Volcaniclastic density currents explain widespread and diverse seafloor impacts of the 2022 Hunga Volcano eruption

Domain: earth_science
Published: 2023-11-30
Editorial outcome: major_revision

Abstract
The impacts of large terrestrial volcanic eruptions are apparent from satellite monitoring and direct observations. However, more than three quarters of all volcanic outputs worldwide lie submerged beneath the ocean, and the risks they pose to people, infrastructure, and benthic ecosystems remain poorly understood due to inaccessibility and a lack of detailed observations before and after eruptions. Here, comparing data acquired between 2015-2017 and 3 months after the January 2022 eruption of Hunga Volcano, we document the far-reaching and diverse impacts of one of the most explosive volcanic eruptions ever recorded. Almost 10 km3 of seafloor material was removed during the eruption, most of which we conclude was redeposited within 20 km of the caldera by long run-out seafloor density currents. These powerful currents damaged seafloor cables over a length of >100 km, reshaped the seafloor, and caused mass-mortality of seafloor life. Biological (mega-epifaunal invertebrate) seafloor communities only survived the eruption where local topography provided a physical barrier to density currents (e.g., on nearby seamounts). While the longer-term consequences of such a large eruption for human, ecological and climatic systems are emerging, we expect that these previously-undocumented refugia will play a key role in longer-term ecosystem recovery.

Introduction
Explosive volcanic eruptions can have global impacts, producing large eruption plumes and volcanic deposits, spawning powerful density currents, triggering far-reaching tsunamis, damaging infrastructure, and leading to loss of human life1–3. The ash and chemicals released by large volcanic eruptions are also a driving factor in global climate variability, and can affect biogeochemical cycles, ecosystem functioning, associated carbon cycling, and local biological communities4–7. Despite their significance, our understanding of the magnitude, extent and diversity of impacts of major eruptions is poor, due to a paucity of pre- and post-eruption observational data8,9. This knowledge gap is particularly acute for submarine volcanoes, where eruptions are often undetected or underreported10. Despite amounting to >75% of all magmatic outputs worldwide, submarine volcanic eruptions account for only 8% of those recorded11–13. This observational bias is severe in regions under-represented by scientific study, such as the South Pacific Ocean where the majority of the world’s 100,000 uncharted underwater volcanoes of >1 km in elevation lie14. In particular, shallow-water, active volcanic centres that are near populated islands expose a major blind spot regarding risk assessment and response preparedness15,16.

This lack of awareness was illustrated by the highly explosive eruption of the partially-submerged Hunga Tonga-Hunga Ha’apai (hereafter referred to as Hunga) Volcano on 15 January 2022. With a Volcanic Explosivity Index of 5.7, the 15 January eruption was the largest eruption since Mount Pinatubo (Philippines) in 1991 and is one of the most explosive submarine eruptions ever recorded. The Hunga Volcano eruption had global impacts, yet came with little warning8. The eruption generated a steam rich eruption plume (a cloud of ash and tephra suspended in gases) that reached the lower mesosphere17. Near-term impacts included atmospheric shockwaves, a tsunami that crossed the Pacific Ocean, loss of human life, and damage to seafloor cables that severed telecommunications for the Kingdom of Tonga at a critical time for disaster response18–21.

The Hunga Volcano eruption highlights the potential risks posed by the 22 mapped volcanoes in the Kingdom of Tonga, hundreds more along the Tonga-Tofua-Kermadec Arc, and many others worldwide. Most submerged volcanoes are poorly mapped, and data from surveys before and after eruptions that can be used to quantify pre- and post-event seafloor conditions are rare22–25. No such data previously existed for an event on the scale of the 2022 Hunga Volcano eruption. This data paucity means that the full range of seafloor impacts of large eruptions remain enigmatic. Here, we address this knowledge gap, integrating a multi-disciplinary dataset acquired just 3 months after the 2022 Hunga Volcano eruption26 with data acquired prior to the eruption27,28. Combining these datasets (including multibeam echo sounder, geological data, video footage and water column samples), we provide an overview of the wide-reaching ocean impacts of such a large eruption, ranging from topographic changes and damage to critical infrastructure, to widespread loss of seafloor life.

Using seafloor mapping surveys, we show that almost 10 km3 of seafloor material was removed following the eruption and document the fate of the material, providing a first budget for seafloor loss/gain after the eruption. Through integration of seafloor mapping, sediment coring, geochemical analyses, and numerical modelling we find the extensive reshaping of the seafloor was caused by long run-out (>100 km) volcaniclastic density currents. Pre-existing seafloor relief funnelled these flows, explaining spatially variable trends in erosion and deposition. Using seafloor video surveys, we document an apparent widespread loss of biological communities, due to physical disruption or smothering of seafloor habitats, and discuss the broader implications of major submarine eruptions for ecosystem structure, function, and recovery.

Results
Major but localised seafloor reshaping resulting from the 2022 Hunga Volcano eruption

The islands of Hunga Tonga and Hunga Ha’apai are remnants of the rim of a roughly 3- to 4-km wide submarine caldera of Hunga Volcano, with eruptions noted in various parts of the caldera throughout the past century. The most recent eruption series (in 2015) formed a 120 m high and 2 km wide near-circular tephra cone which joined the two islands together18,27,28. After this eruption, Hunga Volcano was largely inactive until it began erupting in late 2021, producing a steam-rich gas and ash plume17,18. This eruption culminated with the explosive events on the 15 January 202229. Satellite imagery on 15 January, prior to the largest eruptive episode, showed the islands had been disconnected by the eruption sometime before this29. After the 15 January eruption, only small remnants of the islands of Hunga Tonga and Hunga Ha’apai remained above water (Fig. S5). However, the modification of the submerged fraction of the volcanic edifice (~99% of Hunga Volcano) remained unknown.

A map of bathymetric change (derived by subtracting new bathymetric data from pre-eruption surveys between 2015 - 2017) reveal the seafloor morphologic fingerprint of a major eruption. Significant change occurred on the summit, where an increase in depth >800 m was observed in the caldera (Fig. 1). Integrating depth changes over selected areas of the map (see Methods), this change accounted for at least 6.0 km3 seafloor loss from within the caldera (dense rock volume), equivalent to 20 times the eruptive volume of Mount St Helens (USA) in 198030. An additional 3.5 km3 was lost from the outer flanks, without impacting the general morphology of Hunga Volcano (Fig. 1). Many small features present pre-eruption, such as peaks, ridges and gullies, were visible in the post-eruption bathymetry. The deposited volume report is an uncompacted volume, while the erupted volume was dense rock; hence, direct comparison of these volumes is complicated due to their differing bulk density. Overall, 6.3 km3 of uncompacted material was deposited within 20 km of the caldera rim. Prior studies suggest that ~1.9 km3 of material was ejected into the atmosphere as an eruption plume31. The remaining material that is unaccounted for was likely deposited as widespread thin deposits below the detection limit of what can be resolved with ship-based echosounders. This notion is supported by sediment coring that sampled volcanic material linked to the Hunga Volcano eruption (Supplementary Data 1), with some material likely also deposited beyond our survey area21.

There was a widespread loss of material from Hunga Volcano in water depths shallower than 600 m; however, the seafloor changes in deeper water are more varied and relate to the pre-existing morphology. In water depths deeper than 600 m, which encompasses the flanks of Hunga Volcano and the area around the caldera rim, volume loss is confined to linear chutes that radiate from the caldera rim; all but one of which coincide with pre-existing channels (Fig. 2). Up to 70 m deepening occurred within the incised chutes, which are 5-10 km in length and 2 km in width. At the chute termini, the thickest accumulations of sediment are observed as lobate forms up to 22 m thick, with widespread blanketing of sediment across the pre-eruption seafloor. The most extensive erosion occurred within a new chute, which was not visible in pre-eruption data, that initiated at a topographic low at the NNW edge of the caldera between the two islands (Fig. 2a, Profile A). Most of the measured loss was within 6.5 km of the caldera rim, beyond this deposition dominated. There is no superficial morphology consistent with seafloor landslides (i.e., head scarps and/or scars) on the volcano flanks despite the scale of the eruption and associated seismicity, which makes this event distinct from partially-submerged eruptions that generated large slope collapses such as Anak Krakatau (Indonesia) in 201832.

Volcaniclastic density currents as the primary agents of seafloor change on the volcano flanks

We relate these spatially variable changes in seafloor elevation on the volcano flanks to density currents laden with volcaniclastic material that radiated downslope from Hunga Volcano in multiple directions, exploiting and enhancing pre-existing topography (Figs. 2a, S6; Supplemental Movie 1). We suggest these flows initiated as collapsing eruption column material that plunged into the ocean and mixed with the ambient seawater, becoming turbulent underwater density flows (e.g., turbidity flows) that were steered by the complex topographic relief around the volcano. As we cannot fully discern the point at which flows transitioned from hot gas supported pyroclastic density currents to cooler turbidity currents, we refer to them as volcaniclastic density currents21,33. The volcaniclastic density currents most likely initiated from the partial collapse of the eruptive column of Hunga Volcano, with this hypothesis similarly supported by the timing and intensity of the tsunami waves and images of the base of the column21,34. However, primary magmatic explosivity or hydrovolcanic explosions could also have played a role. Sensitivity tests of our density current model (see Supplementary Material, Fig. S6), however, confirm that whatever initiation mechanism caused them, it most likely occurred within the caldera in order to cause the strong channelisation that was observed on the flanks of Hunga volcano and extends up to the caldera rim.

We observed crescentic scours (up to 30 m deep) and bedforms (up to 1200 m wavelength, 20 m wave height; Fig. 2), commonly seen in other deep-sea settings affected by powerful density flows35, including density flows triggered by large magnitude earthquakes that severed seafloor cables and attained speeds of up to 20 m/s36–38. These observations are further corroborated by seafloor sediment coring and imagery, geochemical analyses and numerical modelling. Multicoring 80 km from the caldera sampled a volcaniclastic deposit with an abrupt basal contact above pre-eruption hemipelagic deposits, overlain by fine volcanic ash deposits tens of centimetres thick (Figs. 3, S1; Table S1). The entire deposit is coarser than the sedimentary layers below, and fines upwards – as expected from a particulate density current deposit. The density flow deposit dominantly comprises sand-sized mixed volcaniclastic material with up to 4 mm-diameter scoria and occasional preserved ripples, which are also observed in seafloor photos (Fig. S9). Similar density current deposits were sampled along a transect approaching the caldera (Fig. S2). Major elemental chemical compositions of volcaniclastic deposits were measured by X-Ray fluorescence spectrometry and overlap with ashfall collected from Tongatapu (main island of Kingdom of Tonga) post-eruption (measured by scanning electron microprobe analysis) as well as recent eruptions from Hunga volcano39, and are distinct from other regional volcanoes (Supplemental Information; Figs. S3, S4; Supplemental Data 1). A lack of bioturbation within the volcaniclastic deposit, and no indication of oxidation, indicate that it is a relatively fresh deposit, as well as the absence of any hemipelagic layering on top of the volcaniclastic deposit. These combined observations support an interpretation of fast-moving and locally erosional flows that blanketed the flanks of the volcano and beyond, resulting in widespread deposition. Bathymetric difference mapping shows that the density currents were predominantly erosional on the upper slopes of the Hunga Volcano edifice, entraining an additional 3.5 km3 of material; more than the volume of the largest documented historical onshore landslide (Mount St Helens in 198040).

Numerical modelling of the submerged density currents (see Methods section for further details) suggest that ~3–4 km3 of flow is required, originating from inside the caldera rim, to replicate the extent of the observed deposits, especially the overtopping of two submarine knolls to the south heading toward the international telecommunications cable that was damaged extensively after the eruption (Fig. 3). While our relatively simple model does not account for seafloor erosion, incorporation of additional material, nor multi-phase flow, the flow pathways that the simulated density currents follow align remarkably well with the areas of most pronounced seafloor change. The model (especially the density current flow paths) is consistent with the locations and extensive damage to the international and domestic telecommunication cables, damaged along 89 km and 105 km length, respectively21 (Fig. 3). Furthermore, the modelled flow paths taken by the faster moving and thicker (in m) parts of the density current correlate well with the observed spatial trends in erosion and flow paths. The most pronounced erosion occurred immediately to the NNW of the caldera, where the modelling shows part of the density current being funnelled through an area of negative seafloor relief between the two islands and continuing downslope to the northwest. Post-eruption bathymetry suggests this part of the density current went well-beyond the northwest limit of our survey area. Overall, as the simulated density currents flow away from the caldera, the model illustrates how these seafloor flows can be steered by the local topography and how, in some locations, separate flows can overcome bathymetric barriers, converge back together and accelerate. These modelled results illustrating the paths of the simulated density currents corroborate the areas of impact observed in the seafloor surveys presented here (Fig. S19). Flow paths like this have previously been observed subaerially but only inferred in submarine settings33.

Wider impacts of the eruption on ocean ecosystems and the role of topographic refugia

The density currents also exerted dramatic, and differential impacts on seafloor ecosystems over a large area. Towed camera footage of the seabed after the eruption revealed an absence of epifaunal biota on the flanks of Hunga Volcano, in the chutes and across most of the surrounding seafloor, indicating widespread loss of life following the eruption (Figs. 3, S9, 10, Supplemental Data 2). Close to the caldera, seafloor substrate was almost completely removed, indicating erosional regimes where animals would have been physically removed with the substrata (Fig. S10).

While the seafloor in most of the remaining survey area was smothered with volcanic deposits linked to volcaniclastic density currents and subsequent ashfall, areas of potential refugia from the impacts of density currents appear to have been created locally by the high and irregular seafloor relief provided by several seamounts (Figs. 3, S11, S19, Supplemental Data 2). We observed relatively undisturbed and diverse benthic communities on seamounts in the southern part of our survey area, at distances >50 km from the caldera. The density and diversity of these ecosystems align with our expectations for the warm, oxygenated and highly productive waters that exist adjacent to seamounts and volcanic islands of the Tonga-Tofua-Kermedec Arc41–43. Closer to Hunga Volcano, abundant benthic invertebrate and fish communities were also observed on seamounts in the area, presumably protected from direct eruption impacts (Fig. 3; Figs. S11, S19). The locations of these refugia match closely the thickest (in m) flow paths of the density current modelling, which shows that deflection of flow pathways around irregular relief and seamounts left some areas relatively unaffected (Fig. 3, S6, S19; Supplemental Movie 1). Pre-eruption deep-sea benthic surveys for the Hunga Volcano edifice and adjacent seamounts were not available to compare with our new post-eruption data, except for discrete observation of mussel beds on a southern seamount within our study area26. The mussel beds visualised in 2007 and 2017 on the southern seamount were still present, and of similar density to prior observations26, indicating that these communities were not affected by the eruption of Hunga Volcano (Fig. S11). Such refugia from pyroclastic density currents have been reported on land, where vegetation and people have been sheltered by local relief44–46; however, to our knowledge, this is the first time such a sheltering effect from volcaniclastic density current has been reported in a marine setting.

The removal and smothering of benthic communities and habitats have been documented following previous volcanic eruptions, albeit in different settings47–52. Ashfall from Mount Pinatubo reduced the density and diversity of foraminifera assemblages nearby in the South China Sea53, while larger animal communities experienced mass mortalities following the underwater eruption of the caldera of Deception Island in 1967-1970 (South Shetland Islands, Antarctica)54. Ocean entering pyroclastic density currents from the 2008 eruption of Kasatochi Volcano (Aleutian Islands, USA) smothered shallow-water benthic communities55. In each of these cases, benthic recovery started within a few years and, in the case of the Deception Island eruption, shallow-water (<200 m) benthic communities were thought to have recovered 14 years after the event56. In a deeper setting (700–750 m) on Vailulu’u Seamount (east of American Samoa) there was higher abundance of hormathiid anemones, hydroids, a demosponge, brittle stars and octocorals on older basaltic seafloor following a series of eruptions52, consistent with ecological succession of benthic communities, which occurred over a period of 12 years. A much slower and less complete recovery of macrofaunal communities was reported at vent-proximal sites on the East Pacific Rise at 2500 m51. We do not know the timescale over which the seafloor communities in the Hunga Volcano may recover, but we speculate that the process may be aided by re-colonisation from the refugia indicated above.

Persistent ocean impacts of Hunga Volcano were also observed within the water column. High turbidity, related to very fine volcanic ash, was recorded 3 months after the eruption at 200 m water depth up to 20 km to the NW of the caldera (Fig. S12). Future research will seek to determine if this water column turbidity is related to ongoing venting from plumes within the caldera. Near-seabed ash layers (e.g., nepheloid layers, a layer of water above the ocean floor with a significant amount of suspended sediment) were detected above the seafloor across many of our sampling sites, in some cases extending hundreds of metres up from the seafloor (Fig. S13). The turbidity anomalies (both mid-water and near-seabed layers) were sampled by Conductivity, Temperature, Depth (CTD) rosette and in all instances were dominated by ash particles, identified by scanning electron microscope imagery, with electron microprobe analysis revealing a similar composition to the volcaniclastic density current deposits and post-eruption ashfall samples from Tongatapu discussed above (Figs. S4, S14–17; Supplemental Data 1). Long-lived particulate suspensions of volcanic material have been reported for many other eruptions, with diverse ecological impacts7. Therefore, assessing the true impacts of a large eruption like the 15 January eruption of Hunga Volcano will require holistic and continued monitoring over several years to determine both short-term resilience, and longer-term recovery from the effects of the eruption.

Our data illustrate how an eruption of a single submerged volcano can have wide-reaching and diverse impacts. Volcaniclastic density currents resulting from the Hunga Volcano eruption were steered by antecedent morphology, further excavating pre-existing chutes and depositing the majority of the erupted volume on the seabed within 20 km of the Hunga caldera rim. Modelling and benthic observations indicate that the majority of the density currents originated within the caldera and flowed around bathymetric relief as they moved to deeper waters, with only local preservation of benthic communities on topographic refugia. Outside of these topographic refugia the seafloor ecosystems around Hunga Volcano were decimated by the eruption and related density currents.

While ocean impacts resulting from volcanic eruptions are typically hidden from view, we show they can have major consequences, including widespread loss of marine life and damage to critical seafloor telecommunication links, with knock-on socioeconomic impacts. The domestic cable that connects the Tongan island groups took 18 months to repair, leaving island communities without high-speed connections. The enduring effects of suspended volcanic material in the water column and the localised decimation of benthic communities may have as-yet unknown implications for ecosystem functioning, which may in turn have wider impacts on food security. Future monitoring (of both the volcanic edifice itself and the surrounding seafloor and habitats) is necessary to robustly determine the resilience and recovery of both human and natural systems to major submarine eruptions, and to more broadly assess the risks posed by the many similar submerged volcanoes that exist worldwide.

Methods
Field survey
Most of the post-eruption data included in this manuscript is from a voyage aboard the RV Tangaroa in April and May 2022. A portion of the bathymetric data is also from a survey by the USV Maxlimer in July-September 2022 as detailed below. These surveys together formed the NIWA-Nippon Foundation Tonga Eruption Seabed Mapping Project (TESMaP). The survey design aboard the RV Tangaroa was a combination of sampling at specific topographic and event locations (e.g., sites of breaks along the international and domestic communications cable and other volcanic seamounts and knolls in the immediate vicinity of Hunga Volcano), and sites along a distance (and impact) gradient on the flanks of Hunga Volcano. These latter sites comprised two transects aligned southwest and southeast from the Hunga Volcano caldera.

Seafloor imagery collection and analysis
Seafloor imagery was collected with NIWA’s Deep Towed Instrument System (DTIS), a battery-powered towed camera frame which records continuous high-definition digital video (1080p, @ 60 fps) and simultaneously takes high definition still images (24 megapixel) at 15 s intervals. The seabed position and depth of DTIS were tracked in real time using the KONGSBERG ultra-short baseline (USBL) transponder system. The seabed position of DTIS was plotted in real time using OFOP software (Ocean Floor Observation Protocol, www.ofop-by-sams.eu), with all navigation data, camera commands, and spatially-referenced observations of seabed type and the occurrence of visible mega-epifaunal invertebrates (animals living on or above the seafloor with sizes ≥2 cm) recorded to OFOP log files and captured by the ship’s Data Acquisition System (DAS). For initial analyses presented here various levels of taxonomic identification were amalgamated into higher taxa: Hexactinellida (glass sponges), Demospongiae (sponges), Alcyonacea (various gorgonian octocorals, whip corals, stony corals (reef-forming), cup corals), Hydrozoa (stylasterid hydrocorals), Pennatulacea (seapens), Holothuroidea (sea cucumbers), Ophiuroidea (brittle stars), Asteroidea (seastars), Crinoidea (crinoids), Bivalvia (vent mussels). The abundance of invertebrate taxa were counted in real time from the live video feed (totalling 31 h over a distance of 32 km) and hence the data included here should only be regarded as provisional estimates of abundance. The abundance data is available in Supplemental Data 2.

Sediment core collection and analysis
Seafloor sediment samples were collected with the Ocean Instruments MC-800 multicorer system. The cores described here were then subsampled from the multicore tubes with push core tubes. These were 80 cm long polycarbonate core liners which were inserted down the core barrel, then sealed with absorbent foam, capped and taped at either end, and stored upright in a 4°C refrigerator on board and onshore until analyses. Sediment core liners were then cut longitudinally, and the sediment parted using cheese wire to create a flat surface on the split core halves. Once split, cores were immediately photographed and subject to sedimentological core logging that comprised visual identification of sedimentary structures, grain size and texture. Grain size was visually characterised through use of a grain size comparator, which has been shown by prior studies to relate to the coarsest 5% of the grain size distribution57. Core sample observations were synthesised in a visual sedimentologic log (Figs. 3, S1, S2). Deposits were visually differentiated on the basis of their composition, colour, sedimentary textures and grain size, which included the identification of two main facies that are linked to the Hunga volcano eruption. These are (i) Volcaniclastic density current deposits, which comprise dominantly sand to granule-sized volcanic material that generally fines upwards and a very sharp basal contact (where it was sampled); and (ii) A thin veneer of orange-brown oxidised ash-rich deposits that has no obvious internal structure and that was sampled at seafloor, consistently overlying the density current deposits. As continued suspension and settling of ash was observed by video and sampling of the water column 3 months after the eruption (Figs. S13–S15), we consider these surficial seafloor deposits to be most likely related to ash fallout; however, it is possible that this facies may relate to the fallout of ash-sized material that was suspended by the volcaniclastic density current as a dilute cloud and subsequently settled out after the deposition of the coarsest load of the current, or that this relates to settling from a surface plume. Regardless, this does not affect the key conclusions of this present study.

To establish if ash preserved in the marine cores collected during the TAN2206 voyage were sourced from Hunga Volcano, nine samples of ash from four cores were analysed for major elements by quantitative bench-top X-Ray Fluorescence (XRF) spectrometry and compared with Tongatapu ash deposits, samples from the water column (both described below) and literature data. Samples were selected from core tops, with an additional four samples picked from different phases of the eruption preserved in core 90, as well as a basal bioturbated layer representative of the seafloor prior to the 2022 eruption.

Samples were prepared by soaking in deionised water (18.2 MΩ) for 5 days, with periodic (1–2 h/day) agitating in a warm ultrasonic bath. As the sample material was very fine the ash was allowed to settle overnight, drained and fresh water added. Salinity was tested each day and after 5 days the water was found to have a conductivity consistent with deionised water. The samples were then drained of water and dried in an oven at 110°C. For sample 90-1, aliquots of sample were taken after 1, 4 and 5 days washing to test the effect of washing on ash composition.

