022b). The raft caused significant clogging of ports, damage to vessels, and impairment of fishing and tourism industries (Ohno et al., 2022; Kyodo News, 2021). Efforts were made to remove pumice clasts from ports using heavy machinery and in some cases oil fences were put up to prevent the pumice from entering harbors (Ohno et al., 2022). By 4 Feb 2022 almost 30,000 m³ of pumice was collected within the Okinawa Prefecture (Okinawa Prefecture, 2022).

### 3.5. Eruptive mass flux

We estimate the total mass flux (kg/s) of the eruption using the plume height method (eq. (1)), volume flux method (eqs. (2)-(3)) with $Q = 1.5 \times 10^9 m^3 s^{-1}$, and total erupted mass method (eq. (4)) where we consider an erupted mass of $1.2 \times 10^{11}$ kg. For the total erupted mass method, we first calculate flux assuming the mass was erupted during the most vigorous and steady eruption phases 1 and 2 (28 hours total). Although eruption phase 3 exhibited unsteady behavior, we also calculate a flux assuming that the mass was erupted over phases 1-3 as defined by Maeno et al. (2022) (51 hours total) (Table 1).

We find significant variation in our estimates of mass flux based on the method applied. The mass flux estimate based on plume top height (eq. (1)) is higher, by at least an order of magnitude, than those determined from volume flux and total erupted volume. The estimates of mass flux from the volume flux method (eqs. (2)-(3)) and total erupted volume method (eq. (4)); 28 hour duration) are consistent. However, we quantified volume flux for the initial and most vigorous stage of plume activity, whereas our observations show that the eruption was unsteady and waned with time (e.g., supplementary video S1). As a result, the eqs. (2)-(3) mass flux estimate may represent a maximum flux. We further distinguish the initial mass flux estimated (eqs. (2)-(3)) from an average mass flux estimated with eq. (4). When the average flux is estimated with eq. (4), the time over which we assume material is being ejected from the vent highly influences our results ($1.2 \times 10^6 kg s^{-1}$ vs. $6.5 \times 10^5 kg s^{-1}$ for eruption durations of 28 and 51 hours, respectively) and puts a lower and upper estimate on average mass flux.

We also note that Maeno et al. (2022) estimated a FOB mass flux of $3-6 \times 10^5 kg s^{-1}$ based on a plume height of 16 km and using a novel implementation of Plumeria. Other than the mass flux estimate from eq. (1), our estimates of mass flux agree within a factor of two ($0.6-1.4 \times 10^6 kg s^{-1}$) and overlap with that from Maeno et al. (2022). We speculate that the discrepancy between the mass flux estimate from eq. (1) and the others may be a result of the water-rich nature of the plume.

The above discrepancies highlight the uncertainty associated with mass flux estimates. In addition, eqs. (1)-(3) were calibrated

based on models and observations of ash rich plumes. In contrast, the volcanic particles largely fell out of the water-rich FOB plume close to the vent, thereby raising questions about the applicability of eqs. (1)-(3). Despite the decoupling of volcanic particles from the FOB plume, we argue that eqs. (1)-(3) can be reasonably applied by assuming that most of the magma thermal energy was transferred to the water vapor plume (i.e., through vaporization and heating of seawater). This is because the scalings in eqs. (1)-(3) are achieved by considering a buoyancy flux at a point source. In general, the thermal energy ejected from the vent (buoyancy flux), together with air entrainment, largely control plume dynamics (e.g., Carey and Bursik, 2015; Morton et al., 1956).

## 4. Discussion

### 4.1. Raft generation processes

Despite minimal real-time observations of pumice raft formation to date, several different processes have been proposed in the literature (Cas and Giordano, 2014). Since vapor-filled clasts can be buoyant, it is possible that fragmented clasts can rise buoyantly to the ocean surface (Allen and McPhie, 2009; Fauria and Manga, 2018). Alternatively, raft-forming clasts can be delivered from ballistic ejecta from shallow submarine eruptions, by aerial plume fallout (Elser et al., 2015), or by pyroclastic currents traveling over water (Dufek et al., 2009; Jutzeler et al., 2017). The satellite and direct visual observations from the initial phase of the FOB eruption provide novel constraints to distinguish among these proposed mechanisms for delivering pumice clasts to the sea surface.