The dried samples were crushed in a ceramic mortar and pestle, which was then cleaned with detergent and dilute citric acid added to the bowl overnight to avoid contamination. Fused glass disks of the samples were prepared by mixing 1 g of sample, 10 g of flux (35% lithium tetraborate, 65% lithium metaborate) and an NH4I tablet in a platinum crucible. The mixture was then heated in a Claisse LeNeo Fluxer at 1075°C for 25 min to produce a fused glass disk suitable for analysis by XRF spectrometry.

Analyses were conducted on a Panalytical Minipal-4 energy-dispersive XRF spectrometer at NIWA, Wellington. Each analytical run consisted of three analyses at different keV energy to optimise analytical conditions for each element. Two international standards (USGS basaltic standards BHVO-2 and BCR) and an internal standard (TP-1) produced at Victoria University of Wellington, New Zealand58 were analysed together with Hunga Volcano samples. The resultant data was processed using a calibration curve derived from nine international standards. Results for samples and standards corrected for loss on ignition (LOI) by summing the elements to 100% are given in Supplementary Data 1. LOI was determined as the difference between the original total and 100% and is also given in the table. Average values for the standards are in close agreement with certified values (generally within 1-3% relative, although P2O5 is up to 9% due to its low abundance), and standard deviation analytical precisions for each element (other than Na2O and P2O5) and are generally <2% (2SD) relative.

Water Column Analysis
Water column data (particle density) was collected with sensors on a conductivity, temperature, depth (CTD) rosette equipped with turbidity optical sensors, offering an insight into the particle makeup of the water. Niskin water sampling bottles attached to the CTD rosette were fired at relevant depths throughout the water column for discrete water samples. This water was then filtered onto 25 mm polycarbonate filters and dried before analysis with the Scanning Electron Microscope.

Shards of material filtered from CTD samples were identified as volcanic glass based on their optical properties. To constrain their origin the samples were analysed for major elements together with a sample of ash collected from Tongatapu (‘Ash’ samples) a few days after the eruption. Analyses were performed at the University of Otago using energy-dispersive spectroscopy (EDS) on a Zeiss Sigma VP FEG SEM, JEOL FE-SEM 6700 scanning electron microscope. The analyses were semi-quantitative with the aim of analysing as many shards as possible to investigate their origin.

Post-eruption multibeam surveys
Complete seabed mapping of the post-eruption seabed around Hunga Volcano was one of the primary goals of TESMaP. The seabed mapping was undertaken in two parts. The first was by the RV Tangaroa (TAN2206) in April-May 2022 using a Kongsberg EM302 multibeam echosounder system which mapped the flanks on Hunga Volcano and the surrounds. The second part of the TESMaP seabed survey was undertaken by an uncrewed surface vessel, the USV Maxlimer (MAX2201), in July-September 2022 utilising a Kongsberg EM710 multibeam echosounder system. USV Maxlimer was operated from a Remote Operating Center in Essex, UK while a team of surveyors from around the world (Ireland, Poland, Egypt, Australia, and New Zealand) remotely operated the EM710 multibeam.

The Kongsberg EM302 multibeam echosounder system operates at a frequency of 30 kHz and comprises 288 beams/432 soundings per swath, with real-time beam steering compensating for ship motion. The beam width is 1° along-track and 2° across-track, producing small acoustic footprints.

Positioning was provided by GPS, differentially corrected by the Fugro SeaStar XP Wide Area Differential GPS (WADGPS). Heave and attitude data were provided by an Applanix POS/MV 320 motion sensor. Measurements of roll, pitch, and heading are accurate to 0.02° or better. Heave measurements supplied by the POS/MV maintain an accuracy of ±5% of the measured vertical displacement or ±5 cm (whichever is larger) for movements that have a measured period up to 20 s. No significant heave artefacts were observed in the processed bathymetric data.

The Kongsberg EM710 multibeam echosounder system operates at a frequency range of 70 to 100 kHz and comprises 200 beams/400 soundings per swath, with real-time beam steering compensating for ship motion. The beam width is 1° along-track and 1° across-track, producing small acoustic footprints. Heave and attitude data were provided by a Seapath 130 with MRU-5+ MK-11 motion sensor.

These multibeam bathymetry data were edited and processed during the voyage using QPS Qimera v2.3.1 software, then gridded to a 50 m cell-size.

Pre-eruption Multibeam surveys
To generate the pre-eruption Digital Elevation Model (herein referred to as bathymetry) we searched all available bathymetry repositories for survey data conducted around Hunga Volcano prior to 2022. These data repositories include the IHO DCDB, the Seabed 2030 Pacific Data Center hosted at NIWA, AusSeabed, EMODnet, and JAMSTEC. For this search we identified three surveys that were suitable to generate this pre-eruption surface (Table 1). These surveys were by the RV Falkor (FK160407) in May 2016 using a Kongsberg EM302, a small boat survey in November 2015 using a WASSP multibeam sounder, and 2017 satellite-derived bathymetry from Land Information New Zealand as part of the Pacific Regional Navigation Initiative.

These multibeam bathymetry data were edited and processed using QPS Qimera v2.3.1 software, then gridded to a 50 m cell-size. Corrections for sound velocity were provided either by the direct SV measurement by the deployment of sound velocity probes or by water sound speed calculated from conductivity, temperature and pressure-depth (CTD) values. Refraction residuals for these surveys were further reduced using the TU Delft Speed Sound Inversion tool within Qimera59.

To determine the spatial extent and vertical change in the seafloor from the 15 January eruption we used the TESMaP and the pre-eruption bathymetry surfaces to perform a change detection analysis. The pre- and post-eruption DTM surfaces are co-registered to further reduce vertical and horizontal uncertainties on any analysis. Analysis of the DTMs was undertaken using ArcPro GIS software where many bathymetric derivative surfaces (for example slope, aspect, and curvature) were generated to understand the geomorphology. To determine the spatial extent and vertical change in the seafloor from the 15 January eruption, the post-eruption bathymetry was subtracted from the pre-eruption bathymetry. Volumetric calculations of differences between the pre- and post-eruptions DTMs were undertaken using the Cut Fill tool, on the summit and flanks of the Hunga Volcano (Fig. S18).

Vertical uncertainties for multibeam acquired bathymetric data are depth dependent and are usually reported as being 1% of depth (ranging from 20 cm to 20 m). The variable uncertainty makes any quantitative volume estimation difficult. However, morphology as measured by multibeam echo sounders is typically not affected by vertical uncertainties, and if so we would expect artifacts in the difference maps which we do not see. While quantitively estimates of gain/loss of seafloor will be impacted by this uncertainty range, morphological changes are not. Pre- and post-eruption digital elevation models can be accessed at 10.5281/zenodo.7456324. To determine the percentage of the volcano area that is subaerially exposed, the area of the submerged edifice was defined by a change in slope of 3° from the base of the edifice. This result was compared to the area of the pre-eruption island area to demonstrate that <1% of the volcano is subaerially exposed.

Volcaniclastic density current modelling
The turbulent gravity-driven current generated when the volcaniclastic density current mixed with seawater and the ocean above was modelled with the Basilisk modelling software60 using a two-layer model with the lower layer representing turbulent density current and the upper layer representing the overlying sea water61. This approach allows us to model the submarine density current flowing over the complex bathymetry at the same time as we model its effects on the sea water above it. The vertically-averaged density current was modelled as a dense incompressible Newtonian fluid using the Saint Venant (Shallow Water) Equations affected by the lighter fluid (the sea water) over top. The vertically-integrated equations for the density current are:

∂h_s / ∂t + ∂(h_s u_s) / ∂x + ∂(h_s v_s) / ∂y = 0 (1)

∂(h_s u_s) / ∂t + ∂(h_s u_s^2 + 1/2 gh_s^2) / ∂x + ∂(h_s u_s v_s) / ∂y + gh_s ∂(ρ_s h_s + z) / ∂x = F_sx + τ_sx / ρ_s (2)

∂(h_s v_s) / ∂t + ∂(h_s u_s v_s) / ∂x + ∂(h_s v_s^2 + 1/2 gh_s^2) / ∂y + gh_s ∂(ρ_s h_s + z) / ∂y = F_sy + τ_sy / ρ_s (3)

Where h_s is the thickness of the volcaniclastic density current and h is the depth of the sea water above it. The top equation is derived by vertically integrating the mass conservation equation and the bottom two equations the momentum. The vertically-averaged velocity of the volcaniclastic density current in the x (east) and y (north) directions are u_s and v_s. ρ_s and ρ are the density of the volcaniclastic density current and sea water respectively, g is the acceleration due to gravity. F_s is the contribution of friction and τ_s shear of the turbidity current with x and y denoting the direction. In these equations the flow rheology is prescribed by the friction, in this model a simple quadratic friction is used and the coefficient is set to 10^-4 for the density current, similar to the water as physical modelling results suggest these volcaniclastic density currents are highly mobile and act very similarly to dense fluids62. We assume a free slip condition in the bottom boundary when deriving these equations, but other boundary conditions can be accommodated by τ_s.

The sea water was modelled using the Saint Venant Equations influenced by the denser submarine density current layer underneath:

∂h / ∂t + ∂(hu) / ∂x + ∂(hv) / ∂y = 0 (4)

∂(hu) / ∂t + ∂(hu^2 + 1/2 gh^2) / ∂x + ∂(huv) / ∂y + gh ∂(h + z) / ∂x = F_x + τ_bx / ρ + h g ∂η / α∂x (5)

∂(hv) / ∂t + ∂(huv) / ∂x + ∂(hv^2 + 1/2 gh^2) / ∂y + gh ∂(h + z) / ∂y = F_y + τ_by / ρ + h g ∂η / α∂y (6)

Where u and v are the vertically averaged velocity in the x and y directions, F is the contribution of friction and τ shear in the x and y directions. η is the water surface. These equations are very similar to the two-layer equations solved by Giachetti et al.63 within the VolcFlow software, however, with a more mobile dense flow following results from physical modelling of aerated granular flows entering water62. The lateral boundary condition for the volcaniclastic density current (the lower layer) is a solid boundary (no flux over the boundary) but the boundary of the model is sufficiently far from the volcano that the density currents that are not stopped by the bathymetry do not reach the boundary within the simulation time (the boundary is beyond the geographic limit of all figures and outputs here, and thus is not shown in any density current model outputs included). The sea water (upper layer) is solved with a radiating boundary condition at the edge of the domain, which allows for waves to propagate out of the domain and not reflect back in.

The density current was initialised as an 80 m thick, 4 km radius column of dense fluid62 (density 1600 kg/m^3 and total volume of 4 km^3 which for instance could represent a density current made up of 1.6 km^3 of volcaniclastic material of density 2500 kg/m^3 mixed with sea water, at a volumetric ratio of 2:3) on top of the Hunga Volcano edifice (Table S2 model simulation 1–4 km^3). This could represent either the point where the eruptive column has collapsed and mixed with the sea water or where an effusive density current has mixed with the sea water. The upper sea water layer is initialised as a flat surface (η=0) which then evolves according to the movement of the density current beneath it.

The TESMaP data was used for the bathymetry data around the edges of the edifice but the centre of the caldera was filled into a maximum depth of 200 m to represent the bathymetry before the caldera collapse. Topography data for the islands of Hunga Tonga and Hunga Ha’apai was taken from the 2017 bathymetry and used to fill in where these islands remain above sea level after the 15 January 2022 eruption. This near-field bathymetry was blended into a coarser bathymetry grid made up of GEBCO 2021, Seabed 2030 soundings, and LiDAR for Tongatapu.

This modelling is a very simple representation of a very complicated process. While it captures some aspects well it is not able to resolve other aspects. The density current is represented as a dense Newtonian fluid flowing over an unerodable bed. It does not erode sediment as it flows down the flanks of the edifice and neither does sediment deposit when the speed decreases, it just flows to the lowest point in the bathymetry. Thus, this model cannot predict where the density current will finally stop (i.e., runout distances), except in depressions or hollows, but it can indicate where the density current may have enough momentum to overcome bathymetric barriers (such as the saddles to the south of Hunga Volcano). If the volcaniclastic density current is created by a column collapsing back into the ocean and mixing with the water, then this model will also underestimate the surface waves generated as it does not consider the effect of the volcanic material falling back into the water. It can, however, capture the motion of the density current as it cascades down the sides of the edifice and the wave generation process in the upper layer that occurs due to this (see Supplemental Movie 2).

We have provided a description in the Supplemental information of the differences and outcomes of the different model runs undertaken to summarize impacts of varied bathymetry, fountaining within the crater or outside, varied volumes, changed densities of the volcaniclastic density current, and changes to friction of volcaniclastic density current (Table S2). To investigate the sensitivity of the density current flow paths to the location of the originating volcaniclastic density current, an alternate initialisation was modelled starting with a annulus of material between five and seven km from the centre of the caldera (thus originating outside the main caldera) and 53 m thick (so also ~4 km^3 volume of material) but with a radial velocity of 10 m/s to ensure that it flowed outwards rather than falling back inward to the caldera. This initialisation shows that the strong channelisation of the flow on the flanks of the Hunga edifice that match with the chutes, erosional scars, and channelisation observed in the bathymetric data from the voyage does not occur when the volcaniclastic density current originates outside the caldera (compare Fig. S7a which is initialised outside the caldera with Fig. S6a which is initialised within). Because this channelisation was clearly observed, that is evidence that the volcaniclastic density current must have originated within the caldera.

The total volume of the volcaniclastic density current specified in our final model was chosen based on the conditions that resulted in a density current that was able to reach the international cable, which is known to have been extensively damaged following the Hunga eruption (at least 89 km was buried or damaged by a volcaniclastic density current21). Small volume density currents that were modelled with an initial volume of 2 km^3 or less had insufficient inertia to overcome the topographic relief (specifically the saddles in the ridges to the south of Hunga edifice; see Fig. S7b) and were thus incapable of reaching the international cable. The model runs presented in the manuscript are based on an initial density current volume of 4 km^3 that resulted in flows that reached, and ran out beyond, the international cable. Sensitivity tests on the density of the density current show that the distance the flow reaches is relatively insensitive to the density of the density current (i.e., the ratio of volcanic material to sea water) with only minor differences being observed for densities of 1400 kg/m^3 and 1800 kg/m^3 (see Fig. S7, c and d).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This arficle describes fimely observafions made of the 2022 erupfion site and accompanying analyses. Fortuitously, mulfibeam sonar data of good quality were available from before the erupfion, collected sufficiently recently that the depth changes are almost certainly due to the erupfion. There has been a lot of interest in using depth changes around volcanic areas at smaller scale to study effects of volcanism (e.g., Casalbore et al.; Wafts), which this study adds to. Seabed photographs from prior expedifions were available also for comparison with new bed photos. The authors document sediment core and boftom photographic evidence that indicate the pathways travelled by the submarine pyroclasfic flows. The area will further become a benchmark for studying how seabed fauna recover from such events.\nGiven the widespread media coverage of this event, I would not be at all surprised if this arficle will become highly viewed. I hope I don't appear to be diminishing the role of the Tonga erupfion and the damage it caused, though the importance to hazards research probably lies with it helping to understand the risks posed by similar magma bodies nearer to major populafion centres, such as the Phlegraean Fields near Naples. To the north of San Miguel Island of the Azores, another field of giant sediment waves can be found (Weiss et al. (2016); Chang et al. (2022)). They lie north of the Furnas suspected magma body of similar concern. The arficle menfions the scale of the erupfion, though it might have been useful to read a magnitude comparison with other known erupfions, rather than leaving that to other publicafions.\nDespite my enthusiasm, I found reading the arficle somewhat frustrafing, as it dips a toe into many topics without exploring them in great detail. At the end of reading it, I would certainly feel happy cifing this arficle for the effect on the seabed around the vent and as another indicator of the scale of the erupfion, though I'm not sure it will get cited so much for specific insights. This is illustrated by the abstract, which emphasizes both the geological event and biological impact. As a geoscienfist, selfishly I might have preferred more focus on the geological event. For example, only the opfimum results of modelling of pyroclasfic density currents are shown, but this seems a rare example where we can be sure the deposits can be associated with an event (almost unequivocally). Sensifivity tests (varying volume and density of the originafing flow) might have been explored and inferrences drawn from comparison of those results with the field data. Although the modelling was very simple as the authors admit, I wondered if the spafial paftern of differences may allow at least a discussion of possible range of scale of the submarine PDCs. The flows evidently lacked sufficient kinefic energy to over-top the surrounding seamounts and fricfion (bed shear and momentum transfer from water entrainment) - this seems a basic constraint on them. Do the mulfibeam data suggest the elevafion that the PDCs reached? The areas of bed loss in Figure 2 around the seamounts suggests that they may have. Equafing potenfial energy gain for kinefic energy implies the velocity would be at least sqrt(2gh) ['at least' because this neglects fricfion]. Given that velocity (from which bed shear stress can be esfimated), is it surprising or unsurprising that widespread erosion occurred over the edifice and along the NW channel? This is just an illustrafion - it seems to me a punchier outcome could be developed.\nDetailed suggesfions:\nLine #\n48 height\n60 data from surveys before and after erupfions that can be used to quanfify ..\nbegan erupfing in late ...\nPut a full stop after 2021.\nShouldn't these polygons be shown in the electronic supplement? How could these results be reproduced?\n90 small features\n99 varied\n100-101 I found it hard working out the details from the maps shown (I can see the areas of sediment waves in Figure 1 roughly correpond with the major depth changes, but not much more than that). It might be useful to plot some depth contours from the post-erupfion survey over the depth changes in Figure 2.\n135 ... of sediment laden flow ...\n? or parficle volume (DRE)?\nFigure 1 - quite hard seeing much detail in these maps. I can see the sediment waves. Can the maps be shown enlarged?\nI also wondered if a tradifional figure with overlain depth contours might also be useful, as the contours can help accentuate the channels and minor gullies.\n366 insert (grey areas) before \"2017\".\nFigure 2 is annotated with the author's interpretafions of processes, rather than features in the map. Tradifionally, readers like to see the evidence presented in a non-genefic way so that they can assess it alongside basic observafions, before arriving at the interpretafion. I would suggest replacing these terms with observafions.\nThe scale bar on the map needs values even if the same as in the cross-secfions graph below (or otherwise say so in the figure capfion).\nFigure 3\n377 modelling results\n381 and each image is located in...\nSediment core is located by ...\n383 with interpreted pre-erupfion ...\n416 please write in the past tense, as it is unclear as wriften if the CAT scans were useful or if useful in general (maybe not there).\nThe areas of compliance and non-compliance of the model results have not really been explored.\n536 no need for \"back\"\nElectronic supplement:\nFigure S10. Please menfion the sonar and frequency used to produce the water column backscafter image lower-right.\n––––––––––––––––––––––––––––\nI very much enjoyed reading the results of this study and look forward to seeing a revised version in print.\nNeil C Mitchell, University of Manchester, January 2023.\nCasalbore D, Di Traglia F, Bosman A, Romagnoli C, Casagli N, Chiocci FL (2021) Submarine and subaerial morphological changes associated with the 2014 erupfion at Stromboli Island. Remote Sensing 13:arficle 2043\nChang Y-C, Mitchell NC, Hansteen TH, Schindlbeck-Belo JC, Freundt A (2021) Volcaniclasfic deposits and sedimentafion processes around volcanic ocean islands: the central Azores. In: Chang Y-C, Mitchell NC, Quartau R, Hübscher C, Rusu L, Tempera F (2022) Why are submarine sediment waves more common on the north sides of the Azores volcanic islands? Marine Geology 449:art. 106837\nde Mousfier C, Kleinrock MC (1986) Bathymetric arfifacts in Sea Beam data: How to recognize them and what causes them. J. Geophys. Res. 91:3407-3424\nDi Capua A, De Rosa R, Kereszturi G, Le Pera E, Rosi M, Waft SFL (eds) Volcanic Processes in the Sedimentary Record: When Volcanoes Meet the Environment. Geol. Soc. Lond., London\nHughes Clarke JE, Mayer LA, Mitchell NC, Godin A, Costello G (1993) Processing and interpretafion of 95 kHz backscafter data from shallow-water mulfibeam sonars. In: OCEANS'93 Proceedings. IEEE, Victoria, B.C., pp 437-442\nSchmift T, Mitchell NC, Ramsay ATS (2008) Characterizing uncertainfies for quanfifying bathymetry change between fime-separated mulfibeam echo-sounder surveys. Cont. Shelf Res. 28:1166-1176\nWafts AB, Nomikou P, Moore JDP, Parks MM, Alexandri M (2015) Historical bathymetric charts and the evolufion of Santorini submarine volcano, Greece. Geochem. Geophys. Geosys. 16:847-869\nWeiss BJ, Hübscher C, Lüdmann T, Serra N (2016) Submarine sedimentafion processes in the southeastern Terceira Rift/São Miguel region (Azores). Marine Geology 374:42-58", "reviewer_2": "This manuscript represents an important contribufion, marking the first documentafion of the impacts to the benthos of the Jan 2022 Hunga Tonga erupfion and providing some key observafions on the physical processes of subsea dispersal of volcanic material. I think the paper could be improved by refining the message about the hazards posed by this erupfion relafive to the potenfial hazards of all submarine volcanoes. In addifion, the authors should consider more descripfion of the data and models and de-emphasizing some of the interpretafion - given that there may not yet be sufficient informafion.\nBelow I describe a number of suggesfions that I think would significantly improve the paper. If addressed, I highly recommend its publicafion. The erupfion itself and its clear impacts on local island communifies were highly publicized and of broad interest to the scienfific community and the public. In addifion, there were well-documented global impacts that captured the interest of the world (e.g., pressure waves). These results will likewise be of high interest because they reveal the subsea impacts in some detail for the first fime. The paper is well-constructed and reports high-quality data that advances our understanding of this important volcanic event and its impacts. Below I describe some of the potenfial improvements to the manuscript.\n1. There are many superlafive claims about the erupfion and data collecfion that I view as distracfing at best and disingenuous at worst. The high interest of the topic speaks for itself does not require the authors to go out on such limbs. Nature should accept some responsibility for this as, intenfionally or not, it seems to encourage such language in its publicafions. When it is accurate, I’m all for it, but in this case I don’t think there is a strong enough case to merit it. Some specific examples…\nline 28 “largest seaborne volcanic erupfion in nearly 150 years” is based on a citafion of a lefter that does not provide any documentafion of erupfion volume, but instead cites a YouTube presentafion. The authors note that a significant porfion of the volume erupted is removed from the exisfing edifice. Perhaps it is the largest submarine erupfion of the last 150 years, but I don’t feel like that it is currently well-supported and it would be simple enough to say “one of the largest documented seaborne volcanic erupfions in perhaps a century or more”.\nline 86 “reveal the seafloor impacts of a major submarine erupfion for the first fime” seems to ignore several decades of work documenfing submarine erupfions. Most noficeably there is no reference to Carey et al., 2014; 2018 that documented seabed impacts on Havre volcano that erupted in 2012.\n2. The authors have made special note of the hazards from this erupfion, with good reason. The impact to the local island communifies through volcanogenic tsunami and failure of the subsea telecom cables are very important. However, I thought that the authors implied that these risks are common to all submarine volcanoes and that by juxtaposing those statements next to the fact that there are so many submarine volcanic features, that they implied that the risks are exponenfially greater than reality. The hazards from this erupfion are very real, but are predicated on it being a very large, shallow-water erupfion in a populated area. Erupfions of this nature can and will happen again and being prepared for that is important. However, most submarine erupfions are too deep and too remote to pose similar hazards. The authors cited the documentafion of 100,000 of seamounts as increasing this risk, but the vast majority of those are exfinct volcanos more than 100 Ma. I would suggest caveafing the discussion of submarine volcanic hazards by poinfing out what, when, and where those hazards are most prominent (e.g., Lines 47-49). In contrast to my statement above, the risks to benthic ecosystems are likely valid wherever submarine volcanic erupfions occur and have been documented in many places. It would be worth cifing some of that literature:\n• Vroom, Peter S., and Brian J. Zgliczynski. “Effects of volcanic ash deposits on four funcfional groups of a coral reef.” Coral Reefs 30.4 (2011): 1025-1032.\n• Angulo-Preckler, Carlos, et al. “Volcanism and rapid sedimentafion affect the benthic communifies of Decepfion Island, Antarcfica.” Confinental Shelf Research 220 (2021): 104404.\n• del Moral, Roger. “The importance of long-term studies of ecosystem reassembly after the erupfion of the Kasatochi Island Volcano.” Arcfic, Antarcfic, and Alpine Research 42.3 (2010): 335-341.\n• Shank, Timothy M., et al. “Temporal and spafial pafterns of biological community development at nascent deep-sea hydrothermal vents (9 50ʹ N, East Pacific Rise).” Deep Sea Research Part II: Topical Studies in Oceanography 45.1-3 (1998): 465-515.\n• Marcus, Jean, Verena Tunnicliffe, and David A. Bufterfield. “Post-erupfion succession of macrofaunal communifies at diffuse flow hydrothermal vents on Axial Volcano, Juan de Fuca Ridge, Northeast Pacific.” Deep Sea Research Part II: Topical Studies in Oceanography 56.19-20 (2009): 1586-1598.\n3. I’m somewhat concerned about the descripfion of the origin of the PDCs. The authors did a very good job of explaining their use of the terminology and that these submarine mass flows are different from terrestrial PDCs given that they are water supported, but they make the claim that they likely originated by collapse of the erupfion column and I don’t think they’ve supported that claim (e.g., Line 114, 118-119). The alternafive, that they originated from the submarine environment either via primary magmafic explosivity or hydrovolcanic explosions or simply mass wasfing of the edifice, is not explored. If there is evidence that these originated from column collapse, the authors should include it, or at least note the alternafives.\n4. No Tongan authors? I’m surprised that there are no Tongan authors on the manuscript. Surely they provided data or observafions or significant science support for the work. I would think they should be on the author list.\n5. More about the cores: The cores collected seem a primary piece of evidence for this study, but the data presented seems limited. There is a single core that is imaged and described in the main paper, but there were more cores taken. For the single core described, the image of the core needs to be much befter. It is impossible to see the structure and a porfion of the community would probably like to see it and interpret the potenfial processes it indicates for themselves. I would suggest that the authors add a visualizafion of ash fall and PDC thickness to the map of stafions where total thickness is indicated by dot size and fracfion of fall, flow in a pie chart at each site. This informafion should be used to help support the arguments in the paper of erosion and deposifion inferred from the bathymetric differencing and modeling. Also, it would be useful to know whether any cores were taken that did not show evidence for deposifion (in the refugia, for example).\nSome more specific comments are detailed below…\nLine Comment\n30-31 Where does the informafion on the length of damaged telecom cable come from. I spoke with the crew who was fixing it and I did not get the impression that they were laying 100km of cable.\n46-49 I would suggest removing the statement about risk in the Pacific Ocean. The vast majority of the 100,000 seamounts are exfinct and are so remote as to not pose any real hazards even where they acfive.\n61-68 There is some missing literature here for studies that look at pre- and post-erupfion seafloor condifions (notwithstanding that this study does not really have a comprehensive pre-erupfion survey of benthic biology). I would include Carey et al, 2014 & 2018; Embley, 2014; Soule et al., 2009; Chadwick et al., 2008; Wafts et al., 2015. It’s up to you if you want to say that it hasn’t been done on an erupfion the scale of HT, but it doesn’t seem necessary.\n114, 118 As menfioned, the authors should explain why they think the ‘PDCs’ originated subaerially.\n182-184 The statement about suspended sediment and hydrothermal discharge needs some more explanafion. First, there should be a reference to the supplement otherwise it is unclear what is being referred to in terms of suspended sediment. I’m also unsure why it is expected that suspended sediment will be a long-lasfing effect. It should clear up in a mafter of weeks to months unless something else is happening. In addifion, I’m not sure why hydrothermal discharge is viewed to have a primarily toxic effect? In most cases it can have a posifive impact on ecosystems around the hydrothermal vents and a prefty limited impact on suppressing ecosystems locally and certainly not regionally.\nFigures\nFigure 2 This is a great figure that gives a very nice view of how density currents progressed and evolved. There is no descripfion of the methods used to make the difference map or uncertainty associated with he esfimates. There is clearly some error associated with rough terrain on features that are known not have changed, such as flank domes, that can arise from minor navigafion offsets. It looks like that difference may not be stochasfic (i.e., distributed around zero), but has a somewhat posifive. It would be worth taking a look at the net gain loss in areas that are known to not have changed to make sure the gain/loss is not biased. I’m also a liftle dubious of the spur to the NW, where pre-erupfion data is sparse. There seems to pafterns in the gain loss that look more like arfifacts than real signal. I would be careful interprefing that and it may be befter to leave it out of the analysis.\nFigure 3 The modeled PDC paths in Figure 3 should be made less prominent. They are best guesses at flow paths and are rather complex and distracfing in the figure. It would be nice to see the presence/absence and abundance of benthic life against the loss-gain map, or a least the average for that area could be shown next to the point. It seems that there are some points with benthic biota present near the caldera (to the east) that seem contradictory to the idea of complete destrucfion of habitat. Also, please use a higher resolufion image of the core. It is impossible to see.\nFigure S3 It is hard to see the symbols on the geochemical plots. A higher resolufion version should be included. Also it seems that the T2206 data is just plunked on an older figure and covers up some of the previous data. Also, the legend is missing a symbol for T2206 shards.", "reviewer_3": "This manuscript compiles observafions from ship-based cruises to describe the seafloor impacts of the January 15, 2022 Hunga Tonga-Hunga Ha’apai erupfion. In addifion, they describe results from a PDC model that they plot against observafions. Observafions include changes in seafloor bathymetry, changes to biologic communifies, and reports on the characterisfics of the sedimentary deposits. Due to the remarkable nature of the 15 January Hunga Volcano erupfion, I suggest that these results are worthy of publicafion in a high impact journal such as Nature Geoscience. The changes in seafloor bathymetry, existence of pyroclasfic density current deposits, and runout distances of these pyroclasfic current deposits will be of widespread interest to the volcanological and oceanographic communifies, in my opinion. That said, some of the interpretafions are not currently well-supported by informafion provided in the ms although I do not doubt that this data was collected. I also have significant quesfions about the PDC model. After these and other changes are addressed, I would highly recommend publicafion as I believe these results are of parficularly high impact and importance. Please see my aftached comments.\n-Kristen Fauria\nReview for Pyroclastic density currents explain far-reaching and diverse seafloor impacts of the\nThis manuscript compiles observations from ship-based cruises to describe the seafloor impacts of the\nJanuary 15, 2022 Hunga Tonga-Hunga Ha’apai eruption. In addition, they describe results from a PDC model\nthat they plot against observations. Observations include changes in seafloor bathymetry, changes to biologic\ncommunities, and reports on the characteristics of the sedimentary deposits. Due to the remarkable nature of\nthe 15 January Hunga Volcano eruption, I suggest that these results are worthy of publication in a high\nimpact journal such as Nature Geoscience. The changes in seafloor bathymetry, existence of pyroclastic\ndensity current deposits, and runout distances of these pyroclastic current deposits will be of widespread\ninterest to the volcanological and oceanographic communities, in my opinion. That said, some of the\ninterpretations are not currently well-supported by information provided in the ms. I also have significant\nquestions about the PDC model. After these and other changes are addressed, I would highly recommend\npublication as I believe these results are of particularly high impact and importance.\nGeneral Comments\nFirst, I want to congratulate the authors on their incredible work surveying the Hunga Volcano post-\neruption. Their findings on PDC runout and the lowering of the caldera floor are remarkable. My\ncomments and suggestions below are primarily aimed at clarifying my own questions and making sure\ntheir observations can be well understood and utilized by readers and scientists studying the Hunga\neruption.\nThe change in bathymetry is a major finding of this manuscript. Because of its importance I would suggest\nclarifying both the bathymetric observations in the text and improving the quality of the difference map in\nFigure 2. Specifically, there is no absolute scale on the colors used in the difference map. I cannot see a\ndifference between the remarkable >800 m loss in the caldera and more modest loss along the flanks. I also\ncannot tell, from the text or figure, how much variation there is in deposition or scour along the flanks. The\nms could also benefit from a brief description of the pre-eruption Hunga Volcano morphology (i.e., that it is\na caldera volcano) before a discussion of the topographic changes is introduced.\nThe authors claim that the areas of deposition on the volcano’s flanks are due to PDC deposition.\nAlthough this is reasonable, the claim is not well-supported by sedimentological observations in the\ntext or supplement. More information along the lines of the stratigraphic section in Fig. 3 would be helpful For\nexample, the authors discount sedimentation by landslides without explaining why or defining what they\nconsider to be a “lack of evidence.” The deposits are interpreted to be PDCs in part due to bedforms and\nsediment cores, but the features within the cores or bedforms that distinguish the deposit as PDC are not\nmentioned or defined. Although thorough work on the sedimentary structures within their cores and from\nvideo observations may be outside the scope of the paper, I suggest that the authors describe\n1\nwhat specific aspects of the cores and seafloor observations allow them to define the deposits as PDC\nrelated (rather than mass flow by other flank collapse, for example). In the end, I agree that these are\nPDC-type flows but based on their spatial distribution rather than the sedimentological evidence\nprovided.\nI found the authors findings on post-eruption venting compelling. That said, the descriptions of\nphenomena such as post-eruption venting are only vaguely described. The ms would benefit for more clear\n(even if in the supplement) descriptions of where venting was and was not observed and how.\nI am pleased that the authors employed a PDC model and compare the model to observations. I have major\nquestions about the model, which I detail at the end of this review. One main concern is that it is not clear\nhowever, what aspect of the PDC model they are comparing to what characteristic of the observations. There\nis some discussion of runout. However, how is PDC runout characterized in the model and in the observations?\nHow do the authors determine the flow direction (arrows on Figs. 2 and 3) from the model? The authors\nacknowledge that the model does not include deposition. How, then, is the ultimate flow distance in the\nmodels determined (what process stops the PDC in the model)? I agree with the authors approach of\nemploying a general model, but it is not at all clear how their model works.\nLine by line comments\nLine 63: I suggest being specific about the types of data used earlier in the ms and on this line.\nLine 68: Add “mass” to the description of “loss.”\nLine 79: The eruption on Jan 13-14 could be described as a single large event rather than a series\nof events before the 15th (e.g., Gupta et al., 2022).\nLine 87-91: Here the Hunga summit is mentioned for the first time and it would benefit with a brief\nsummary of context. I suggest briefly describing the pre-eruption bathymetry to give the readers\ncontext for where bathymetric change occurs.\nOne of the most notable findings is the >800 m change in the caldera. It is mentioned here, but the\nreader would not know Hunga is a caldera volcano, without prior knowledge, and would not know the\npre-eruption caldera depth. I suggest adding those things here. Could you also comment on whether\nthis change in caldera floor elevation is uniform across the caldera? It also seems crater and caldera are\nused interchangeably. Is there a difference between the crater and caldera? What kind of change\noccurred along the caldera rim?\nLines 89-91: Could you expand upon the volume loss along the caldera rim. Was the mass loss uniform\nalong the flanks or localized? If uniform, what was the average change in vertical elevation along the flanks?\nHow far from the caldera rim did the mass loss extend? That is,\n2\nwhere was the transition from erosional to depositional features and how uniform was that distance\nfrom the caldera rim?\nLine 93-94: Ashfall is mentioned here and subsequently. However, the ashfall portion of this\neruption is not described in your ms. Could you briefly describe what is known about the presence\nof ashfall?\nLine 95-97: The supplementary data table is cited as evidence here. I am so glad you have included\nthese supplementary data. However, what aspect of the data are you suggesting makes this point?\nCould you perhaps provide a plot of the data in the supplement to make this point?\nLine 98: “There is consistent loss” is rather vague. What is consistent about the mass loss (spatially,\nmean change in elevation?). Why choose a water depth threshold? Does this correspond to a mean\ndistance from the caldera rim? If so, what is that distance?\nLine 102-103: I assume here you mean chute termini. Could you make more clear the transition in\ndiscussion from erosional to depositional features.\nLine 106: What do you consider an absence of evidence of landsliding? Could you briefly list the\nfeatures you would look for to indicate landsliding?\nHow much erosion and deposition occurs outside of the “chutes”. Could you also describe the change\nin bathymetry in the areas not defined by “chutes”.\nLines 113-114: The process-based description of PDC formation here is a suggestion or interpretation rather\nthan an observation. I suggest, therefore, caveating this sentence with a phrase such as “we suggest”. I also\nwonder the extent to which the flows may have never entered the atmosphere and remained entirely\nsubmerged since the vent was submarine? This possibility could be acknowledged even if only for a portion\nof the flows.\nLine 118: Please describe the plume collapse as “partial” since the plume attained a height of 57 km.\nLine 120-121: Could you please remind the reader how these observations were made. Overall, these\nscours are remarkable and could be discussed or emphasized more than they are at present\n(purely optional).\nLine 126: Could you please remind the reader the scale of a granule? Using what methods was\ngrainsize evaluated and quantified?\nLine 124-125: How far was this from the vent?\nagainst to determine they were from Hunga Tonga?\nOverall, the finding that the PDCs go from erosional to depositional is very interesting. As mentioned earlier I\nwould love more information on the dimensions (thicknesses, distance from the caldera) of the erosional and\ndepositional components. Could you also put the vent location in the context of these observations. It seems\nthat you assume all material was sourced at the vent, which was on one side of the caldera, yet the erosional\nand depositional features are evenly distribution around the caldera? Could you comment, confirm, or\nclarify in the ms?