For the FOB eruption evidence suggests that the raft was, at least partially, created by ballistic delivery of clasts to the ocean surface by collapsing tephra jets and by near vent fallout of clasts from the eruption column. Satellite imagery shows that the raft was created at or very close to the vent, contrary to a scenario in which a diffuse raft would be created from fallout from a spreading aerial plume (e.g., Elser et al., 2015). A low-resolution video from the Japan Coast Guard on August 13 shows a collapsing jet near the base of a large aerial plume (Submarine Volcano Database, 2021). The August 15 Japan Coast Guard video more clearly reveals the ejection of pyroclastic material in classic cocks'tail tephra jets that quickly collapse to the sea surface (Houghton et al., 2015; Kokelaar, 1983). We therefore posit that tephra jets, ballistics, and near-vent fallout from an unstable eruption column conjointly formed islands and pumice raft. However, we cannot fully rule out contributions to the pumice raft from buoyant rise of clasts to the ocean surface or the possibility that proximal fallout created short-lasting pyroclastic surges that fed the raft and island. Processes that distributed volcanic material below the ocean surface (for example, submarine density currents) were also unobservable by satellite methods and submarine surveys are needed to understand the volume and dispersal distances of submarine products. Regardless, observations presented here show that tephra jets and near-vent fallout delivered a significant fraction of clasts to the ocean surface at FOB. Tephra jets and near-vent fallout should therefore be regarded as plausible pumice raft formation mechanisms and are likely prevalent in some eruptions such as FOB.

The role of magma-water interactions in fragmenting the FOB pumice clasts cannot be discerned from our remote-sensing based
analyses. We point out, however, that the FOB raft was made of pumice lapilli and bombs (centimeter to decimeter size clasts) (Yoshida et al., 2022a) while efficient molten fuel coolant interactions (MFCI) have traditionally been thought to cause intense fragmentation that produces large proportions of <1 mm clasts (Sheridan and Wohletz, 1983; Walker, 1973). Seafloor surveys are important because an unknown fraction of clasts did not float and therefore the total grain size distribution cannot be discerned from the raft alone. Analysis of clasts from the raft (e.g., Yoshida et al., 2022a) and from seafloor surveys taking place in 2022 (Tani et al., 2022) will be important for addressing the primary causes of fragmentation.

Regardless of the fragmentation mechanism, the existence of the water-rich plume suggests that magma-water interactions played a significant role in the dynamics of this eruption. We expect, for example, that the heating and vaporization of seawater by pyroclasts and magma supplied significant water vapor to the aerial plume. This is in part because ejected pyroclasts traveled through at most 40 m of water, thereby facilitating some degree of heat transfer. In addition, the clasts that made the shallow tuff ring island were likely hot at emplacement time and a portion of their thermal energy may have gone into the heating and vaporization of seawater.

## 4.2. Particle-plume coupling

The Japan Coast Guard observations show that centimeter- to decimeter-size pumice clasts collapsed from the eruption column and were not carried upwards in the water-rich plume, which indicates that the rise speed of the plume was slow, specifically less than the fall speed of the clasts (Carey and Sparks, 1986; Lippert and Woods, 2018). Prior modeling work has shown that external water can suppress the rise speed of plumes within several kms of the vent because of the energy associated with heating and vaporization of water (Mastin, 2007; Van Eaton et al., 2020). We suggest therefore, that volcanic clasts could be transported upwards within a high intensity jet region, but that the rise speed of the buoyancy-driven plume region immediately above the jet region was slow, thereby allowing the clasts to fall out. This scenario shares similarities with the 1963 basaltic eruption of Surtsey that created classic cock's tail tephra jets and steam-rich eruption columns that rose ~9 km to the tropopause (Thorarinsson et al., 1964). Like the 2021 FOB eruption, the 1963 Surtsey eruption displayed "rapid decoupling and fallout of large poorly fragmented clasts" and "the remaining steam-rich parts of the plume rise as a series of thermals" (Carey and Bursik, 2015).

We did not document fine ash within the aerial plume or see evidence for ash fallout (e.g., no discolored water below the aerial plume; Supplementary Material). In contrast, ash fallout from the umbrella cloud of the 15 Jan 2022 Hunga Tonga-Hunga Ha'apai eruption discolored a > 60,000 km² area of ocean water (Barone et al., 2022). It is possible, however, that small amounts of ash were shielded from satellite sensors and fundamental questions remain regarding the extent to which the FOB plume, and water-rich plumes more generally, are truly tephra poor (especially in fine ash particles). That is, we cannot completely rule out the possibility of fine ash fallout from FOB despite a lack of evidence for far-field ash transport. For context, eruptions from Bogoslof in 2017 created water-rich eruption clouds but also generated downwind fall deposits (Schneider et al., 2020). The water-rich 2018 Anak-Krakatau eruption created a thin tephra deposit, albeit in the opposite direction of dispersal to its water-rich plume (Cutler et al., 2022). In general, constraining the nature and amount of fine ash in plumes is critical for assessing aviation and environmental impacts of submarine eruptions. Finally, unlike ash-rich plumes, vapor-plumes

may not create far field deposits and thus are likely missing from the geological record.