\nWhat were the maximum and characteristic runout distances of the PDC for both the\nchannelized and unchannelized components?\nLine 135-136: Please describe the basic attributes of your model here. Was the model for a dense or\ndilute flow? What were the basic assumptions or initial conditions? Did you model a PDC in water or in air, for\nexample? Here you mention replicating the extent, are you referring to the depositional extent? How did\nyou do this comparison in practice, especially if the PDC model does not include deposition?\nLine 137: Here you introduce the concept of simulated flow pathways. How are these defined? I am\nunfamiliar with PDC models being simplified into the arrows shown on your figures. On what basis\nare you defining these pathways?\nLine 140: What is meant by broken cables along >30 km and >100 km lengths? Is this distance from a\nlocation? If the cables were not broken in one location, could you please describe?\nLine 144: I do not understand this comparison. Are you comparing the model results to\nobservations from areas other than Hunga?\nLine 144: What was the longest PDC runout?\nLine 163: What aspect of the PDC model is being compared to locations of the refugia?\nLine 185-186: How was turbidity measured and where and when? Was there excessive turbidity everywhere\nor in specific locations (both spatially and in water depth)? Could you expand upon the definition of\nnepholoid layers as this may be a new concept to many volcano readers? Could you make Fig. S10 easier to\ninterpret to substantiate this point. For example, the acoustic image is not annotated and has no scale,\ndate/time, or specific location (lat/lon). The ongoing venting is of particular interest and currently it is only\nvaguely described. Where and when was venting found? Can you determine if/when the venting shut off?\nIn the conclusions, I suggest providing a brief summary of your bathymetry change and\nPDC\nerosion/deposition observations.\ncomposition. It could be more clear what sample they compare against to determine the true Hunga\ncomposition.\nFig. 1: Can you please make clear if the two A to A’ transects and B to B’ transects are the same or\ndifferent. If location A is location B and location A’ is location B’, I suggest using the same label for both.\nFig. 2: How do you choose how many flow trajectories to plot here from the model, why not 3 or 300?\nPlease provide an absolute scale for mass gain/loss. What is meant by “diffusive flow”? I also don’t see any evidence\nfor interaction with topography as annotated on the figure.\nFig. 3: Is this sedimentary log based on a single point or a general summary? To what extent is this\nlog representative? Overall, this is a nice figure. I have the same issue with the flow pathways. I would\nlike to see scale bars on A-C and the other panels labelled with letters as well.\nLine 406-407: How much footage (distance or time)?\nLine 505: Why 50 cm gridding of the data. This seems very high resolution for ship-based\nbathymetry. Wouldn’t the natural resolution be much lower.\nPlease add scales to all images.\nCould you please make sure there are geographic coordinates included in all sampling locations (e.g.,\nFig. S1 and data tables).\nPDC Modeling Comments:\nPDC modeling is challenging and I think it is neat that the authors undertook this challenge in addition\nto their substantial ship-based work. Overall, I cannot understand the model based on the information\nprovided. Please see the questions below.\n- Did you consider the current dilute or dense in your modeling effort? Your choice would result\nin different conservation equations and assumptions.\n- I am unfamiliar with this modeling software for PDCs. Can you describe how common it is for PDCs\ncompared to other types of models such as VolcFlow, Titan2D, and other known codes? Why did\nyou choose this modeling software?\n- What are the governing equations for mass, momentum, and energy in your model?\n- The two-layer choice is interesting, but I don’t understand how it is set up. Why two layers\nwith a water layer on top? The water layer is the ambient fluid so shouldn’t be considered\npart of the flow. How does this work?\n- How was flow through water dealt with since most PDC models consider flow through air.\nThat is, how did you treat the ambient fluid (as water or air)?\n5\n- Was the model 1D, 2D, or 3D?\n- Is the model single-phase?\n- What controls runout distance in the model. How does the model stop? This is\nparticularly important since runout is one of the features you compare.\n- How is the model coupled to topography?\n- Was the initial volume of PDC material placed on the seafloor, at the water surface?\n- How did you parameterize key aspects of the model (e.g., friction coefficient, Froude number,\nRichardson number, or entrainment coefficient)? I don’t understand enough about your model\nto know what you needed to parameterize, but models that aren’t 3D include these types of\nparameterizations.\n- Can you clarify your boundary conditions.\n6", "reviewer_author_response": "We thank the reviewers for their time spent reviewing this manuscript. We have endeavoured to address every comment, with responses in bold and italicized text following each comment. We have in-line track changes in the revised manuscript, with corresponding text addition and line numbers indicated here. We have provided additional supplemental information as well as the code (in a zip file) for your review as well. We thank the reviewers for their useful feedback which we believe has made this manuscript clearer, more robust, and better all around.\n\nRESPONSE TO REVIEWER COMMENTS\n\nReviewer #1 (Remarks to the Author):\nThis article describes timely observations made of the 2022 eruption site and accompanying\nanalyses. Fortuitously, multibeam sonar data of good quality were available from before the\neruption, collected sufficiently recently that the depth changes are almost certainly due to the\neruption. There has been a lot of interest in using depth changes around volcanic areas at smaller\nscale to study effects of volcanism (e.g., Casalbore et al.; Watts), which this study adds to.\nSeabed photographs from prior expeditions were available also for comparison with new bed\nphotos. The authors document sediment core and bottom photographic evidence that indicate the\npathways travelled by the submarine pyroclastic flows. The area will further become a benchmark\nfor studying how seabed fauna recover from such events.\nWe thank R1, Dr Mitchell, for recognizing the importance of this study and the contributions that\nthis work will make to the field.\nGiven the widespread media coverage of this event, I would not be at all surprised if this article\nwill become highly viewed. I hope I don't appear to be diminishing the role of the Tonga eruption\nand the damage it caused, though the importance to hazards research probably lies with it helping\nto understand the risks posed by similar magma bodies nearer to major population centres, such\nas the Phlegraean Fields near Naples. To the north of San Miguel Island of the Azores, another\nfield of giant sediment waves can be found (Weiss et al. (2016); Chang et al. (2022)). They lie\nnorth of the Furnas suspected magma body of similar concern. The article mentions the scale of\nthe eruption, though it might have been useful to read a magnitude comparison with other known\neruptions, rather than leaving that to other publications.\nWe agree that there may be larger populations exposed to such hazards in other regions and we\nrecognize the importance of this work to understanding potential events at similar magma bodies\nof concern. Therefore we have highlighted this further in lines 52-54 to expand on the wider\nimplications to more populous areas.\n“In particular, shallow water active volcanic centres that are near populated islands expose a major\nblind spot regarding risk assessment and response preparedness3–5.”\nTo place this in a more useful context, we have also added the Volcanic Explosivity Index to line 57\nto provide a magnitude comparison to other known eruptions.\n“With a Volcanic Explosivity Index of 5.7, the 15 January eruption was the largest eruption since\nMount Pinatubo in 1991 and is one of the most explosive submarine eruptions ever recorded and\nlargest documented in a century or more.”\nHowever, we are also keen not to remove too much focus from small island developing states in\nthe South Pacific that may contain smaller numbers of people, but may be far more exposed and\nvulnerable, due to them lying so close to sea level, with a far greater reliance on one or relatively\nfew telecommunications connections, and with a much reduced or a total lack of hazards\nmonitoring capacity (thus inhibiting early warning).\nDespite my enthusiasm, I found reading the article somewhat frustrating, as it dips a toe into\nmany topics without exploring them in great detail. At the end of reading it, I would certainly feel\nhappy citing this article for the effect on the seabed around the vent and as another indicator of\nthe scale of the eruption, though I'm not sure it will get cited so much for specific insights. This is\nillustrated by the abstract, which emphasizes both the geological event and biological impact. As a\ngeoscientist, selfishly I might have preferred more focus on the geological event.\nAs R1 indicates, a key aim of this paper is to highlight the wider impacts of the Hunga eruption,\nencapsulating geologic and biologic impacts and shining a new light on the diverse, and typically-\nignored, seafloor impacts of submarine eruptions. The broad interest that we seek to attract with\nthis manuscript construction is why we think it is an ideal fit for Nature Communications, with\nmore specific insights about the Hunga eruption saved for follow-up papers in a more specialist\ngeologic journal. The multidisciplinary data acquired from this event will enable a host of follow\non papers, but we believe it is important that this paper, which synthesises our new\nunderstanding of those diverse impacts, is published first to set the scene for more discipline-\nspecific and specialised studies.\nFor example, only the optimum results of modelling of pyroclastic density currents are shown, but\nthis seems a rare example where we can be sure the deposits can be associated with an event\n(almost unequivocally). Sensitivity tests (varying volume and density of the originating flow) might\nhave been explored and inferrences drawn from comparison of those results with the field data.\nThis is an entirely reasonable comment and we have now provided further information on the PDC\nmodelling, which includes the outcomes of different model runs in the supplementary material\nthat provide a sensitivity assessment as asked for by the reviewer. We investigate the sensitivity\nof the results on the initial volume and the density of the PDC gravity current and an alternative\nfountaining initialisation. We would not claim that the results we show are optimum as the model\nis very simple and the constraints fairly wide, but it is a plausible scenario and we have expanded\non what we can infer from the modelling results, as seen throughout the revised text.\nAlthough the modelling was very simple as the authors admit, I wondered if the spatial pattern of\ndifferences may allow at least a discussion of possible range of scale of the submarine PDCs. The\nflows evidently lacked sufficient kinetic energy to over-top the surrounding seamounts and friction\n(bed shear and momentum transfer from water entrainment) - this seems a basic constraint on\nthem.\nTo the south of the Hunga edifice, the PDCs did in fact overcome bathymetric constraints and\nmake it over submarine knolls into the valley where the international cable lay. We have added a\nsentence to clarify this point in the main text (lines 172-173: “… and the overtopping of two\nsubmarine knolls to the south heading towards the international cable”).\nSmaller PDCs that were modelled did not reach that far and mainly ended up in the valley that the\ndomestic cable was in, this can be seen in the different model run outcomes that we have provided\nin supplemental material (Figure S6 top right panel).\nDo the multibeam data suggest the elevation that the PDCs reached? The areas of bed loss in\nFigure 2 around the seamounts suggests that they may have. Equating potential energy gain for\nkinetic energy implies the velocity would be at least sqrt(2gh) ['at least' because this neglects\nfriction]. Given that velocity (from which bed shear stress can be estimated), is it surprising or\nunsurprising that widespread erosion occurred over the edifice and along the NW channel? This is\njust an illustration - it seems to me a punchier outcome could be developed.\nDue to the water depths in which the multibeam data were acquired, the resolution of these data\ndoes not permit the robust identification of trim lines on the seamounts to infer flow thickness nor\nto confidently resolve the absolute elevation that the PDCs reached (i.e. not in the same manner\nas has been done on other studies such as Stevenson et al., 2018. Reconstructing the sediment\nconcentration of a giant submarine gravity flow. Nature communications, 9(1), p.2616. There the\nacquisition of side-scan sonar data and greater coverage of sediment cores enabled mapping of\ntrimlines. We agree with the reviewer that this is an interesting area to explore and we do\npropose to perform more detailed analysis on the sedimentological characteristics of the PDCs in a\nfollow-up paper that is under review. In time, we may be able to provide deeper insight into some\nof these points; however, this is beyond the scope of this present manuscript as we are seek to\nprovide a broad, multi-disciplinary view of the seafloor impacts of the Hunga eruption and\ndemonstrate the widespread but otherwise unseen impacts of the eruption.\nDetailed suggestions:\nLine #31 Is it possible to be more specific concerning biological communities? Fauna? To be\npedantic, if sampling for bacteria, there may have been some carried in by the ocean. On the\nother hand, it might not be surprising not to see whales as they are not so abundant.\nWe have adjusted the text accordingly (lines 34-35): Biological (mega-epifaunal invertebrate)\nseafloor communities…\nLine 43 now reads: The ash and chemicals from large volcanic…\n48 height\nLine 52 now reads: …volcanoes of >1km in elevation…\n60 data from surveys before and after eruptions that can be used to quantify ..\nLines 67-68 now read: ...and data from surveys before and after eruptions that can be used to\nquantify…\nbegan erupting in late ...\nPut a full stop after 2021.\nLines 94-96 now read: After this eruption, Hunga Volcano was largely inactive until it began\nerupting in late 2021, producing a steam-rich gas and ash plume. Following this, Hunga Volcano\ncontinued erupting intermittently until two large eruptions…\n\"majority\"\nThe following sentence has been added to supplementary material (Pre-eruption multibeam\nsection in the methods, lines 656-659) to explain how the quantity 99% was measured:\n“To determine the percentage of the volcano area that is subaerially exposed the area of the\nsubmerged edifice defined by a change in slope of 3° the base of the edifice. This was compared to\nthe area of the pre-eruption island area to demonstrate that <1% of the volcano is subaerially\nexposed”.\nLine 101-102 now reads: A map of bathymetric change (derived by subtracting new bathymetric\ndata from pre-eruption surveys in 2015-20176,7) reveal the seafloor morphologic fingerprint of a\nmajor eruption.\nLines 104-105 now read: Integrating depth changes over selected areas of the map, this change\naccounted for 6.0km3 seafloor loss from within the caldera..\nShouldn't these polygons be shown in the electronic supplement? How could these results be\nreproduced?\nA shapefile with the polygons used has been included as a figure within the supplemental\ninformation (Fig S18) as indicated now in the methods section : Line 641: “…for select areas of the\nmap (shown in Fig. S18).”\n90 small features\nLine 108 now reads: with many small features, such as..\ndoes that change this assessment?\nWe refer to the initial estimates of volume ejected from the volcano. We acknowledge there are\nupdated values and uncertainty in these values, and we have modified the sentence to reflect this:\n(Line 111: Prior studies suggest that ~1.9km3… remaining material (~1.3km3)…). This uncertainty\nalso includes the thin deposit on the seabed that we cannot resolve using modern echosounder\ntechniques, which we have further clarified in this paragraph (Lines 112-113: widespread thin\ndeposits below the detection limit of what can be resolved with modern echosounder techniques)\n99 varied\nWe have addressed this in the manuscript, line 117 now.\n100-101 I found it hard working out the details from the maps shown (I can see the areas of\nsediment waves in Figure 1 roughly correspond with the major depth changes, but not much more\nthan that). It might be useful to plot some depth contours from the post-eruption survey over the\ndepth changes in Figure 2.\nWe have added contour lines to Figure 1. Given the broad readership of Nature Communications\nwe prefer to keep Figure 2 as a difference map without contour lines as we worry contour lines\nover the difference map could add confusion.\nLine 120 now reads: Up to 70m entrenchment was..\nocurred in the area marked as \"diffusive flow\" in Figure 2? They look difficult to rule out. Maybe\ninstead say that there is no superficial morphology typical of landslides remaining, e.g.,\ncrown/headwall.\nWe have addressed this in the manuscript. The sentence (lines 126-129) now reads:\n“There is no superficial morphology consistent with seafloor landslides (i.e., headscarps and/or\nscars) on the volcano flanks despite the scale of the eruption and associated seismicity, which\nmakes this event distinct from partially-submerged eruptions that generated large slope collapses\nsuch as Anak Krakatau (Indonesia) in 201837.”\nmagnitude. Can the scale (wave spacing and height) be compared with features created by known\n(measured) currents or even inferred from cable breaks? The gravel waves produced by the Grand\nBanks failure have been well studied for example.\nThe scale of bedforms is comparable to those that have been formed by large volume and fast-\nmoving sediment density flows such as the 1929 Grand Banks and the 2016 Kaikoura Canyon\nevents. We therefore infer that these flows were also similarly powerful. We also know that they\nwere locally capable of running up slope so must have had sufficient inertia to do so. We are\nworking on a more detailed follow up paper that provides further insight into the initiation timing,\ncable breaks and relates the bedform morphodynamics to likely flow velocities and properties and\nthere is not sufficient space here to properly expand. To address this point we have added the\nfollowing text to explain by what is meant by “powerful”, providing context with published studies\non the Kaikoura and Grand Banks events\nLines 149 to 154 : “We observe crescentic scours (up to 30 m deep) and bedforms (up to 1,200 m\nwavelength, 20 m wave height; Fig. 2), commonly observed in other deep-sea settings affected by\npowerful density flows (including those triggered by large magnitude earthquakes, that severed\nseafloor cables, comprised a dense basal layer, and have been shown to attain speeds of up to 20\nm/s), and further corroborated by seafloor sediment coring and imagery, geochemical analysis\nand numerical modelling”.\nlikely) or an earlier eruption. Is there anything further about the deposit in the core that indicates\nit is young? Perhaps lack of any iron-manganese coating of grains near the surface, lack of\nbioturbation, glass shards are unaltered, etc?\nWe have added a sentence (lines 165-167) to clarify points that indicate the deposit are from this\nmost recent eruption: “A lack of bioturbation within the volcaniclastic deposit and no indication of\noxidation indicate that it is a relatively fresh deposit, as well as the absence of any hemipelagic\nlayering on top of the volcanoclastic deposit.”\nIt is also worth noting that we have sampled locations where there is clearly resolvable change\nfrom the bathymetric difference mapping (up to 22 m thickness of new deposits);\nhence any\nseafloor deposit there cannot pre-date the eruption.\nWe have addressed this in the manuscript, lines 169-170.\n135 ... of sediment laden flow ...\n? or particle volume (DRE)?\nLine 174 now reads: ...of sediment laden flow\nbetween appearance of the atmospheric eruption column and these breaks? Does that tell us\nanything about how the base of the eruption column collapsed?\nThis is a very good question and is the subject of ongoing research that specifically focuses on this\naspect of the event (rather than the compounded impacts as a whole). As a result, this cannot be\nexplored in detail in this present manuscript.\nWe have modified the text to reflect the model outputs as well (lines 179-183):\n“The model is consistent with the locations and extensive damage to the international and\ndomestic telecommunication cables, damaged along 89 km and 105 km length, respectively (Fig.\n3), and explains the observed spatial trends in erosion and flow runout.”\nmodelling, not broader social issues that have not really been developed in the article.\nThe conclusion section has been modified to address this comment.\nLines 263-271: “Our data illustrate how an eruption of a single submerged volcano can have wide-\nreaching and diverse impacts. PDCs resulting from the Hunga Volcano eruption were steered by\nantecedent morphology, further excavating pre-existing chutes and depositing the majority of the\nerupted volume on the seabed within 20 km of the Hunga caldera rim. Modelling and benthic\nobservations indicate that the majority of the PDCs originated within the caldera and flowed\naround bathymetric relief as they moved to deeper waters, with only local preservation of benthic\ncommunities on topographic refugia. Outside of these topographic refugia the seafloor ecosystems\naround Hunga Volcano were decimated by the eruption and related PDCs.\nFigure 1 - quite hard seeing much detail in these maps. I can see the sediment waves. Can the\nmaps be shown enlarged?\nThe intention of this overview map is to illustrate that the dominant change to the volcano is\nwithin the caldera and that the changes to the flank are much more subtle. Given the scale of the\neruption, the relatively minor morphological changes to the flanks, compared to the caldera, are a\nkey result – an initial surprise to us and in stark contrast to other submerged volcanoes such as the\nhalf-island collapse of Anak Krakatau in 2019. The details on the local changes on the flank are\nprovided in Figure 2. However, in order to make it easier to see, we have increased the size of\npanels b and c so that they occupy a full-page width.\nI also wondered if a traditional figure with overlain depth contours might also be useful, as the\ncontours can help accentuate the channels and minor gullies.\nWe have added depth contours to Figure 1.\n366 insert (grey areas) before \"2017\".\nWe have addressed this in the manuscript, with a sentence added within the figure caption for\nFigure 1 at line 485: This is overlain on GEBCO_2022 bathymetric grid (greyscale..).\nFigure 2 is annotated with the author's interpretations of processes, rather than features in the\nmap. Traditionally, readers like to see the evidence presented in a non-genetic way so that they\ncan assess it alongside basic observations, before arriving at the interpretation. I would suggest\nreplacing these terms with observations.\nWe have removed the annotations from Figure 2\nThe scale bar on the map needs values even if the same as in the cross-sections graph below (or\notherwise say so in the figure caption).\nWe have added values to the scale bar on the map in Figure 2\nmotion of a virtual particle in the flow or flow front?\nWe have clarified this in the figure caption, lines 491-493: “Indicative PDC flow paths (i.e., the\npassage of the flow fronts as estimated from model animations and snapshots (see\nSupplemental Video 1 and Fig. S6))”\nFigure 3\n377 modelling results\nWe have addressed this in the manuscript (line 497-498): “with indicative PDC flow paths”\n381 and each image is located in...\nSediment core is located by ...\nWe have addressed this in the manuscript, lines 502-503: The sediment core is located by light blue\nstar.\n383 with interpreted pre-eruption ...\nWe have addressed this in the manuscript, line 488.\nindividuals?\nWe have clarified this in the text, lines 527-528. “The biological assemblages are mega-epifaunal\ninvertebrates, living on or above the seafloor with sizes >2cm.” and lines 533-534 “ abundance of\ninvertebrate taxa were counted”\n416 please write in the past tense, as it is unclear as written if the CAT scans were useful or if\nuseful in general (maybe not there).\nWe removed this text from the manuscript as we agree it was confusing as written and not\nnecessary. The removed text is seen between lines 542-547.\ncarbon (well below threshold) - Chang et al. (2021). It seems obvious that there wouldn't be,\nthough was organic carbon detected? This is a minor further check.\nOrganic Carbon was not analysed from the cores. For this study only major elements were\nmeasured to establish that the material was sourced from HTHH. Full analyses of the cores,\nincluding carbon, will be undertaken as part of a more detailed study on the composition of the\nash layers.\naccuracy.\nThis is provided in the methods section, line 649 “uncertainties for multibeam acquired\nbathymetric data are depth dependent and are usually reported as being 1% of depth (ranging\nfrom 20 cm to 20 m).”\nstudying impact of turbidity on pelagic fauna.\nYes, and this will be looked at in works in preparation and released for public use.\n(Figure 2) does not contain any obvious known artifacts (e.g., Hughes Clarke et al.), which is\ngood.\nthem in the change map is reassuring.\nTo address the two points above we added to lines 636-637: .. “However, morphology as\nmeasured by multibeam echo sounders is typically not affected by vertical uncertainties, and if so\nwe would expect artifacts in the difference maps which we do not see.”\nshame the volume and density were not explored further with sensitivity tests (ie., what range of\nthese values leads to a breakdown in match to the observations and what can we learn about the\nflow (or limits of the modelling) from that comparison?).\nThe PDC modelled here is actually a underwater density current made up of a mixture of the\ndense part of the PDC and water. The density is taken to be 1,600kg/m3 which assumes that the\nPDC flow is 40% (by weight) dense pyroclastic material and 60% water. Sensitivity tests were\nundertaken and the results of some of these are shown in the supplementary material (SI Fig 6,7).\nThe model is fairly insensitive to changes in density of the PDC flow. The bottom two panels of\nFigure S6 show model runs with lower and higher density at time t=2,400s. It can be seen that the\nflow does not reach quite as far for a less dense flow (1,400kg/m3, 26.7% pyroclastic material,\nbottom left panel of Figure S6) and reaches slightly further for a denser flow (1,800kg/m3, 53.3%\npyroclastic material, bottom right panel of Figure S7) but in both cases the differences are fairly\nsubtle.\nThe areas of compliance and non-compliance of the model results have not really been explored.\nThe simplicity of the model means that there is limited use in more than a qualitative comparison\nbetween the model and the observations, but we now discuss some of the implications of where\nversions of the model do or do not agree with the observations in the Methods Section. (lines 710-\n730)\n“To investigate the sensitivity of the PDC flow paths to the location of the originating PDC, an\nalternate initialisation was modelled starting with a annulus of material between five and seven\nkm from the centre of the caldera (thus originating outside the main caldera) and 53m thick (so\nalso approximately 4km3 volume of material) but with a radial velocity of 10m/s to ensure that it\nflowed outwards rather than falling back inwards to the caldera. This initialisation shows that the\nstrong channelisation of the flow on the flanks of the Hunga edifice that was observed during the\nvoyage does not occur when the PDC originates outside the caldera (see Fig. S7 top left panel).\nBecause this channelisation was clearly observed, that is evidence that the PDC must have\noriginated within the caldera.\nThe total volume of PDC material in this model was chosen so that the gravity current reached the\ninternational cable with sufficient force to be able to cause the damage observed. Because of the\nsimplicity of the initialisation, we have to be careful with interpretation, but smaller volumes were\nalso modelled and volumes of 2 km3 or less do not extend that far (see Fig. S7). This suggests that a\nsignificant proportion of the volcanic material must have been ejected within a short period of\ntime as a slower, more continuous, rate of PDC formation would not have the energy to reach the\ninternational cable. Sensitivity tests on the density of the PDC show that the distance the flow\nreaches is relatively insensitive to the density of the gravity current with only minor differences\nbeing observed for densities of 1,400kg/m3 and 1,800kg/m3 (see Fig. S7, bottom panels).”\n536 no need for \"back\"\nLines 688-690 : “This could represent either the point where the eruptive column has collapsed and\nmixed with the sea water or where an effusive PDC has mixed with the seawater.”\nElectronic supplement:\nFigure S10. Please mention the sonar and frequency used to produce the water column\nbackscatter image lower-right.\nThis has been added to the supplemental figure caption : “(collected with the Kongsberg EM710,\nwhich has 65-100 kHz frequency).”\n––––––––––––––––––––––––––––\nI very much enjoyed reading the results of this study and look forward to seeing a revised version\nin print.\nNeil C Mitchell, University of Manchester, January 2023.\nCasalbore D, Di Traglia F, Bosman A, Romagnoli C, Casagli N, Chiocci FL (2021) Submarine and\nsubaerial morphological changes associated with the 2014 eruption at Stromboli Island. Remote\nSensing 13:article 2043\nChang Y-C, Mitchell NC, Hansteen TH, Schindlbeck-Belo JC, Freundt A (2021) Volcaniclastic\ndeposits and sedimentation processes around volcanic ocean islands: the central Azores. In:\nChang Y-C, Mitchell NC, Quartau R, Hübscher C, Rusu L, Tempera F (2022) Why are submarine\nsediment waves more common on the north sides of the Azores volcanic islands? Marine Geology\n449:art. 106837\nde Moustier C, Kleinrock MC (1986) Bathymetric artifacts in Sea Beam data: How to recognize\nthem and what causes them. J. Geophys. Res. 91:3407-3424\nDi Capua A, De Rosa R, Kereszturi G, Le Pera E, Rosi M, Watt SFL (eds) Volcanic Processes in the\nSedimentary Record: When Volcanoes Meet the Environment. Geol. Soc. Lond., London\nHughes Clarke JE, Mayer LA, Mitchell NC, Godin A, Costello G (1993) Processing and interpretation\nof 95 kHz backscatter data from shallow-water multibeam sonars. In: OCEANS'93 Proceedings.\nIEEE, Victoria, B.C., pp 437-442\nSchmitt T, Mitchell NC, Ramsay ATS (2008) Characterizing uncertainties for quantifying\nbathymetry change between time-separated multibeam echo-sounder surveys. Cont. Shelf Res.\n28:1166-1176\nWatts AB, Nomikou P, Moore JDP, Parks MM, Alexandri M (2015) Historical bathymetric charts and\nthe evolution of Santorini submarine volcano, Greece. Geochem. Geophys. Geosys. 16:847-869\nWeiss BJ, Hübscher C, Lüdmann T, Serra N (2016) Submarine sedimentation processes in the\nsoutheastern Terceira Rift/São Miguel region (Azores). Marine Geology 374:42-58\nReviewer #2 (Remarks to the Author):\nThis manuscript represents an important contribution, marking the first documentation of the\nimpacts to the benthos of the Jan 2022 Hunga Tonga eruption and providing some key\nobservations on the physical processes of subsea dispersal of volcanic material. I think the paper\ncould be improved by refining the message about the hazards posed by this eruption relative to\nthe potential hazards of all submarine volcanoes. In addition, the authors should consider more\ndescription of the data and models and de-emphasizing some of the interpretation - given that\nthere may not yet be sufficient information.\nBelow I describe a number of suggestions that I think would significantly improve the paper. If\naddressed, I highly recommend its publication. The eruption itself and its clear impacts on local\npopulations were highly publicized and of broad interest to the scientific community and the public.\nIn addition, there were well-documented global impacts that captured the interest of the world\n(e.g., pressure waves). These results will likewise be of high interest because they reveal the\nsubsea impacts in some detail for the first time. The paper is well-constructed and reports high-\nquality data that advances our understanding of this important volcanic event and its impacts.\nBelow I describe some of the potential improvements to the manuscript.\nWe thank Reviewer 2 for their review of this manuscript, their constructive feedback that has\nimproved the quality of this manuscript, and their encouragement. We have done our best to\naddress all comments as detailed below.\n1. There are many superlative claims about the eruption and data collection that I view as\ndistracting at best and disingenuous at worst. The high interest of the topic speaks for itself does\nnot require the authors to go out on such limbs. Nature should accept some responsibility for this\nas, intentionally or not, it seems to encourage such language in its publications. When it is\naccurate, I’m all for it, but in this case I don’t think there is a strong enough case to merit it. Some\nspecific examples…\nline 28 “largest seaborne volcanic eruption in nearly 150 years” is based on a citation of a letter\nthat does not provide any documentation of eruption volume, but instead cites a YouTube\npresentation. The authors note that a significant portion of the volume erupted is removed from\nthe existing edifice. Perhaps it is the largest submarine eruption of the last 150 years, but I don’t\nfeel like that it is currently well-supported and it would be simple enough to say “one of the largest\ndocumented seaborne volcanic eruptions in perhaps a century or more”.\nline 86 “reveal the seafloor impacts of a major submarine eruption for the first time” seems to\nignore several decades of work documenting submarine eruptions. Most noticeably there is no\nreference to Carey et al., 2014; 2018 that documented seabed impacts on Havre volcano that\nerupted in 2012.\nWe have adjusted statements in the text with these points in mind, in order to reduce superlative\nclaims. We have changed “largest” to “most explosive” where required (lines 30 and 58) to\nseparate this eruption from other large volume but less explosive events and we have added a\nreference for this explosivity in line 30. We have also qualified the section around revealing the\nimpacts with regards to other eruptions. What makes this study unique is the availability of pre\nand post eruption bathymetry, which allows an analysis for an eruption this size that has not\npreviously been possible. This is now stated in lines 67-69: “Most submerged volcanoes are poorly\nmapped, and data from surveys before and after eruptions that can be used to quantify pre- and\npost-event seafloor conditions are rare29–32. No such data currently exist for an event on the scale\nof the 2022 Hunga Volcano eruption. “\nOther related changes:\nLines 57-58: We added reference to the VEI of the eruption and reworded the original sentence so\nthat it now reads: ” With a Volcanic Explosivity Index of 5.7, the 15 January eruption was the\nlargest eruption since Mount Pinatubo in 1991 and is one of the largest documented seaborne\neruptions in a century or more.”\nWe removed “for the first time” from line 103.\nWe added reference to Carey et al., 2018 in line 68, and reference to Carey et al., 2014 in line 48.\n2. The authors have made special note of the hazards from this eruption, with good reason. The\nimpact to the local island communities through volcanogenic tsunami and failure of the subsea\ntelecom cables are very important. However, I thought that the authors implied that these risks\nare common to all submarine volcanoes and that by juxtaposing those statements next to the fact\nthat there are so many submarine volcanic features, that they implied that the risks are\nexponentially greater than reality. The hazards from this eruption are very real, but are predicated\non it being a very large, shallow-water eruption in a populated area. Eruptions of this nature can\nand will happen again and being prepared for that is important. However, most submarine\neruptions are too deep and too remote to pose similar hazards. The authors cited the\ndocumentation of 100,000 of seamounts as increasing this risk, but the vast majority of those are\nextinct volcanos more than 100 Ma. I would suggest caveating the discussion of submarine\nvolcanic hazards by pointing out what, when, and where those hazards are most prominent (e.g.,\nLines 47-49).\nWe have added to lines 50-54 statements to clarify these points, as well as additional citations this\nnow reads:\n“This observational bias is severe in regions under-represented by scientific study, such as the\nSouth Pacific Ocean where the majority of the world’s 100,000 uncharted underwater volcanoes of\n>1 km in elevation lie. In particular, shallow water active volcanic centres that are near populated\nislands expose a major blind spot regarding risk assessment and response preparedness3–5.”\nIn contrast to my statement above, the risks to benthic ecosystems are likely valid wherever\nsubmarine volcanic eruptions occur and have been documented in many places. It would be worth\nciting some of that literature:\n• Vroom, Peter S., and Brian J. Zgliczynski. “Effects of volcanic ash deposits on four functional\ngroups of a coral reef.” Coral Reefs 30.4 (2011): 1025-1032.\n• Angulo-Preckler, Carlos, et al. “Volcanism and rapid sedimentation affect the benthic\ncommunities of Deception Island, Antarctica.” Continental Shelf Research 220 (2021): 104404.\n• del Moral, Roger. “The importance of long-term studies of ecosystem reassembly after the\neruption of the Kasatochi Island Volcano.” Arctic, Antarctic, and Alpine Research 42.3 (2010): 335-\n341.\n• Shank, Timothy M., et al. “Temporal and spatial patterns of biological community development at\nnascent deep-sea hydrothermal vents (9 50′ N, East Pacific Rise).” Deep Sea Research Part II:\nTopical Studies in Oceanography 45.1-3 (1998): 465-515.\n• Marcus, Jean, Verena Tunnicliffe, and David A. Butterfield. “Post-eruption succession of\nmacrofaunal communities at diffuse flow hydrothermal vents on Axial Volcano, Juan de Fuca Ridge,\nNortheast Pacific.” Deep Sea Research Part II: Topical Studies in Oceanography 56.19-20 (2009):\n1586-1598.\nWe added the impact to benthic ecosystems to line 27. We have added these references as well as\nmore detail and a few other relevant references to the section between lines 194 – 271 and\nthroughout the text as appropriate.\n3. I’m somewhat concerned about the description of the origin of the PDCs. The authors did a very\ngood job of explaining their use of the terminology and that these submarine mass flows are\ndifferent from terrestrial PDCs given that they are water supported, but they make the claim that\nthey likely originated by collapse of the eruption column and I don’t think they’ve supported that\nclaim (e.g., Line 114, 118-119). The alternative, that they originated from the submarine\nenvironment either via primary magmatic explosivity or hydrovolcanic explosions or simply mass\nwasting of the edifice, is not explored. If there is evidence that these originated from column\ncollapse, the authors should include it, or at least note the alternatives.\nWe have adjusted the language to reflect the alternative mechanisms, now lines 139-142, and\nadded supporting evidence from a recent publication (Purkis et al., 2023)\n“The PDCs most likely initiated from the partial collapse of the eruptive column of Hunga Volcano,\nwith this hypothesis similarly supported by the timing and intensity of the tsunami waves39.\nHowever primary magmatic explosivity, hydrovolcanic explosions, or mass wasting of the edifice\ncould have also been formative mechanisms.”\nWe would like to note that sensitivity tests of our PDC model (see supplementary material, Fig. S6\nand S7) confirm that whatever initialisation mechanism caused these PDCs it must have occurred\nwithin the caldera in order to cause the strong channelisation that was observed on the flanks of\nHunga volcano. An initialisation mechanism such as a fountaining PDC where the PDC and water\nmixed outside of the caldera (i.e. on the flanks of the Hunga edifice or even further out) would not\nhave caused this channelisation. Additionally, this possibility has been suggested in relation to\ndata regarding the tsunami waves in a recent publication which we have now cited (indicated\nabove, Purkis et al., 2023).\nOn the basis of the eroded channel / flanks that we see on the flanks – footprint of the cladera; point to\nshanes paper and the video\n4. No Tongan authors? I’m surprised that there are no Tongan authors on the manuscript. Surely\nthey provided data or observations or significant science support for the work. I would think they\nshould be on the author list.\nWe agree with R2 on this point, and have since included Mr. Taaniela Kula, the Deputy Secretary\nof the Tonga Geological Services, as a co-author. Mr. Kula has remained a key part of planning,\ncoordination, and implementation of the field research programmes that has led to this work and\nhe should have been included as a co-author from the beginning.\n5. More about the cores: The cores collected seem a primary piece of evidence for this study, but\nthe data presented seems limited. There is a single core that is imaged and described in the main\npaper, but there were more cores taken. For the single core described, the image of the core\nneeds to be much better. It is impossible to see the structure and a portion of the community\nwould probably like to see it and interpret the potential processes it indicates for themselves. I\nwould suggest that the authors add a visualization of ash fall and PDC thickness to the map of\nstations where total thickness is indicated by dot size and fraction of fall, flow in a pie chart at\neach site. This information should be used to help support the arguments in the paper of erosion\nand deposition inferred from the bathymetric differencing and modeling. Also, it would be useful to\nknow whether any cores were taken that did not show evidence for deposition (in the refugia, for\nexample).\nWe have added a figure in the supplemental information which details locations of sediment\ncores, detailed visual sedimentological core logs and core photography along a transect that\nextends to the south-west from the caldera, with distance away from caldera indicated. This\nsupplemental figure shows that even 80.5 km away from the caldera, we were still sampling\ndensity flow deposits 23 cm thick. Closer to the caldera we were unable to retrieve long enough\ncores to constrain the extent of the flow deposit (the base was not able to be sampled). In the\nareas of refugia, we were unable to successfully retrieve cores due to the steepness of the seafloor\nand the rocky terrain. A follow-up paper (in review now) contains a more extensive record of\nsediment cores collected during the survey, and specific focus on the behaviour and evolution of\nthe pyroclastic density currents. This level of detail is beyond this scope of this manuscript which is\nintended to be a multidisciplinary overview of the impacts of the event, rather than specifically\nfocusing on specific sedimentological aspects. Additionally, we have added an improved image of\nthe core to Figure 3.\nSome more specific comments are detailed below…\nLine Comment\nsubmarine environment, or more preciscely more than 3/4 of all volcanic output. It would be good\nto cite Crisp et al., 1984 and maybe White et al., 2006 for this statement.\nWe have addressed this in the manuscript, lines 25-276 now reads: “..three quarters of all volcanic\noutput worldwide…”\nknow that the erupted volumes have been accurately determined and I’m not sure if I would count\nedifice loss in the magmatic output that would make it a VEI 5.\nLine 30 now reads: “…what some have suggested is the largest seaborne volcanic eruption in\nnearly…”\nSimilarly,\nLines 56-60 now reads: “With a Volcanic Explosivity Index of 5.7, the 15 January eruption was the\nlargest eruption since Mount Pinatubo in 1991 and is one of the most explosive submarine\neruptions ever recorded and largest documented in a century or more.”\n30-31 Where does the information on the length of damaged telecom cable come from. I spoke\nwith the crew who was fixing it and I did not get the impression that they were laying 100km of\ncable.\nTonga Cable Ltd (the cable owner) has recently commented that “The actual break distance\nbetween the 2 recovered ends of the international cable was actually 89km rather than 35km”.\nThe domestic cable was damaged along a stretch of 105 km and has not yet been repaired as of\nthe writing of this response. We have updated the numbers in the manuscript accordingly (lines\n179-183: The model is consistent with the locations and extensive damage to the international and\ndomestic telecommunication cables, damaged along 89 km and 105 km length, respectively (Fig.\n3), and explains the observed spatial trends in erosion and flow runout.)\nliterature for documenting global submarine volcanic output.\nWe have addressed this in the manuscript.\n46-49 I would suggest removing the statement about risk in the Pacific Ocean. The vast majority\nof the 100,000 seamounts are extinct and are so remote as to not pose any real hazards even\nwhere they active.\nWe have modified the statement to this (lines 50-54):\n“This observational bias is severe in regions under-represented by scientific study, such as the\nsouth Pacific Ocean where the majority of the world’s 100,000 uncharted underwater volcanoes of\n>1 km in elevation. In particular, the shallow water active volcanic centres that are near populated\nislands expose a major blind spot regarding risk assessment and response preparedness.”\n61-68 There is some missing literature here for studies that look at pre- and post-eruption seafloor\nconditions (notwithstanding that this study does not really have a comprehensive pre-eruption\nsurvey of benthic biology). I would include Carey et al, 2014 & 2018; Embley, 2014; Soule et al.,\n2009; Chadwick et al., 2008; Watts et al., 2015. It’s up to you if you want to say that it hasn’t\nbeen done on an eruption the scale of HT, but it doesn’t seem necessary.\nWe have added more references including these throughout the document. We were constrained\nby reference limitations in original submission but have asked for additional references to be\nallowed.\nstages of this eruptive episode.\nWe have added this reference, now line 88.\n114, 118 As mentioned, the authors should explain why they think the ‘PDCs’ originated\nsubaerially.\nEye witness imagery has shown collapses from within the plume below the umbrella cloud shortly\nafter the first major eruption on 15th January, which generated subaerial pyroclastic density\ncurrents; hence, we assume this is the most likely origin mechanism. However, we recognise this is\nnot definitive, and as discussed above, we have now revised the language to be more speculative\nin the initiation of the flows (i.e.: lines 139-148 “The PDCs most likely initiated from the partial\ncollapse of the eruptive column of Hunga Volcano, with this hypothesis similarly supported by the\ntiming and intensity of the tsunami waves39. However primary magmatic explosivity,\nhydrovolcanic explosions, or mass wasting of the edifice could have also been formative\nmechanisms. Sensitivity tests of our PDC model (see Supplementary Material, Fig. S6),\nhowever, confirm that whatever initialisation mechanism caused these PDCs it most likely occurred\nwithin the caldera in order to cause the strong channelisation that was observed on the flanks of\nHunga volcano. An initialisation mechanism such as a fountaining PDC where the PDC and water\nmixed outside of the caldera (i.e. on the flanks of the Hunga edifice or even further out) would not\nhave caused this channelisation.”)\norigin of the PDCs from column collapse? It might be better to just say ‘We observe’.\nWe have addressed this in the manuscript, now line 149-154: “We observe crescentic scours (up to\n30 m deep) and bedforms (up to 1,200 m wavelength, 20 m wave height; Fig. 2), commonly seen in\nother deep-sea settings affected by powerful density flows40, (including those triggered by large\nmagnitude earthquakes, that severed seafloor cables, comprised a dense basal layer, and have\nbeen shown to attain speeds of up to 20 m/s)41–43, and further corroborated by seafloor sediment\ncoring and imagery, geochemical analysis and numerical modelling.”\nclarified. The model itself is not described, which is fine in the context that it is used - to define\nflow pathways. Because of this, I’m not sure the volume estimate is particularly relevant.\nHowever, if this would be considered a minimum estimate because of the model assumptions,\nthe authors could indicate that and leave it in.\nWe have added clarification on this point to lines 173-182: “Numerical modelling of the submerged\nPDC (see Methods section for further details) suggest that approximately 3-4 km3 of sediment\nladen flow is required originating from inside the caldera rim to replicate the extent of the\nobserved PDC deposits, especially the erosion channels on the flanks of Hunga edifice and the\novertopping of two submarine knolls to the south heading towards the international cable (Fig. 3).\nWhile our relatively simple model does not account for seafloor erosion, incorporation of\nadditional material, nor multi-phase flow, the flow pathways that the simulated PDC follows align\nremarkably well with the areas of most pronounced seafloor change. The model is consistent with\nthe locations and extensive damage to the international and domestic telecommunication cables,\ndamaged along 89 km and 105 km length, respectively (Fig. 3), and explains the observed spatial\ntrends in erosion and flow runout.”\nIn this we have also directed readers to the Methods section for further details. Importantly, in\nour model simulations, smaller volumes of sediment laden flow did not recreate what was\nobserved following the eruption, including the fact that the PDCs appear to have overtopped two\nsubmarine cols in the south before reaching the international cable. We have added other model\nsimulations to the supplemental to allow the reader to see these differences.\ngroup that visited the Lau Basin and found evidence for HT ash deposition there, but nothing in the\nwork described in the paper would necessarily predict that. To avoid undercutting the observations\nfrom the other group, it would be appropriate to simply say that the flows likely went well beyond\nthe study area and not include mention of the Lau Basin.\nThe flow clearly extended towards the northwest towards in the direction of the Lau Basin which\nlies downslope, but we have no data to indicate how much further it went. As a result, we have\nremoved mention of the Lau Basin from the document as suggested.\nof mussel beds.\nCorrected wording, now lines 217-222: Pre-eruption deep-sea benthic surveys for the Hunga\nVolcano edifice and adjacent seamounts were not available to compare with our new post-\neruption data, except for discrete observation of mussel beds on a southern seamount within our\nstudy area33. The mussel beds visualised in 2007 and 2017 on the southern seamount were still\npresent, and of similar density to prior observations33, indicating that these communities were not\naffected by the eruption of Hunga Volcano (Fig. S11)..\n182-184 The statement about suspended sediment and hydrothermal discharge needs some more\nexplanation. First, there should be a reference to the supplement otherwise it is unclear what is\nbeing referred to in terms of suspended sediment. I’m also unsure why it is expected that\nsuspended sediment will be a long-lasting effect. It should clear up in a matter of weeks to months\nunless something else is happening. In addition, I’m not sure why hydrothermal discharge is\nviewed to have a primarily toxic effect? In most cases it can have a positive impact on ecosystems\naround the hydrothermal vents and a pretty limited impact on suppressing ecosystems locally and\ncertainly not regionally.\nThis sentence has now been removed.\nFigures\nFigure 2 This is a great figure that gives a very nice view of how density currents progressed and\nevolved. There is no description of the methods used to make the difference map or uncertainty\nassociated with he estimates. There is clearly some error associated with rough terrain on features\nthat are known not have changed, such as flank domes, that can arise from minor navigation\noffsets. It looks like that difference may not be stochastic (i.e., distributed around zero), but has a\nsomewhat positive. It would be worth taking a look at the net gain loss in areas that are known to\nnot have changed to make sure the gain/loss is not biased. I’m also a little dubious of the spur to\nthe NW, where pre-eruption data is sparse. There seems to patterns in the gain loss that look\nmore like artifacts than real signal. I would be careful interpreting that and it may be better to\nleave it out of the analysis.\nRev 2 is correct that the color range we previously used was heavier on the gain, and so we have\nadjusted the color bar to be evenly distributed about zero. Additionally, the neutral color that was\nused in the previous figure to show no change was a warmer color that appeared to be more\ntowards the orange side and skewed the image. We have corrected this. If you view the histogram\nincluded below you can see that the gain/loss appears roughly evenly distributed and does not\nappear to be biased.\nAdditionally, in the manuscript, we speak mostly of qualitative morphological change, not\nquantitative change. The only quantification that we provide is on the order of cubic kilometers. In\nthis case, the error associated with the echosounders, even in the most erroneous scenarios,\nwould be 1% of that quantification. The maximum error in the deepest portion of our survey area\n(associated with echosounder error) would be 2m vertically, and in the shallowest portions of our\nsurvey area it would be 0.1m. This error would not significantly change the estimates we have\nprovided on the order of cubic kilometers and we consider the specific uncertainties associated\nwith the difference map estimates (which would require a depth variable uncertainty grid) to be\nbeyond the scope of this manuscript, and not significant enough to impact the cubic kilometer\nvolumes that we report in this manuscript.\nFigure 3 The modeled PDC paths in Figure 3 should be made less prominent. They are best\nguesses at flow paths and are rather complex and distracting in the figure. It would be nice to see\nthe presence/absence and abundance of benthic life against the loss-gain map, or a least the\naverage for that area could be shown next to the point. It seems that there are some points with\nbenthic biota present near the caldera (to the east) that seem contradictory to the idea of\ncomplete destruction of habitat. Also, please use a higher resolution image of the core. It is\nimpossible to see.\nThe modelled PDC paths have been made less prominent in Figure 3. We think it is best to keep\nthe benthic biota observations in Fig 3 and not include with the loss/gain map, however we have\nincluded a simplified abundance table as a supplemental table to show what groups of\ninvertebrates were present in each location, and the shift in diversity of invertebrate groups, e.g.,\nmore diversity in areas of refuge. Additionally, this table shows orders of magnitude difference in\nthe abundance of invertebrates near to the caldera and in areas of refugia. Further, these points\nwere there are some benthic biota still present are within areas that have minimal seafloor\nchange noted in Figure 2 (and appear outside of PDC paths).\nWe now include a higher resolution core image as well in Fig. 3 and in the Supplemental core log.\nFigure S3 It is hard to see the symbols on the geochemical plots. A higher resolution version\nshould be included. Also it seems that the T2206 data is just plunked on an older figure and covers\nup some of the previous data. Also, the legend is missing a symbol for T2206 shards.\nWe have made a higher resolution version of Figure S4 (updated numbering) and corrected issues\nwith data plotting which was resulting from the original figure being corrupted during conversion\nto a PDF.\nReviewer #3 (Remarks to the Author):\nThis manuscript compiles observations from ship-based cruises to describe the seafloor impacts of\nthe January 15, 2022 Hunga Tonga-Hunga Ha’apai eruption. In addition, they describe results\nfrom a PDC model that they plot against observations. Observations include changes in seafloor\nbathymetry, changes to biologic communities, and reports on the characteristics of the\nsedimentary deposits. Due to the remarkable nature of the 15 January Hunga Volcano eruption, I\nsuggest that these results are worthy of publication in a high impact journal such as Nature\nGeoscience. The changes in seafloor bathymetry, existence of pyroclastic density current deposits,\nand runout distances of these pyroclastic current deposits will be of widespread interest to the\nvolcanological and oceanographic communities, in my opinion. That said, some of the\ninterpretations are not currently well-supported by information provided in the ms although I do\nnot doubt that this data was collected. I also have significant questions about the PDC model. After\nthese and other changes are addressed, I would highly recommend publication as I believe these\nresults are of particularly high impact and importance. Please see my attached comments.