## 4.3. Utility of satellite remote sensing observations

The August 2021 FOB eruption is unique, in part, because a combination of satellites facilitated integrated analysis to a degree that is largely unprecedented for submarine eruptions [Fig. 6]. Eruption processes that evolve within hours and that are at approximately the meter scale are still not resolvable with current satellite data, however are important for understanding eruption produce delivery mechanisms to the ocean and atmosphere [Fig. 6]. Although our study has relied primarily on visual satellite imagery, future studies can take advantage of the full multispectral information from the various instruments to get more detailed constraints on plume dynamics and raft composition (Williams and Ramsey, 2022), especially high-resolution SAR datasets (e.g., Sentinel-1, NI-SAR, commercial satellites). We strongly recommend that medium and ultra-high resolution satellites should be tasked to repeatedly image known submarine volcanoes to determine background conditions, identify and monitor eruptions and landslide events, and be part of coordinated rapid responses.

## 5. Conclusions

A clear view of the August 2021 eruption of Fukutoku-Oka-no-Ba, Japan was enabled by data from a combination of satellite sensors and complemented by direct observations by the Japan Coast Guard. We determined an initial volume flux of $1.5 \times 10^9$ m³ s⁻¹ in the aerial plume using geostationary satellite Himawari-8 data, measured plume heights of 16 km, and documented that the aerial plume contained SO₂ and abundant ice. Central to questions about the timing and style of raft creation, we concluded that the 0.1 km³ pumice raft and 16 km high aerial plume formed at the same time. Furthermore, satellite observations of a water-rich aerial plume and video from the Japan Coast Guard, together demonstrated that the pyroclasts were largely decoupled from the aerial plume and delivered to the ocean surface by tephra jets and near vent fallout. We therefore suggested that the rise speed of the buoyant plume near the vent was slow enough to facilitate the fallout of lapilli clasts. We could not find evidence for ash in the eruption plume, however we highlight that it is still uncertain the degree to which water-rich plumes like FOB lack volcanic ash. Finally, we used ultra-high resolution imagery to observe complex structures within pumice rafts, variation in raft thickness, and effects of wind and waves on raft dispersal.

### Open research

This study relied on satellite remote sensing data from Himawari-8 and the satellites listed in table S1. Code and processed data used in this analysis will be made available in the supplemental materials and on zenodo.org.

### CRediT authorship contribution statement

Kristen E. Fauria: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Supervision, Visualization, Writing – original draft. **Martin Jutzeler:** Conceptualization, Investigation, Methodology, Project administration, Visualization, Writing – original draft. **Tushar Mittal:** Conceptualization, Investigation, Methodology, Project administration, Supervision, Visualization, Writing – original draft. **Ashok Kumar Gupta:** Investigation, Methodology, Visualization, Writing – original draft. **Liam J.**
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** (A) Estimates of relevant timescales and spatial scales for important processes related to submarine volcanism. (B) Temporal and spatial resolutions of satellite data that can be used to examine volcanic processes. Data volume as plotted is estimated as proportional to 1/(resolution² × revisit time). That is, data volume as estimated is not dependent here on global coverage area or the spectral resolution/number of bands.</figcaption></figure>

**Kelly:** Investigation, Visualization, Writing – review & editing. **John Rausch:** Investigation, Methodology. **Ralf Bennartz:** Funding acquisition, Methodology, Supervision, Writing – review & editing. **Brent Delbridge:** Investigation, Methodology, Writing – review & editing. **Lise Retailleau:** Investigation, Methodology, Visualization, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

All satellite data used in this study are publicly accessible, with the exception of MAXAR and Planet data. Supporting code and data files are available at https://doi.org/10.5281/zenodo.7689653.

Acknowledgements

DigitalGlobe/Maxar data were provided by the Commercial Archive Data for NASA investigators (cad4nasa.gsfc.nasa.gov) under the National Geospatial-Intelligence Agency's NextView license agreement. This work was funded by National Aeronautics and Space Administration grant 80NSSC20K1450 (K.E.F., R.B., A.G.,
L.J.K.). TM was supported by a Massachusetts Institute of Technology Crosby fellowship. We thank the Massachusetts Institute of Technology Library for providing access to imagery from Planet Labs. The Himawari-8 radiance data are obtained from the National Institute of Information and Communications Technology (NICT) Science Cloud, Japan. The authors wish to thank the World Wide Lightning Location Network (http://wwlln.net), a collaboration among over 50 universities and institutions, for providing the lightning location data used in this paper. The local earthquakes location was extracted from the USGS earthquakes catalog (earthquake.usgs.gov/earthquakes). The seismic recordings were accessed through the IRIS datacenter (http://www.iris.edu/mda) with the module Obspy. We thank Alexa Van Eaton and an anonymous reviewer for their constructive comments that greatly improved this manuscript. We also thank editor Chiara Petrone for handling this manuscript and Michael Manga and three anonymous reviewers for commenting on an earlier version. Finally, we acknowledge the Japan Meteorological Agency and Japan Coast Guard for their important roles responding to this eruption.

## Appendix A. Supplementary material

Supplementary material related to this article can be found online at https://doi.org/10.1016/j.epsl.2023.118076.

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