\n-Kristen Fauria\nWe thank Reviewer 3 for their time spent in reviewing our manuscript and offering constructive\nfeedback and encouragement. We have addressed concerns and made changes as indicated\nbelow.\nGeneral Comments (from attached document)\nFirst, I want to congratulate the authors on their incredible work surveying the Hunga Volcano post-\neruption. Their findings on PDC runout and the lowering of the caldera floor are remarkable. My\ncomments and suggestions below are primarily aimed at clarifying my own questions and making\nsure their observations can be well understood and utilized by readers and scientists studying the\nHunga eruption.\nWe thank the reviewer for their positive feedback.\nThe change in bathymetry is a major finding of this manuscript. Because of its importance I would\nsuggest clarifying both the bathymetric observations in the text and improving the quality of the\ndifference map in Figure 2. Specifically, there is no absolute scale on the colors used in the\ndifference map. I cannot see a difference between the remarkable >800 m loss in the caldera and\nmore modest loss along the flanks.\nWe have added an absolute scale to the colours used in the difference map, this is +/- 30m as you\nwill now see in Figure 2. We are intentionally trying to not focus on the >800m loss in the caldera,\ninstead focusing instead on the loss on the flanks. The loss within the caldera is a focus of other\nstudies by our colleagues. We have included the cross-section showing the loss within the caldera\nin Figure 1 to allow the reader to appreciate the massive loss within the caldera, which compares\nto the more modest changes on the flanks. We have also made additional changes to Figure 2\nfollowing recommendations of Reviewer 2 which should make some of the loss/gain that we have\nmeasured more obvious.\nI also cannot tell, from the text or figure, how much variation there is in deposition or scour along\nthe flanks.\nThe changes we have made to Figure 2 should make the variation in deposition/scour along the\nflanks more obvious.\nThe ms could also benefit from a brief description of the pre-eruption Hunga Volcano morphology\n(i.e., that it is a caldera volcano) before a discussion of the topographic changes is introduced.\nWe have added the following sentences / reworded text in lines 87-96 with this in mind:\n“The islands of Hunga Tonga and Hunga Ha’apai are remnants of the rim of a roughly 3- to 4-km\nlarge submarine caldera of Hunga Volcano, with eruptions noted in various parts of the caldera\nthroughout the past century. The most recent eruption series (in 2015) formed a 120m high and\n2km wide near-circular tephra cone which joined the two islands together6,7,26. After this eruption\nthe Hunga Volcano was largely inactive until it began erupting in late 2021, producing a steam-\nrich gas and ash plume. Following this, Hunga Volcano continued erupting intermittently until two\nlarge eruptions 14-15 January 2022, with the largest on the evening of 15 January34.”\nThe authors claim that the areas of deposition on the volcano’s flanks are due to PDC deposition.\nAlthough this is reasonable, the claim is not well-supported by sedimentological observations in the\ntext or supplement. More information along the lines of the stratigraphic section in Fig. 3 would be\nhelpful For example, the authors discount sedimentation by landslides without explaining why or\ndefining what they consider to be a “lack of evidence.” The deposits are interpreted to be PDCs in\npart due to bedforms and sediment cores, but the features within the cores or bedforms that\ndistinguish the deposit as PDC are not mentioned or defined. Although thorough work on the\nsedimentary structures within their cores and from video observations may be outside the scope of\nthe paper, I suggest that the authors describe 2 what specific aspects of the cores and seafloor\nobservations allow them to define the deposits as PDC related (rather than mass flow by other flank\ncollapse, for example). In the end, I agree that these are PDC-type flows but based on their spatial\ndistribution rather than the sedimentological evidence provided.\nWe have modified the sentence in lines 126-129 to be more inclusive of the possibility of land\nsliding:\n“There is no superficial morphology consistent with seafloor landslides (i.e., headscarps and/or\nscars) on the volcano flanks despite the scale of the eruption and associated seismicity, which\nmakes this event distinct from partially-submerged eruptions that generated large slope collapses\nsuch as Anak Krakatau (Indonesia) in 201837”\nAs we state in lines 132-134, it is the spatially variable change that we are relating to the PDCs first\nand foremost, as well as the crescentic scours that are indicative of a large sediment-laden flow\n(lines 149-150). The sediment coring and other analysis corroborate this, but detailed analysis of\nthe sediment cores collected as part of this broader work is beyond the scope of the manuscript\nand will be included in a follow-up manuscript that is in review.\nI found the authors findings on post-eruption venting compelling. That said, the descriptions of\nphenomena such as post-eruption venting are only vaguely described. The ms would benefit for more clear\n(even if in the supplement) descriptions of where venting was and was not observed and how.\nWe have adjusted the text to reduce the focus on potential venting. Instead, in lines 251-252:\n(Future research will seek to determine if this water column turbidity is related to ongoing venting\nfrom plumes within the caldera) and in the figure caption for Supplemental Fig. S12 we specifically\nsay that the particle layer in the water column, and the ‘plume’ image indicated in Fig. S12, are\npotentially a result of ongoing venting within the caldera, with the nature of this to be determined\nby future research. We are specifically trying to not describe these phenomena as there is ongoing\nwork to comprehensively characterise plumes detected within the caldera that we are trying to\nnot supersede.\nI am pleased that the authors employed a PDC model and compare the model to observations. I\nhave major questions about the model, which I detail at the end of this review. One main concern is\nthat it is not clear however, what aspect of the PDC model they are comparing to what characteristic\nof the observations. There is some discussion of runout. However, how is PDC runout characterized\nin the model and in the observations?\nMinimum runout is best constrained by the coring and cable damage. The model (as described in\nthe Methods Section) is a fairly simple two-layer model where the turbidity current from the PDC\nis modelled as a dense Newtonian fluid underneath the water layer. As such it will not correctly\npredict the maximum extent of the runout but will over-estimate this. The model is, however, able\nto show that a minimum volume of PDC is required to over-top two saddles to the south of Hunga\nto reach the international cable. We have clarified this in the text.\nHow do the authors determine the flow direction (arrows on Figs. 2 and 3) from the model? The\nauthors acknowledge that the model does not include deposition. How, then, is the ultimate flow distance in the\nmodels determined (what process stops the PDC in the model)? I agree with the authors approach of\nemploying a general model, but it is not at all clear how their model works.\nThe flow directions in the figures are merely indicative. They were added to the figures from\nanimations of the flows. The supplemental material contains snapshots of these flows at various\ntimes (Figure S6,S7) as well as an animation of the flow in the electronic supplementary material\n(Supplemental Video 1). As mentioned above, the model provides no information as to the\nultimate flow distance as the PDC is modelled as a Newtonian fluid and therefore only stops when\nit reaches a depression. The model is, however, able to distinguish between the volume of PDC\nrequired to overtop the saddles to the south of Hunga and reach the international cable, which we\nsee from the sea-floor observations.\nDetails of the equations used in the model are now given in the Methods Section.\nLine by line comments\nLine 63: I suggest being specific about the types of data used earlier in the ms and on this line.\nWe have addressed this in the manuscript, the sentence now reads (now lines 73-76):\n“Combining these datasets (including multibeam echosounder and geological data, video footage\nand water column samples), reveals the wide-reaching ocean impacts of such a large eruption,\nranging from topographic changes and damage to critical infrastructure, to widespread loss of\nseafloor life.”\nLine 68: Add “mass” to the description of “loss.”\nWe have addressed this in the manuscript (line 75-76: widespread loss of seafloor life.).\nLine 79: The eruption on Jan 13-14 could be described as a single large event rather than a series\nof events before the 15th (e.g., Gupta et al., 2022).\nWe have clarified this in the manuscript (lines 93-100: “After this eruption, Hunga Volcano was\nlargely inactive until it began erupting in late 2021, producing a steam-rich gas and ash plume.\nFollowing this, Hunga Volcano continued erupting intermittently until two large eruptions on 13-\nprior to the largest eruptive episode in the evening, showed the islands had been disconnected by\nthe eruption on 13-14 January 34 (Fig. S5). After the 15 January eruption, only small remnants of\nthe islands of Hunga Tonga and Hunga Ha'apai remained above water. However, the modification\nof the submerged 99% of the volcano edifice remained unknown.”)\nLine 87-91: Here the Hunga summit is mentioned for the first time and it would benefit with a brief\nsummary of context. I suggest briefly describing the pre-eruption bathymetry to give the readers\ncontext for where bathymetric change occurs.\nWe have added lines at the beginning of this paragraph (starting at line 82) describing that Hunga\nVolcano is a caldera volcano that has had various eruptive phases, that most recent of which\nformed a 120m high and 2km wide near-circular tephra cone which joined the two islands\ntogether. It is this difference between the tephra cone and the new caldera depth that we describe\nin the next paragraph and illustrate in Figure 1. The depth of the remainder of the volcano that\nwas submerged is shown in Figure 1, with contour lines added to aid in the visual analysis of\ndifferences in depth pre- and post eruption.\nIt also seems crater and caldera are used interchangeably. Is there a difference between the crater\nand caldera? What kind of change occurred along the caldera rim?\nWe have changed all instances of crater to caldera. With regards to change along the caldera rim,\nchange on the caldera rim or within the caldera is not the focus on this manuscript but will be\nextensively detailed in follow-on work. This manuscript is focused on change on the flanks of the\nvolcano and the surrounding seafloor, and we want to keep this focus in the manuscript.\nLines 89-91: Could you expand upon the volume loss along the caldera rim. Was the mass loss\nuniform along the flanks or localized? If uniform, what was the average change in vertical elevation\nalong the flanks? How far from the caldera rim did the mass loss extend? That is,\n2\nwhere was the transition from erosional to depositional features and how uniform was that distance\nfrom the caldera rim?\nWe have adjusted the colour-scale of Figure 2 which makes the mass loss / gain along the rim of\nthe caldera and along the flanks of the volcano more obvious. We have added a sentence at line\n124 to indicate how far from the caldera rim mass loss extended:\n“Most of the measured loss was within 6.5 km of the caldera rim, with this distance greatest in\nthe NW direction, beyond 6.5 km there is predominantly a depositional environment.”\nLine 93-94: Ashfall is mentioned here and subsequently. However, the ashfall portion of this\neruption is not described in your ms. Could you briefly describe what is known about the presence\nof ashfall?\nNow line 155-156 – we have updated to : “..overlain by fine volcanic ash deposits tens of\ncentimetres thick..” as the ash is not necessarily from ash fall but could also be from submarine\nash release and deposition.\nLine 95-97: The supplementary data table is cited as evidence here. I am so glad you have included\nthese supplementary data. However, what aspect of the data are you suggesting makes this point?\nCould you perhaps provide a plot of the data in the supplement to make this point?\nA plot of the data is provided in the Supplementary Material (under Expanded Geochemical\nResults). This section shows plots of the data compared to previously analyses of material from\nHunga volcano (Brenna et al., 2022) as well as other nearby volcanoes. The data match those from\nBrenna et al (2022) and are distinct from other volcanic material sourced from other nearby\nvolcanoes, indicating a Hunga volcano source. A brief statement reflecting this has now been\nadded to the main text (specifically lines 161-165: “Scanning electron microprobe analysis and X-\nRay fluorescence spectrometry indicate that PDC and ashfall deposits have the same chemical\ncomposition as samples previously analysed from Hunga volcano44, suggesting that they were\nsourced from the Hunga Volcano eruption (Supplemental Information; Figs. S3, S4; Supplemental\nData 1).”)\nLine 98: “There is consistent loss” is rather vague. What is consistent about the mass loss (spatially,\nmean change in elevation?). Why choose a water depth threshold? Does this correspond to a mean\ndistance from the caldera rim? If so, what is that distance?\nThe word “consistent” has been changed to “widespread” (now line 116). This is more of an\nobservation, above 600 m we see erosion and below we see deposition. We have also included\nreference to the distance (6.5km) from the caldera rim that we see a transition from\npredominantly mass loss to mass deposition (lines 124-126: “Most of the measured loss was\nwithin 6.5 km of the caldera rim, with this distance greatest in the NW direction, beyond 6.5 km\nthere is predominantly a depositional environment.”)\nLine 102-103: I assume here you mean chute termini. Could you make more clear the transition in\ndiscussion from erosional to depositional features.\nWe have added the word “chute” here to clarify (now line 121), as well as distance as indicated\nabove (6.5km). Additionally, the changes made in Figure 2 should make this transition more\nobvious.\nLine 106: What do you consider an absence of evidence of landsliding? Could you briefly list the\nfeatures you would look for to indicate landsliding?\nWe have added: “There is no superficial morphology consistent with no seafloor landslides (i.e.,\nheadscarps and/or scars)” to lines 126-127.\nLines 113-114: The process-based description of PDC formation here is a suggestion or\ninterpretation rather than an observation. I suggest, therefore, caveating this sentence with a phrase\nsuch as “we suggest”. I also wonder the extent to which the flows may have never entered the\natmosphere and remained entirely submerged since the vent was submarine? This possibility could\nbe acknowledged even if only for a portion of the flows.\nWe have modified this sentence (line 134-136) to read: “We suggest these flows initiated as PDCs\nthat mixed with the ambient seawater, becoming turbulent underwater density flows..”\nAdditionally, in response to Reviewer 2 we have also adjusted the language elsewhere to\nreflect the alternative mechanisms, now lines 139-142.\n“The PDCs most likely initiated from the partial collapse of the eruptive column of Hunga Volcano,\nwith this hypothesis similarly supported by the timing and intensity of the tsunami waves39.\nHowever primary magmatic explosivity, hydrovolcanic explosions, or mass wasting of the edifice\ncould have also been formative mechanisms.”\nLine 118: Please describe the plume collapse as “partial” since the plume attained a height of 57 km.\nWe have modified this sentence accordingly (now lines 139-140: The PDCs most likely initiated\nfrom the partial collapse of the eruptive column of Hunga Volcano..).\nLine 120-121: Could you please remind the reader how these observations were made. Overall,\nthese scours are remarkable and could be discussed or emphasized more than they are at present\n(purely optional).\nThis is the focus of follow-on work so we do not analyse them in detail here.\nLine 126: Could you please remind the reader the scale of a granule? Using what methods was\ngrainsize evaluated and quantified?\nGrain size was identified using a visual grain size comparator as outlined in the Methods section.\nWe now specify “up to 4 mm-diameter scoria” rather than “granule-sized” for clarity (line 159).\nLine 124-125: How far was this from the vent?\nHave added distance (now line 154 : “Multicoring 80 km from the caldera”)\nLine 128: Here an earlier description of ashfall could be helpful. What sample did you compare\nagainst to determine they were from Hunga Tonga?\nThe data was compared to that of Brenna et al (2022) who analysed volcanic material from the\nislands, the data match extremely well (see Supplementary Material – “Expanded Geochemical\nResults”. In addition, the data was compared to other nearby volcanoes and is distinct from them.\nOverall, the finding that the PDCs go from erosional to depositional is very interesting. As mentioned\nearlier I would love more information on the dimensions (thicknesses, distance from the caldera)\nof the erosional and depositional components. Could you also put the vent location in the context of these observations. It seems\nthat you assume all material was sourced at the vent, which was on one side of the caldera, yet the erosional\nand depositional features are evenly distribution around the caldera? Could you comment, confirm, or\nclarify in the ms?\nWe are treating the caldera as the centre, with the flows radiating out. This intensity of this\nradiation does vary around the caldera rim, with some pre-existing chutes (e.g. the NW direction)\nhaving more focused erosion.\nWhat were the maximum and characteristic runout distances of the PDC for both the\nchannelized and unchannelized components?\nWe cannot state this with certainty as the model is not appropriate to define runout for areas\nwhere it is not bathymetrically constrained. We can infer from observations (i.e. cores and cable\ndamage) where the PDCs definitely reached and runout distance were likely considerably further\nthan these locations based on the depths of deposits measured and the amount of damage. From\nthis we can estimate minimum volumes of the PDC required to reach this far (the international\ncable lies over two submarine saddles). However, our interpretation remains speculative (e.g.,\nlines 190-192: The longest observed PDC runout likely exploited bathymetric lows to the NW of the\ncaldera. Modelling and post-eruption bathymetry indicate this PDC went well-beyond the northwest\nlimit of our survey area). This will be investigated in more detail in a follow-up manuscript.\nLine 135-136: Please describe the basic attributes of your model here. Was the model for a dense or\ndilute flow? What were the basic assumptions or initial conditions? Did you model a PDC in water or\nin air, for example? Here you mention replicating the extent, are you referring to the depositional\nextent? How did you do this comparison in practice, especially if the PDC model does not include\ndeposition?\nWe describe the model in the Methods section. What we actually model is the underwater flow\ncharacteristics of the dense current created from the mixture of the dense PDC and the ambient\nwater. The PDC and the water above it are both modelled as Newtonian fluids which is why we\nare not able to make predictions about the maximum extent of the PDC. When we refer to\nreplicating the event we are referring to the fact that we are able to replicate the flow paths seen\nin the observations and our maximum flow depths compare well with locations where benthic\nfauna was destroyed. We also specifically refer to the fact that a minimum volume of PDC was\nrequired to overtop the two submarine knolls to the south of Hunga and reach the international\ncable.\nLine 137: Here you introduce the concept of simulated flow pathways. How are these defined? I am\nunfamiliar with PDC models being simplified into the arrows shown on your figures. On what basis\nare you defining these pathways?\nThese flow path lines are simply an attempt to illustrate complex time varying flows over 2D\ntopography in a single diagram. Snapshots of the modelling are also shown in the supplementary\nmaterial as well as an animation being provided (Supplemental Video 1). These show where the\nflow fronts and deepest material flowed in the simulation.\nLine 140: What is meant by broken cables along >30 km and >100 km lengths? Is this distance from a\nlocation? If the cables were not broken in one location, could you please describe?\nWe have modified the sentence for clarity (now lines 180-182: “extensive damage to the\ninternational and domestic telecommunication cables, damaged along 89 km and 105 km length,\nrespectively”). These lengths are the points between which the cables were damaged. As\nmentioned earlier, we have since received further clarification from the cable owner that the\ncables were actually broken along 89 km (international) and 105 km (domestic cable) length.\nLine 144: I do not understand this comparison. Are you comparing the model results to\nobservations from areas other than Hunga?\nWe have clarified this in text by saying: “As the submarine PDC flows away from the caldera, it is\nsteered by the local bathymetry and in some locations separate PDC flows converge back together\nand accelerate. This has previously been observed subaerially but only inferred in submarine\nsettings” (now lines 184-190).\nLine 144: What was the longest PDC runout?\nAs we indicate above, the model does not allow us to accurately provide this information, nor do\nwe have sufficient observational data to estimate the longest PDC runout (as this likely went\nbeyond our survey area as we indicate).\nLine 163: What aspect of the PDC model is being compared to locations of the refugia?\nWe have modified this to read: The locations of these refugia match closely the thickest flow paths\nof the PDC modelling, which shows that deflection of flow pathways around irregular relief and\nseamounts, leaving some areas relatively unaffected (Fig 3, S6; Supplemental Video 1).” (now lines\n215-217).\nLine 185-186: How was turbidity measured and where and when? Was there excessive turbidity\neverywhere or in specific locations (both spatially and in water depth)? Could you expand upon the\ndefinition of nepholoid layers as this may be a new concept to many volcano readers?\nTurbidity was measured with a transmissometer on a CTD rosette. This had been added to the\nfigure caption of Fig S12 and the new Fig S13. The locations of nepheloid layers are now shown in\nthe new Fig S13, with the spatial and water depth variation shown. We have expanded upon the\ndefinition of nepheloid layers in the main text as well as added clarity to this section (lines 251-\n257: Future research will seek to determine if this water column turbidity is related to ongoing\nventing from plumes within the caldera Near-seabed ash layers (e.g., nepheloid layers, a layer of\nwater above the ocean floor with a significant amount of suspended sediment) were detected\nabove the seafloor across many of our sites, in some cases extending hundreds of metres up from\nthe seafloor (Fig. S13). In all instances (mid-water and near-seabed ash layers), the turbidity\nanomalies were identified as being ash particles with SEM imaging and analysis (Figs. S14-17,\nSupplemental Table 1).).\nCould you make Fig. S10 easier to interpret to substantiate this point. For example, the acoustic\nimage is not annotated and has no scale, date/time, or specific location (lat/lon). The ongoing\nventing is of particular interest and currently it is only vaguely described. Where and when was\nventing found? Can you determine if/when the venting shut off?\nWe have added additional text, and SEM images to support the beam transmission anomalies\nshown in Fig S12 and reported in the manuscript.\nIn the conclusions, I suggest providing a brief summary of your bathymetry change and\nPDC\nerosion/deposition observations.\nWe have added this to the conclusions, lines 263-271: Our data illustrate how an eruption of a\nsingle submerged volcano can have wide-reaching and diverse impacts. PDCs resulting from the\nHunga Volcano eruption were steered by antecedent morphology, further excavating pre-existing\nchutes and depositing the majority of the erupted volume on the seabed within 20 km of the\nHunga caldera rim. Modelling and benthic observations indicate that the majority of the PDCs\noriginated within the caldera and flowed around bathymetric relief as they moved to deeper\nwaters, with only local preservation of benthic communities on topographic refugia. Outside of\nthese topographic refugia the seafloor ecosystems around Hunga Volcano were decimated by the\neruption and related PDCs.”\nI am not a petrologist and so cannot evaluate the details of their methods for determining the ash\ncomposition. It could be more clear what sample they compare against to determine the true Hunga\ncomposition.\nThis is provided in the Supplemental Material ‘Expanded Geochemical Results’ which shows the\ndata compared to other studies\nFig. 1: Can you please make clear if the two A to A’ transects and B to B’ transects are the same or\ndifferent. If location A is location B and location A’ is location B’, I suggest using the same label for\nboth.\nWe have made this change.\nFig. 2: How do you choose how many flow trajectories to plot here from the model, why not 3 or\n300? Please provide an absolute scale for mass gain/loss. What is meant by “diffusive flow”? I also\ndon’t see any evidence for interaction with topography as annotated on the figure.\nPer recommendation of Reviewer 1 we have removed the text entirely that was describing the\nflow types. An absolute scale for mass/gain loss has been added as well. The flow trajectories are\nsimply indicative flow paths interpreted from model animations and snapshots (see Figure S6 and\nSupplemental Video 1) and they represent the main channels of flow seen in these. The number\nwas basically chosen as a minimum thought to be able to represent the time-varying movement of\nthe flow on a static map. We have clarified this in the caption. Lines 491-494: “Indicative PDC flow paths (i.e., the\npassage of the flow fronts as estimated from model animations and snapshots (see\nSupplemental Video 1 and Fig. S6)) are indicated with dashed grey lines plotted over the difference\nmap with arrows indicating flow direction. “\nFig. 3: Is this sedimentary log based on a single point or a general summary? To what extent is this\nlog representative? Overall, this is a nice figure. I have the same issue with the flow pathways. I\nwould like to see scale bars on A-C and the other panels labelled with letters as well.\nThe sedimentary log is based on a single point and is representative of the general findings. The\ndifferences in deposits measured at each core site is indicated in Table S1, to compare with the\ndetailed core log in Fig 3 and Fig S1. We have added scale bars to A-C, but prefer to keep the other\npanels without letter labels. We have also added a description about how the flow pathways\nshown were drawn (Lines 504-508 “Indicative PDC flow paths taken from modelling results (see\nSupplemental Video 1 and Fig. S6) are shown in solid black lines and locations of damaged\nsubmarine cable are depicted with dashed pink lines. In order to reach the international cable,\nthe PDC must overtop two knolls directly E and ESE of location C, which partially constrains the\nminimum volume of material in the PDC.”)\nLine 406-407: How much footage (distance or time)?\nWe have added this (now lines 533-536: The abundance of invertebrate taxa were counted in real\ntime from the live video feed (totalling 31 hours over a distance of 32km) and hence the data\nincluded here should only be regarded as provisional estimates of abundance).\nLine 505: Why 50 cm gridding of the data. This seems very high resolution for ship-based\nbathymetry. Wouldn’t the natural resolution be much lower.\nThe gridding was to a 50 m resolution, not 50 cm. This is the standard gridding that we use.\nPlease add scales to all images.\nCould you please make sure there are geographic coordinates included in all sampling locations (e.g.,\nFig. S1 and data tables).\nWe have added scales to all images. Additionally, we have added geographic coordinates to Table\nS1 (which also covers Fig S1) and the Supplemental data table. We also have locations for the\ninvertebrate observations in the new Supplemental Table 2.\nPDC Modeling Comments:\nPDC modeling is challenging and I think it is neat that the authors undertook this challenge in addition\nto their substantial ship-based work. Overall, I cannot understand the model based on the information\nprovided. Please see the questions below.\n- Did you consider the current dilute or dense in your modeling effort? Your choice would result\nin different conservation equations and assumptions.\n- I am unfamiliar with this modeling software for PDCs. Can you describe how common it is for PDCs\ncompared to other types of models such as VolcFlow, Titan2D, and other known codes? Why did\nyou choose this modeling software?\n- What are the governing equations for mass, momentum, and energy in your model?\n- The two-layer choice is interesting, but I don’t understand how it is set up. Why two layers\nwith a water layer on top? The water layer is the ambient fluid so shouldn’t be considered\npart of the flow. How does this work?\n- How was flow through water dealt with since most PDC models consider flow through air.\nThat is, how did you treat the ambient fluid (as water or air)?\n5\n- Was the model 1D, 2D, or 3D?\n- Is the model single-phase?\n- What controls runout distance in the model. How does the model stop? This is\nparticularly important since runout is one of the features you compare.\n- How is the model coupled to topography?\n- Was the initial volume of PDC material placed on the seafloor, at the water surface?\n- How did you parameterize key aspects of the model (e.g., friction coefficient, Froude number,\nRichardson number, or entrainment coefficient)? I don’t understand enough about your model\nto know what you needed to parameterize, but models that aren’t 3D include these types of\nparameterizations.\n- Can you clarify your boundary conditions.\n132-139 (We relate these spatially variable changes in seafloor elevation on the volcano flanks to\nparticle-laden gravity flows that radiated downslope from Hunga Volcano in multiple directions,\nexploiting and enhancing pre-existing topography (Figs. 2A, S6; Supplemental Video 1). We\nsuggest these flows initiated as PDCs that mixed with the ambient seawater, becoming turbulent\nunderwater density flows (e.g., turbidity flows) that were steered by the complex topographic\nrelief around the volcano. For simplicity, and due to their formative mechanism, here we also refer\nto the underwater component of these flows as PDCs, although we recognise that their behaviour\nchanges as they travel underwater38.), what we are actually modelling is not a classic PDC but a\nturbidity current generated by the mixing of the dense portion of the PDC with the sea water. The\nmodel is a two-layer model which, by its nature, must have the more dense layer below the less\ndense layer. The simplicity of the model means that we must initialise it at the point where the\nPDC has already mixed with the water to create the turbidity current. \n\nREVIEWER COMMENTS\n\nReviewer #2 (Remarks to the Author):\nI’m speaking on behalf of reviewers 1 & 2, since reviewer 1 was unable to re-review.\nThis paper describes the inifial submarine observafions of the 2022 Hunga Tonga erupfion and will be an\nimportant touchstone for ongoing and future studies of this and other similar shallow water explosive\nerupfions. The reviewers had a lot of comments seeking clarificafion, addifion, jusfificafion, and the\nauthors did a great job of addressing those comments, somefimes directly and somefimes by addifion or\nsubtracfion of informafion. The comments were wide-ranging and, I presume, not an easy task to\naddress them all. Kudos! Overall I think the paper is nearly ready for publicafion, and only have a few\nminor points that should be addressed, along with a few specific wording suggesfions. The authors\nshould consider these comments as burnishing of a well-wriften paper and can feel free to address or\nignore as they see fit.\n• I found the first few sentences of the introducfion a bit confusing. Are the impacts of large explosive\nerupfions on land and in the marine environment being described, or just terrestrial erupfions? I was\nthrown off by the menfion of tsunamis (which of course could refer to terrestrial erupfions - but are not\na common impact) and the inclusion of reference 20. Perhaps I was influenced by the first two sentences\nof the abstract which more clearly stated ‘we know a lot about large erupfions on land, but very liftle\nabout those in the marine environment’. If this could be clarified just a bit to befter reflect the\nstatements in the abstract it would be much more powerful.\n• line 51 - add comma ‘shallow-water, acfive’\n• line 83 - replace ‘large’ with ‘wide’\n• line 88 - replace ‘unfil’ with ‘culminafing with’\n• line 90 - remove ‘in the evening’\n• line 91 - remove ’99% of the volcano’. Given the comment by reviewer 1 (although addressed in the\nauthors rebuftal) it doesn’t seem necessary to put a number on this.\n• line 96 - replace ‘a decrease of >800m elevafion’ with ‘an increase in depth >800m’. There is a range of\nterminology related to seafloor change in the paper with volume and seafloor being used\ninterchangeably and depth and elevafion being subsfituted. When referring to the seafloor change, I\nwould sfick with referring to changes in depth and addifion or removal of volume or material.\n• line 101 - ‘post erupfion’ I think this is referring to features present in pre- and post-erupfion data, but\nthe wording is unclear.\n• line 101 - replace ‘following’ with ‘during’\n• line 105 - replace ‘modern echo sounder techniques’ with ‘ship-based echo sounders’. Modern echo\nsounders at high frequency operated near the seafloor could indeed resolve small changes.\n• line 109 - ‘elevafion’ to ‘depth’\n• line 110 - replace ‘seafloor’ with ‘volume’\n• line 111 - replace ‘was focused within’ to ‘coincide with’\n• line 114 - perhaps add a clause that the ‘new chute’ was not present in pre-erupfion data.\n• line 116-117 - ‘distance greatest in the NW direcfion’ I know what you’re saying here, but I think most\nwill find it confusing. Could leave out. Possible new sentence ‘Most of the measured loss was within\n6.5km of the caldera rim, beyond which deposifion dominated.’\n• line 130 - could also cite ‘Cas, R. A., & Wright, J. V. (1991). Subaqueous pyroclasfic flows and\nignimbrites: an assessment. Bullefin of Volcanology, 53, 357-380’ although I know you’re running short\non references.\n• line 133-134 - remove ‘or mass wasfing of the edifice’ as there is now a descripfion of why this was\nunlikely.\n• line 136 - rephrase to indicate channelizafion at the rim since that is the key piece of evidence\nsupporfing the hypothesis (e..g., channelizafion that was observed on the flanks of the volcano and\nextends up to the caldera rim).\n• Line 138-139 - I don’t think this last sentence is necessary given the argument made above. It’s\nplacement is a bit odd and reads like a response to reviewer comment.\n• Line 140-144 - This very long sentence should be broken up.\n• Line 148 - remove ‘which we interpret to be a result of PDC’s’. It doesn’t seem that people need to be\nreminded that your talking about the PDCs.\n• Line 151-154 - I would reword the geochemistry descripfion for clarity…’Major element chemical\ncomposifion of PDC deposits was measured by XRF and overlap with those reported for recent erupfions\nfrom Hunga-Tonga Volcano (Brenna et al., 2022) and disfinct from other regional volcanoes.’ I think this\nimplies the source of the erupfion and you don’t need to spell it out more explicitly. I also think you\ncould leave out the SEM EDS analyses of the CTD-recovered ash shards here and move it to the secfion\nthat talks about turbidity plumes/layers.\n• Line 158 - ‘that eventually decelerated’ seems unnecessary. I see that you use it as a link to\n‘widespread deposifion’, but you could instead say ‘that blanketed the flanks of the volcano and beyond’.\n• Line 161 - I don’t really see the point of comparing the volume to a subaerial landslide?\n• Line 164 - Saying the model replicates the erosion channels is a bit misleading since the model, as I\nunderstand it’, does not allow for erosion. I would remove this or rephrase.\n• Line 169 - remove ‘extensive’ seems prefty binary to me damaged/undamaged. If you do remove it,\nyou should also remove ‘damaged’ on line 170.\n• Line 173 - replace ‘submarine’ with ‘simulated’.\n• Line 186 - replace ‘occurred’ with ‘were observed’\n• Line 188 - reccomend breaking up this sentence, e.g., ‘…of our survey area, at distances >50km from\nthe caldera. The density and diversity of these ecosystems aligns with expectafions for the warm,\noxygenated…\n• Line 193 - what is meant by ‘thickest flow paths’? Widest, deepest?\n• Line 211 - confusing wording ‘covering 700-750m’. Perhaps remove and put (700-750m) after\nSeamount.\n• Line 228 - Need a period after caldera.\n• Line 232 - clarify. ‘The turbidity anomalies were sampled by CTD and the filtrate was dominated by ash\nparficles as idenfified by SEM imagery and with a similar composifion to the PDC deposits as measured\nby SEM EDS.’\n• Line 248 - possibly replace ‘substanfial’ with ‘potenfially substanfial’ or just remove ‘substanfial’.\n• Line 250 - Perhaps add ‘locally decimated benthic communifies’ to the suspended parficles as impacts\nwith potenfial enduring effects.\nReviewer #3 (Remarks to the Author):\nReview for Pyroclasfic density currents explain far-reaching and diverse seafloor impacts of the 2022\nHunga Tonga Hunga Ha’apai erupfion\nThank you to the authors for their thoughfful comments and revisions on the previous ms. Most of my\ncomments focus on the modeling aspects of the work and making the figures and data as useable for a\ngeneral audience as possible. I have not provided many specific comments on the biological destrucfion\naspects because these are outside my realm of experfise.\nAlthough the new methods secfion has clarified some important aspects of the model, my opinion is\nthat more clarity is needed in regard to both how the model works and towards how the authors are\ndrawing specific conclusions from the modeling work (see further comments below). That is, I sfill do not\nunderstand many aspects of the model and specific model conclusions are not supported well by\ninformafion provided in the ms, in my opinion. For example, I think Figs. Like S6 and S7 are valuable, but\nI do not see general differences between these two figures and that support the conclusion that flow\ninifializafion is needed inside the caldera to create channelized flows.\nOverall, I think that this work is of extremely high scienfific value to the community. The data sets\nincluded in this ms are large and I applaud the efforts to share them and make them broadly available\nwith the scienfific community.\nLine by line comments:\nLine 32: I suggest making it clear that you are inferring or providing evidence that transport and\ndeposifion was by submarine pyroclasfic currents in this ms.\nLines 50-51: Would there be a reference for the claim of 100,000 uncharted submarine volcanoes?\nLine 74: Add the word “submarine” before “loss/gain”\nLine 76: Was the runout truly greater than 100s of kms or simply >100 km?\nLine 87: Would there be a reference for the start of the erupfion in late 2021 and composifion?\nLine 92-93: Perhaps modify to something like, “However, the modificafion of the submerged fracfion of\nthe volcanic edifice (~99% of Hunga volcano) remained unknown.”\nLine 100: Perhaps modify to, “An addifional 3.5 km3 was lost from the outer flanks, without impacfing\nHunga’s general morphology (Fig. 1). Many small features, such as peaks, ridges and gullies, were visible\nin the bathymetry post-erupfion.”\nLine 105 – 107: I am unclear what this sentence is saying or suggesfing, could you please clarify.\nLine 108: Could you make clear that this 600 m threshold refers to the flanks of Hung and the area\naround the caldera rim? Currently, the depth threshold doesn’t make a lot of sense in terms of what\nspafial area it is refering to.\nLine 111: What is meant by the term “entrenchment”? I am not familiar with the use of this term in this\nway.\nLines 113-114: Modify to “widespread blankefing of sediment”\nLine 138-139: Is the rafionale here that the channels begin close to or at the Caldera rim, and therefore\nthe flows must have been fully formed by the fime they reach the caldera rim?\nLine 141: Can you point these scours out in Fig. 2 with an arrow?\nLine 149-150: Are the ripples on the seafloor or within the deposit?\nLine 153: Are you saying the submarine deposits have the same composifion as ashfall from 2022 Hunga.\nOr, that both submarine and ashfall deposits have the same composifion as previous Hunga erupfions.\nLine 158: What was the evidence for local erosion? The negafive difference in part of the\nbathymetry differencing map or something else?\nLine 188-189: What evidence is there that the warm, oxygenated and highly producfive nature of the\nwater above the seamounts was relevant for the survival of species there, rather than the observafion\nthat currents may simply not have toppled these higher areas?\nFig 3: Could you please put scale bars in parts A-C. Could you please label the other parts of\nthe subsubfigure as well.\nPDC Modeling Comments\nI am extremely supporfive of this manuscript overall, but have crifiques and quesfions about the\nmodeling effort as it currently stands.\nFirst, it is not at all clear what aspect of the modeling result is being compared to the seafloor data or\nhow the authors are evaluafing the model. For example, what do the arrow in Fig. 2 and 3 represent\nfrom the model, a simplified version of the velocity field? If so, could so much be stated and could the\nauthors explain how they performed this simplificafion? The authors claim that the model “explains the\nobserved spafial trends in erosion and flow runout” but do not explain what aspect of the model they\nare comparing to field data to come to this conclusion. Indeed, the overall runout result from the model\nis not reported and the authors state that their model does not include erosional processes, which\nseems to contradict the claim that the model can be used to explain runout and erosional trends. More\nspecific line by line comments are below.\nLines 171-177: In these secfions, there is generally blurring between results and interpretafions\nextrapolated from seafloor data vs. model results. Could the authors clarify what conclusions and results\nare supported by the data vs. models and/or clarify how they are integrafing the two.\nModeling Methods secfion:\nI think that the authors modeling may be interesfing to a broad audience, but their methods are sfill\nunclear. There are general quesfions that the authors could answer specifically as they describe their\nmodel such as:\n⁃ Why a two layer model?\n⁃ Can the boftom layer entrain water from the upper layer and thus evolve in density?\n⁃ I believe that flow rheology is prescribed by their fricfion coefficient. If so, could the authors state this\nexplicitly and tell the reader what they select for a fricfion coefficient.\n⁃ I also can’t recall if the model flow eventually stops, or not? Could the authors explain whether the\nmodel gives a final runout distance and what that is. If not, could they explain why not.\nCould the authors please label their equafions and comment on which equafions correspond to which\nconservafion equafions (i.e., mass, momentum, and energy for which layer in their model).\nLine 476-478: This sentence is helpful in pufting their modeling in context of VolcFlow, but I do not know\nwhat is meant by “with a more mobile dense flow following.” Could the authors explain what they mean\nby “mobile dense flow following” and be specific about how their model is different from VolcFlow?\nThat is, why not just use VolcFlow? What did they have to modify and why?\nLines 478-481: I am very glad that the authors have included a descripfion of boundary condifions, but I\ncannot understand what they are based on what is wriften here.\nFor example, I thought at first from lines 478-480 that the authors were referring to the lower boundary\nthat the flow is travelling over, but the later part of the sentence makes me think it is a lateral boundary.\nIf a lower boundary, the no flux condifion makes sense, but do the authors have a no-slip boundary\ncondifion as well?\nLines 480-481: I do not understand what is meant by this sentence. Are the authors talking about the\nupper layer of their model? The water mixed with parficles in the lower layer? What is a radiafing\nboundary condifion?\nLines 482-487: Thank you for providing inifial condifions here. This makes sense for the boftom layer.\nWhat about the inifial condifions for the upper layer? Did the authors run models for more than one set\nof inifial condifions, however? It would seem so based on your conclusion that the flow had to inifialize\ninside the caldera. Perhaps a table that lists the condifions for each model run would be useful. Then you\ncould refer to how specific model runs (as listed in the table) allowed you to draw specific conclusions.\nLines 496-499: Thank you for this informafion. It is helpful.\nLines 499-500: I am somewhat confused. The model cannot erode or deposit material except in hollows?\nHow does this work? Can you clarify? What is the model doing to allow for deposifion only in certain\nareas?\nLines 500-501: I do not understand this sentence. What is meant by “waves generated by this\nmechanism?”\nLines 503: What is the wave generafion process that the model is simulafing? How does it assess wave\ngenerafion? That is, what you do examine in the model to make conclusions about wave generafion?\nHow do you define wave generafion?\nLines 511-512: What are the “outputs” of the model that allow you to draw conclusions about\nchannelizafion. Could you be clear about model outputs (in addifion to the inifial condifions you listed\npreviously).\nLine 514-515: How did you determine what was sufficient force to cause the observed cable damage?\nWhat do you consider to be the observed cable damage? From my understanding, our understanding of\nhow the cables were damaged is quite vague (they may have been buried, broken, had their sheaths\ndamaged, or some combinafion) (Clare et al., in review). How did you fine tune your model to this\ncondifion? That, are there other runs you aren’t sharing with the reader. Did those runs have other\nresults. If so, I think more jusfificafion is needed for showing only one (or two) selected model runs.\nPDC Modeling Animafion Video: This video is neat! Could you please put units on your fimescale, x and y\naxes, and scale bar.\nSupplemental Material:\nTable S1: This is a terrific data set! I am skepfical, however, of the interpretafion of ashfall vs. dilute\nflows. Could you please explain here or elsewhere the text how PDC deposits were disfinguished from\nashfall and why you interpret these as ashfall rather than something like dilute PDCs? The 90/23 sample,\n80 km from the Caldera, has ashfall thickness 2-3 fimes greater than those from Tongatapu (max\nthickness about 3 cm) and Tongataup is less than 80 km from the Caldera, I believe.\nPlease remove highlighted text from the supplement.\nLine 110 (supplement): Add figure number\nFigures S6 and S7: Please include units in figure axes, scale bars, and descripfions.\nFigures S6 and S7: I don’t understand the conclusion that the PDC couldn’t have inifiated outside the\ncaldera based on comparisons between these figures. The figures look broadly similar. I do not see what\nis different between the two sets of figures.\nFigs S9 - S11: These are super neat! I am glad they are included. Could you please include scale bars in\nthese figures.\nCongrats again on your ms. I am extremely excited about this work. Although some of my comments on\nthe modeling secfion may appear crifical, they are primarily aimed at understanding your work.\nWe thank the team from Nature Communications Ecology for their time reviewing our\nmanuscript. Our responses to their queries and suggested changes are below.\nThe claim that the benthic fauna survived the pyroclastic flow-induced currents only on\ntopographic refugia makes perfect sense and is discussed nicely on pp. 7-8, but we would like\nto ask you to give a bit more caution, given that \"Pre-eruption deep-sea benthic surveys [...]\nwere not available to compare with our new post-eruption data, except for discrete\nobservation of mussel beds on a southern seamount within our study area\"; in particular also\nsince you don’t really show data on those mussel beds*, just some (not super clear) pictures\nin the SI. We would ask you to change \"the important role of refugia\" to \"the role of\ntopographic refugia\" in the subheading, and L201-202 to something more cautious like \"to\nour knowledge, this is the first report suggesting a sheltering effect from PDCs in a marine\nsetting\".\n*or are these in suppl table 2? I couldn't tell, there is no legend of what sites the rows\nrepresent.\nWe have made this suggested change to the subheading which now reads: “Wider\nimpacts of the eruption on ocean ecosystems and the role of topographic refugia” and\nfrom previous edits L201-202 reads as suggested “however, to our knowledge, this is the\nfirst time such a sheltering effect from PDCs has been reported in a marine setting.”\nThen, in L216-221 you discuss previous studies on microfauna like foraminifera, but here it's\nall about macrofauna, so that doesn't seem particularly relevant. You also claim that the\ncommunities on the refugia were \"unaffected\", which seems questionable in the lack of\nproper data (just because they weren't destroyed doesn't mean nothing happened). So, we\nsuggest to condense L216-221 as something like: \"We do not know the timescale over which\nthe seafloor communities in the Hunga Volcano may recover, but we speculate that the\nprocess may be aided by re-colonisation from the refugia indicated above\".\nWe have made this suggested change, removing lines 216-221, and using the\nrecommended sentence to end the final paragraph of the section (see lines 220-223: “We\ndo not know the timescale over which the seafloor communities in the Hunga Volcano may\nrecover, but we speculate that the process may be aided by re-colonisation from the refugia\nindicated above.”) \n\nWe thank both Reviewers for their time reviewing our revisions for our manuscript,\nand for their helpful comments and suggestions. We note, as in our Cover Letter, that\nwe have made a change in terminology from ‘Pyroclastic Density Currents’ to\nVolcanoclastic Density Currents’. We have made this change following revisions in an\ncomplementary manuscript by our co-authors (Clare et al., 2023, Science) where this\nchange in nomenclature was requested, and this was supported by extended\nconversations with colleagues in the field. The terminology of Volcaniclastic Density\nCurrents seems to be emerging as the most accepted by the field to refer to this\nphenomenon, particularly as this does not discriminate between where flows are hot\nand gas-supported (i.e. pyroclastic density currents) and where they mix with seawater\nand become cold turbidity currents. As a result, we think this modification will enhance\nthe broader reach of this paper and ensure consistency with other literature. Our\nresponses are below, line by line.\n\nReviewer #2 (Remarks to the Author):\nI’m speaking on behalf of reviewers 1 & 2, since reviewer 1 was unable to re-review.\nThis paper describes the initial submarine observations of the 2022 Hunga Tonga eruption\nand will be an important touchstone for ongoing and future studies of this and other similar\nshallow water explosive eruptions. The reviewers had a lot of comments seeking clarification,\naddition, justification, and the authors did a great job of addressing those comments,\nsometimes directly and sometimes by addition or subtraction of information. The comments\nwere wide-ranging and, I presume, not an easy task to address them all. Kudos! Overall I\nthink the paper is nearly ready for publication, and only have a few minor points that should\nbe addressed, along with a few specific wording suggestions. The authors should consider\nthese comments as burnishing of a well-written paper and can feel free to address or\nignore as they see fit.\nThank you very much, we appreciate this feedback and the suggestions that follow.\n• I found the first few sentences of the introduction a bit confusing. Are the impacts of large\nexplosive eruptions on land and in the marine environment being described, or just terrestrial\neruptions? I was thrown off by the mention of tsunamis (which of course could refer to\nterrestrial eruptions - but are not a common impact) and the inclusion of reference 20.\nPerhaps I was influenced by the first two sentences of the abstract which more clearly stated\n‘we know a lot about large eruptions on land, but very little about those in the marine\nenvironment’. If this could be clarified just a bit to better reflect the statements in the abstract\nit would be much more powerful.\n• line 51 - add comma ‘shallow-water, active’\nLine 51 now reads: ‘In particular, shallow-water, active volcanic centres’\n• line 83 - replace ‘large’ with ‘wide’\nLine 83 now reads: ‘roughly 3- to 4-km wide submarine’\n• line 88 - replace ‘until’ with ‘culminating with’\nLine 88 now reads: ‘intermittently culminating with two large eruptions’\n• line 90 - remove ‘in the evening’\nThis has been removed\n• line 91 - remove ’99% of the volcano’. Given the comment by reviewer 1 (although\naddressed in the authors rebuttal) it doesn’t seem necessary to put a number on this.\nTo address this, and Reviewers 3 comment on this line, we have made a compromise\nbetween the options and line 93 now reads: ‘However, the modification of the submerged\nfraction of the volcanic edifice (~99% of Hunga Volcano) remained unknown.\n• line 96 - replace ‘a decrease of >800m elevation’ with ‘an increase in depth >800m’. There\nis a range of terminology related to seafloor change in the paper with volume and seafloor\nbeing used interchangeably and depth and elevation being substituted. When referring to the\nseafloor change, I would stick with referring to changes in depth and addition or removal of\nvolume or material.\nLine 96 now reads: ‘where an increase in depth >800 m was observed’\n• line 101 - ‘post eruption’ I think this is referring to features present in pre- and post-eruption\ndata, but the wording is unclear.\nLine 106 now reads: ‘visible in the post-eruption bathymetry.’\n• line 101 - replace ‘following’ with ‘during’’\nLine 107 now reads: ‘removed during the eruption’\n• line 105 - replace ‘modern echo sounder techniques’ with ‘ship-based echo sounders’.\nModern echo sounders at high frequency operated near the seafloor could indeed resolve\nsmall changes.\nLine 112 now reads: ‘resolved with ship-based echosounders’\n• line 109 - ‘elevation’ to ‘depth’\nLine 116 now reads: ‘the seafloor changes’\n• line 110 - replace ‘seafloor’ with ‘volume’\nLine 118 now reads: ‘Below 600 m water depth, which encompasses the flanks of Hunga\nVolcano and the area around the caldera rim, volume loss’\n• line 111 - replace ‘was focused within’ to ‘coincide with’\nLine 119 now reads: ‘which coincide with pre-existing’\n• line 114 - perhaps add a clause that the ‘new chute’ was not present in pre-eruption data.\nLine 123-124 now reads: ‘within a new chute, which was not visible in pre-eruption data,\nthat initiated’\n• line 116-117 - ‘distance greatest in the NW direction’ I know what you’re saying here, but I\nthink most will find it confusing. Could leave out. Possible new sentence ‘Most of the\nmeasured loss was within 6.5km of the caldera rim, beyond which deposition dominated.’\nLines 125-126 now read: ‘Most of the measured loss was within 6.5 km of the caldera rim,\nbeyond this deposition dominated.’\n• line 130 - could also cite ‘Cas, R. A., & Wright, J. V. (1991). Subaqueous pyroclastic flows\nand ignimbrites: an assessment. Bulletin of Volcanology, 53, 357-380’ although I know\nyou’re running short on references.\nThough applicable, we are already well over on references so we will refrain from citing\nthis one.\n• line 133-134 - remove ‘or mass wasting of the edifice’ as there is now a description of why\nthis was unlikely.\nLines 146 to 147 now read: ‘magmatic explosivity or hydrovolcanic explosions could\nhave..’\n• line 136 - rephrase to indicate channelization at the rim since that is the key piece of\nevidence supporting the hypothesis (e..g., channelization that was observed on the flanks of\nthe volcano and extends up to the caldera rim).\nLines 150-151 now read: ‘..strong channelisation that was observed on the flanks of Hunga\nvolcano and extends up to the caldera rim.’\n• Line 138-139 - I don’t think this last sentence is necessary given the argument made above.\nIt’s placement is a bit odd and reads like a response to reviewer comment.\nWe agree with this comment and have removed this sentence as such.\n• Line 140-144 - This very long sentence should be broken up.\nWe broke up this section as such: “We observe crescentic scours (up to 30 m deep) and\nbedforms (up to 1,200 m wavelength, 20 m wave height; Fig. 2), commonly seen in other\ndeep-sea settings affected by powerful density flows40, including density flows triggered by\nlarge magnitude earthquakes that severed seafloor cables and attained speeds of up to 20\nm/s.41–43. These observations are further corroborated by seafloor sediment coring and\nimagery, geochemical analysis and numerical modelling.”\n• Line 148 - remove ‘which we interpret to be a result of PDC’s’. It doesn’t seem that people\nneed to be reminded that your talking about the PDCs.\nWe have made this change and lines 163 now read: “…as expected from a particulate\ndensity flow deposit.”\n• Line 151-154 - I would reword the geochemistry description for clarity…’Major element\nchemical composition of PDC deposits was measured by XRF and overlap with those\nreported for recent eruptions from Hunga-Tonga Volcano (Brenna et al., 2022) and\ndistinct from other regional volcanoes.’ I think this implies the source of the eruption and you\ndon’t need to spell it out more explicitly. I also think you could leave out the SEM EDS analyses\nof the CTD-recovered ash shards here and move it to the section that talks about turbidity\nplumes/layers.\nWe think this is a reasonable suggestion and have reworded the sentences in lines 166-\n172 as such: “Major elemental chemical compositions of volcaniclastic deposits were\nmeasured by X-Ray fluorescence spectrometry and overlap with ashfall collected from\nTongatapu post-eruption (measured by scanning electron microprobe analysis) as well as\nrecent eruptions from Hunga volcano44, and are distinct from other regional volcanoes\n(Supplemental Information; Figs. S3, S4; Supplemental Data 1).” We still include the EDS\nanalysis here, though, but only for the correlation with Tongatapu ashfall samples that\nwere collected post-eruption as we think this is a useful comparison. The water column\nsamples we discuss later on, as suggested, with reference to the data mentioned here.\n• Line 158 - ‘that eventually decelerated’ seems unnecessary. I see that you use it as a link to\n‘widespread deposition’, but you could instead say ‘that blanketed the flanks of the volcano\nand beyond’.\nWe have made this change and line 177 now reads” .. blanketed the flanks of the volcano\nand beyond, resulting in widespread deposition.”\n• Line 161 - I don’t really see the point of comparing the volume to a subaerial landslide?\nWe make this comparison to provide an example of volume that readers may be able to\nvisualize easier as the St Helens landslide was very well documented by aerial\nphotography.\n• Line 164 - Saying the model replicates the erosion channels is a bit misleading since the\nmodel, as I understand it’, does not allow for erosion. I would remove this or rephrase.\nWe have removed this as such, and the sentence now reads “..approximately 3-4 km3 of\nflow is required originating from inside the caldera rim to replicate the extent of the observed\ndeposits, especially the overtopping of two submarine knolls to the south heading towards the\ninternational cable (Fig. 3).’\n• Line 169 - remove ‘extensive’ seems pretty binary to me damaged/undamaged. If you do\nremove it, you should also remove ‘damaged’ on line 170.\nWe prefer to have extensive remain in this sentence as we are trying to indicate the\nnotability of the length of damage to the cable, which in our follow up work we illustrate\nand discuss in more detail.\n• Line 173 - replace ‘submarine’ with ‘simulated’.\nWe have made this change and lines 194-197 now read “As the simulated density currents\nflow away from the caldera, the model illustrates how this seafloor flow can be steered by the\nlocal bathymetry and how, in some locations, separate flows can converge back together and\naccelerate.”\n• Line 186 - replace ‘occurred’ with ‘were observed’\nLine 217 now read: ‘We observed relatively undisturbed and diverse benthic communities\non seamounts”\n• Line 188 - reccomend breaking up this sentence, e.g., ‘…of our survey area, at distances\n>50km from the caldera. The density and diversity of these ecosystems aligns with\nexpectations for the warm,\noxygenated…\nLines 218-221 now read: ‘The density and diversity of these ecosystems aligns with our\nexpectations for the warm, oxygenated and highly productive waters that exist adjacent to\nseamounts and volcanic islands of the Tonga-Tofua-Kermedec Arc.’\n• Line 193 - what is meant by ‘thickest flow paths’? Widest, deepest?\nBy thickest we are referring to the output of the modelling simulation, which shows the\ndensity current ‘thickness’ in meters. We have added clarification after mentions of\nthickest to include (in m) for enhanced clarity.\n• Line 211 - confusing wording ‘covering 700-750m’. Perhaps remove and put (700-750m)\nafter Seamount.\nWe have reworded and line 262-263 now reads: ‘In a deeper setting (700 -750 m) on\nVailulu’u Seamount…’\n• Line 228 - Need a period after caldera.\nWe have made this change\n• Line 232 - clarify. ‘The turbidity anomalies were sampled by CTD and the filtrate was\ndominated by ash particles as identified by SEM imagery and with a similar composition to\nthe PDC deposits as measured by SEM EDS.’\nWe have reworded this, with the edits to lines 154-157 in mind, and lines 287-291 now\nread: “The turbidity anomalies (both mid-water and near-seabed layers) were sampled by\nCTD and in all instances were dominated by ash particles, identified by scanning electron\nmicroscope imagery, and electron microprobe analysis revealed a similar composition to the\nPDC deposits and post-eruption ashfall samples from Tongatapu discussed above (Figs. S4,\nS14-17; Supplemental Table 1). “\n• Line 248 - possibly replace ‘substantial’ with ‘potentially substantial’ or just remove\n‘substantial’.\nWe removed substantial\n• Line 250 - Perhaps add ‘locally decimated benthic communities’ to the suspended particles\nas impacts with potential enduring effects.\nLines 313-314 now read: “The enduring effects of suspended volcanic material in the water\ncolumn and the localised decimation of benthic communities…”\nReviewer #3 (Remarks to the Author):\nReview for Pyroclastic density currents explain far-reaching and diverse seafloor impacts of\nthe 2022 Hunga Tonga Hunga Ha’apai eruption\nThank you to the authors for their thoughtful comments and revisions on the previous ms.\nMost of my comments focus on the modeling aspects of the work and making the figures and\ndata as useable for a general audience as possible. I have not provided many specific\ncomments on the biological destruction aspects because these are outside my realm of\nexpertise.\nAlthough the new methods section has clarified some important aspects of the model, my\nopinion is that more clarity is needed in regard to both how the model works and towards\nhow the authors are drawing specific conclusions from the modeling work (see further\ncomments below). That is, I still do not understand many aspects of the model and specific\nmodel conclusions are not supported well by information provided in the ms, in my opinion.\nFor example, I think Figs. Like S6 and S7 are valuable, but I do not see general differences\nbetween these two figures and that support the conclusion that flow initialization is needed\ninside the caldera to create channelized flows.\nOverall, I think that this work is of extremely high scientific value to the community. The\ndata sets included in this ms are large and I applaud the efforts to share them and make them\nbroadly available with the scientific community.\nWe thank Reviewer 3 for their time spent on our manuscript and for the helpful\ncomments and discussion that they have provided. We have done our best to take into\naccount all points and enhance clarity and robustness of our manuscript whenever\npossible. We also thank Reviewer 3 for recognizing the contribution of this work to the\nscientific community, and the value of the dataset we are sharing with this manuscript.\nResponses are line by line below.\nLine by line comments:\nLine 32: I suggest making it clear that you are inferring or providing evidence that transport\nand deposition was by submarine pyroclastic currents in this ms.\nWe have modified lines 32-33 as such: “Almost 10 km3 of seafloor material was removed\nduring the eruption, most of which we conclude was redeposited within 20 km of the caldera\nby long run-out seafloor density currents.”\nLines 50-51: Would there be a reference for the claim of 100,000 uncharted submarine\nvolcanoes?\nThis citation had been lost when the sentence was restructured, we have corrected that\nand have provided the Wessel et al., 2010 citation. Thank you for noting this.\nLine 74: Add the word “submarine” before “loss/gain”\nWe have made this change, using seafloor though instead over submarine, so that line\n82 now reads: “for seafloor loss/gain”\nLine 76: Was the runout truly greater than 100s of kms or simply >100 km?\nWe have modified this so that it reads >100 km as this encapsulates both and is indeed\nmore to the point of what we show here.\nLine 87: Would there be a reference for the start of the eruption in late 2021 and\ncomposition?\nWe have added two appropriate references here for the start of the eruption and the\ncomposition.\nLine 92-93: Perhaps modify to something like, “However, the modification of the submerged\nfraction of the volcanic edifice (~99% of Hunga volcano) remained unknown.”\nWe like this suggested change, and it helps to resolve a comment from Reviewer 2 as\nwell. As such, lines 102-104 now read “However, the modification of the submerged\nfraction of the volcanic edifice (~99% of Hunga Volcano) remained unknown.”\nLine 100: Perhaps modify to, “An additional 3.5 km3 was lost from the outer flanks, without\nimpacting\nHunga’s general morphology (Fig. 1). Many small features, such as peaks, ridges and gullies, were visible\nin the bathymetry post-erupfion.”\nCombining this suggestion with a comment from Reviewer 2, lines 110-113 now read:\n“An additional 3.5 km3 was lost from the outer flanks, without impacting the general\nmorphology of Hunga Volcano (Fig. 1). Many small features present pre-eruption, such as\npeaks, ridges and gullies, were visible in the post-eruption bathymetry.”\nLine 105 – 107: I am unclear what this sentence is saying or suggesting, could you please\nclarify.\nWe made changes to lines 113-119 to enhance clarity in this section. These lines now\nread: “Of the total 9.5 km3 of material removed during the eruption, 66% (6.3 km3) of this\nmaterial was deposited within 20 km of the caldera rim and 3.2 km3 remains unaccounted for.\nPrior studies suggest that ~1.9 km3 of this material that remains unaccounted for was ejected\ninto the atmosphere as an eruption plume37. The remaining material that is unaccounted for\n(~1.3 km3) was likely deposited as widespread thin deposits below the detection limit of what\ncan be resolved with ship-based echosounders.”\nLine 108: Could you make clear that this 600 m threshold refers to the flanks of Hung and the\narea around the caldera rim? Currently, the depth threshold doesn’t make a lot of sense in\nterms of what spatial area it is refering to.\nWe have added this clarification so that lines 124-126 now read: “Below 600 m water\ndepth, which encompasses the flanks of Hunga Volcano and the area around the caldera rim,\nvolume loss is confined to linear chutes that radiate from the caldera rim..”\nLine 111: What is meant by the term “entrenchment”? I am not familiar with the use of this\nterm in this way.\nWe have replaced entrenchment with deepening for more clarity so lines 127-128 now\nread: “Up to 70 m deepening occurred within the incised chutes”\nLines 113-114: Modify to “widespread blanketing of sediment”\nWe have made this change and line 130 now reads: “with widespread blanketing of\nsediment across the pre-eruption seafloor.”\nLine 138-139: Is the rationale here that the channels begin close to or at the Caldera rim, and\ntherefore the flows must have been fully formed by the time they reach the caldera rim?\nWe have removed this sentence following further discussions for enhanced clarity, and\nmodified the sentence prior so that lines 157-160 now read: “Sensitivity tests of our\ndensity current model (see Supplementary Material, Fig. S6), however, confirm that whatever\ninitiation mechanism caused them, it most likely occurred within the caldera in order to cause\nthe strong channelisation that was observed on the flanks of Hunga volcano and extends up to\nthe caldera rim.”\nLine 141: Can you point these scours out in Fig. 2 with an arrow?\nWe attempted this but at the scale was difficult to see well, however we hope that the\nenhanced colour mapping that we have provided in response to the previous revisions,\nand the two A and B profile lines showing vertical gain/loss help to clarify the location\nof some of these features.\nLine 149-150: Are the ripples on the seafloor or within the deposit?\nThe ripples we refer to here were within the volcaniclastic (PDC) deposit and also\nvisible in the seafloor photos that we provide in Fig. S9\nLine 153: Are you saying the submarine deposits have the same composition as ashfall from\nprevious Hunga eruptions.\nFollowing suggestions by Reviewer 2 and this comment we have modified this part so\nthat lines 177-182 now read: “Major elemental chemical compositions of volcaniclastic\ndeposits were measured by X-Ray fluorescence spectrometry and overlap with ashfall\ncollected from Tongatapu post-eruption (measured by scanning electron microprobe analysis)\nas well as recent eruptions from Hunga volcano45, and are distinct from other regional\nvolcanoes …” We hope that these changes clarify that we are referring to the\ncomposition of the volcaniclastic (PDC) deposit, which matches the ashfall collected\nfrom Tongatapu post-eruption and data from previous eruptions, with these\ncompositions all distinct from other regional volcanoes. We have removed comparison\nto the water column data from this section and just discuss it later on for clarity\npurposes.\nLine 158: What was the evidence for local erosion? The negative difference in part of the\nbathymetry differencing map or something else?\nYou are correct that the evidence for local erosion here is the negative difference in the\nbathymetry difference mapping.\nLine 188-189: What evidence is there that the warm, oxygenated and highly productive\nnature of the water above the seamounts was relevant for the survival of species there, rather\nthan the observation that currents may simply not have toppled these higher areas?\nWe edited this section slightly in response to a comment from Reviewer 2 so that lines\n203-213 now read: “While the seafloor in most of the remaining survey area was smothered\nwith volcanic deposits linked to volcaniclastic density currents and subsequent ashfall, areas\nof potential refugia from the impacts of density currents appear to have been created locally\nby the high and irregular seafloor relief provided by several seamounts (Fig. 3, S11, S19,\nSupplemental Data 2). We observed relatively undisturbed and diverse benthic communities\non seamounts in the southern part of our survey area, at distances greater than 50 km from the\ncaldera. The density and diversity of these ecosystems aligns with our expectations for the\nwarm, oxygenated and highly productive waters that exist adjacent to seamounts and volcanic\nislands of the Tonga-Tofua-Kermedec Arc 47–49. Closer to Hunga Volcano, abundant benthic\nand fish communities were also observed on seamounts in the area, presumably protected\nfrom direct eruption impacts (Fig 3; Fig. S11, S19).\nWe hope that this provides more clarity to what we are referring to here, which is that\nthese areas appear to have not been impacted by the volcaniclastic deposits (PDC’s),\nmost likely because the currents did not reach them due to the topographic refugia\nprovided by the seamounts.\nFig 3: Could you please put scale bars in parts A-C. Could you please label the other parts of\nthe subsubfigure as well.\nWe have added these labels and scale bars to the figure.\nPDC Modeling Comments\nI am extremely supportive of this manuscript overall, but have critiques and questions about\nthe modeling effort as it currently stands.\nThank you, it is useful to hear what aspects you don’t understand as it helps us explain\nthe model better. We have made changes to provide more clarity, as detailed below.\nFirst, it is not at all clear what aspect of the modeling result is being compared to the seafloor\ndata or how the authors are evaluating the model. For example, what do the arrow in Fig. 2\nand 3 represent from the model, a simplified version of the velocity field? If so, could so\nmuch be stated and could the authors explain how they performed this simplification? The\nauthors claim that the model “explains the observed spatial trends in erosion and flow runout”\nbut do not explain what aspect of the model they are comparing to field data to come to this\nconclusion. Indeed, the overall runout result from the model is not reported and the authors\nstate that their model does not include erosional processes, which seems to contradict the\nclaim that the model can be used to explain runout and erosional trends. More specific line by\nline comments are below.\nWhile our model can’t predict specifically where erosion or deposition will occur as the\ngravity current is simply modelled as a dense fluid, it can predict where the gravity\ncurrent will flow, and erosion and deposition due to the gravity current can only occur\non that flow path. These flow paths (and the thickness and speed of the gravity current\nalong them) compare well with sea floor locations where erosion, deposition, cable\ndamage and loss of fauna occur. We have updated wording in the manuscript for\nincreased clarity on this, though, for example removing reference to the erosional\nchannels on the flank of the Hunga edifce so that now lines 169-172 say:\n“…approximately 3-4 km3 of flow is required originating from inside the caldera rim to\nreplicate the extent of the observed deposits, especially the overtopping of two submarine\nknolls to the south heading towards the international cable…” More specific changes and\nclarifications follow.\nThe arrows on Figures 2 and 3 are an attempt to distil the time varying paths of the\nmodelled density (PDC) currents onto a map which also includes other information.\nThey were sketched onto the map from the animation of the density current (PDC)\n(which shows the evolution of its thickness, h). We have attempted to make this clearer\ns\nby stating “Indicative density current flow paths (i.e., an accessible visualisation of the\ngeneral flow pathways simplified from the model output animation provided (see\nSupplemental Video 1 and Fig. S6)) are indicated with dashed grey lines plotted over the\ndifference map to enable comparison of density current flow paths with the difference map\nresults. We also removed the arrows from Fig. 2 to decrease confusion and we have\nmodified Fig. 2 to include snapshots from the modelling animation at 120, 300, and 600\nseconds into the modelling run to allow for direct comparison in the figure, as well as\nstating that comparisons can be made with the Supplemental Video which shows the\nentirety of the modelling animation. This is stated as such: “For further context, select\nsnapshots from the model animation (provided in Supplemental Video 1) are provided at 120,\n300, and 600 seconds into the modelling output. For these density current modelling\nsnapshots, the map frame extent is the same as for the difference map, and the background\nimagery is 2022 multidirectional hill shade.” After careful consideration, we want to leave\nthe lines of the indicative flow paths in Fig 2 and Fig 3, however, because we think they\nare very important to the readers ability to see how the differences shown in the\ndifference map of the bathymetry correlate (or not) with the main areas crossed by the\nsimulated PDC flows.\nFor Fig. 3, which shows these same indicative flow paths from the modelling animation\nbut over a larger map frame, we have left the arrows but removed the size differences in\nthe flow path lines. We have left the arrows because they are important for the readers\nability to understand the directionality of the lines, which we can accurately interpret\nfrom the modelling animation as the PDC simulation moves through times, and also the\nfact that the PDC would not be ending at the end of the line provided. We agree that the\nsize difference between the lines was confusing and unnecessary though, so this has been\nremoved to allow for enhanced clarity. We have also provided more detail in the figure\ncaption to make this interpretation clearer: “Indicative density current flow paths taken\nfrom modelling results (as in Fig. 2, see Supplemental Video 1 and Fig. S6) are shown in\nsolid black lines with the arrows indicating directionality of the flow paths interpreted from\nthe modelling simulation. The locations of damaged submarine cable are depicted with\ndashed pink lines.”\nLines 171-177: In these sections, there is generally blurring between results and\ninterpretations extrapolated from seafloor data vs. model results. Could the authors clarify\nwhat conclusions and results are supported by the data vs. models and/or clarify how they are\nintegrating the two.\nWe have tried to make it clearer which parts of this refer to modelling results and which\nto seafloor data. The general message is that locations where the model shows the\ngravity current went are where sea floor effects were seen. These changes span the\nsections with lines 204-219:\n“Furthermore, the modelled flow paths taken by the faster moving and thicker (in m) parts of\nthe density current correlate well with the observed spatial trends in erosion and flow runout.\nThe most pronounced erosion occurred immediately to the NNW of the caldera, where the\nmodelling shows part of the density current being funnelled through an area of negative\nseafloor relief between the two islands. As the simulated density currents flow away from the\ncaldera, the model illustrates how this seafloor flow can be steered by the local bathymetry and\nhow, in some locations, separate flows can converge back together and accelerate. These\nmodelled results illustrating the paths of the simulated density currents corroborate the areas\nof impact observed in the seafloor surveys presented here. The longest observed flow runout\nlikely exploited bathymetric lows to the NW of the caldera. Post-eruption bathymetry\n(corroborated by modelling results) indicates this density current went well-beyond the\nnorthwest limit of our survey area. Flow paths like this have previously been observed\nsubaerially but only inferred in submarine settings39.\n.”\nFig 3: Could you please put scale bars in parts A-C. Could you please label the other parts of\nthe subsubfigure as well.\nWe added the scale bar to be in all A-C photos in this inset, and have added labels to all\nthree parts of the figure (i-iii). We adjusted the figure caption to have more clarity with\nthese changes, as well as to have more clarity in relation to the use of the lines in the\nfigure.\n“Figure 3: (i) Sediment core log with interpreted pre-eruption, volcaniclastic and ash deposits\nlabelled, from the sediment core indicated with the light blue star in (ii). (ii) Compilation map\nshowing seafloor multibeam survey and geologic and biological sampling after the 2022 Hunga\nVolcano eruption. Indicative density current flow paths taken from modelling results (as in Fig.\n2, see Supplemental Video 1 and Fig. S6) are shown in solid black lines with the arrows\nindicating directionality of the flow paths interpreted from the modelling simulation. The\nlocations of damaged submarine cable are depicted with dashed pink lines. In order to reach\nthe international cable, the density current must overtop two knolls directly E and ESE of\nlocation C, which partially constrains the minimum volume of material in the volcaniclastic\ndensity current. The extent of the post-eruption bathymetry survey is shown in colour and was\ncollected by RV Tangaroa and USV Maxlimer. Dark blue graduated points and black crosses\nshow locations where seafloor video footage documented invertebrate abundance (in count)\nduring RV Tangaroa survey in April 2022. (iii) Example images of seabed across the study\narea are shown in insets A-C, and each image is located in main figure. The white bar scale in\neach A-C inset is 10 cm, as indicated in the middle inset (B).”\nModeling Methods section:\nI think that the authors modeling may be interesting to a broad audience, but their methods\nare still unclear. There are general questions that the authors could answer specifically as they\ndescribe their model such as:\n⁃ Why a two layer model?\nThe lower layer of the model is the density current and the upper layer is the overlying\nwater. This allows us to model the flow of the volcaniclastic density current over\ncomplex bathymetry with the effects of the water above them also taken into account.\nThe assumption is that both these layers are vertically integrated. This allows for the\ndensity current to be influenced by the water above it and the water to be influenced by\nthe movement of the density current below but there is not mixing This model was\noriginally developed to try and understand the tsunami generated by the density\ncurrent.\n⁃ Can the bottom layer entrain water from the upper layer and thus evolve in density?\nNo, in this simple model the two layers are impermeable.\n⁃ I believe that flow rheology is prescribed by their friction coefficient. If so, could the\nauthors state this explicitly and tell the reader what they select for a friction coefficient.\nYes, that is correct. We have now stated this in line 528: “in this model a simple quadratic\nfriction is used and the coefficient is set to 10-4 for the gravity current…”\n⁃ I also can’t recall if the model flow eventually stops, or not? Could the authors explain\nwhether the model gives a final runout distance and what that is. If not, could they explain\nwhy not.\nWe have clarified this further in the paragraph starting at line 569 this simple model\nassumes that the density current is a dense Newtonian fluid following physical\nexperiments by Bouguin et al which showed that this captures many of the aspects of a\ndensity current. One of the outcomes of this is that the flow only comes to a stop when it\nreaches a depression. This is why we were careful to state that the model is not able to\npredict the final runout distance of the density current. This is provided in lines 569-575: “This modelling is a very simple representation of a very complicated process. While it\ncaptures some aspects well it is not able to resolve other aspects. The gravity current is\nrepresented as a dense Newtonian fluid flowing over an unerodable bed. It does not erode\nsediment as it flows down the flanks of the edifice and neither does sediment deposit when\nthe speed decreases, it just flows to the lowest point in the bathymetry. Thus, this model\ncannot predict where the density current will finally stop, except in depressions or hollows.”\nCould the authors please label their equations and comment on which equations correspond\nto which conservation equations (i.e., mass, momentum, and energy for which layer in their\nmodel).\nWe have modified these sections to provide extensive further explanation of each\nvariable in the equation means, how it is used together, and what is gleaned from each\nportion.\nLine 476-478: This sentence is helpful in putting their modeling in context of VolcFlow, but I\ndo not know what is meant by “with a more mobile dense flow following.” Could the authors\nexplain what they mean by “mobile dense flow following” and be specific about how their\nmodel is different from VolcFlow?\nThat is, why not just use VolcFlow? What did they have to modify and why?\nThank you for pointing out that this doesn’t make sense – the sentence was supposed to\nread “… following Bougouin et al” but the referencing system messed it up. We have\nreworded this to make it clear. The results of Bougouin et al show that aerated granular\nflows entering water behave very similarly to dense fluids. As discussed during our\nmeeting, we did not use VolcFlow because we do not have access to this software.\nLines 478-481: I am very glad that the authors have included a description of boundary\nconditions, but I cannot understand what they are based on what is written here.\nFor example, I thought at first from lines 478-480 that the authors were referring to the lower\nboundary that the flow is travelling over, but the later part of the sentence makes me think it\nis a lateral boundary. If a lower boundary, the no flux condition makes sense, but do the\nauthors have a no-slip boundary condition as well?\nWe think that it was unclear that this is a vertically-averaged equation and therefore\nonly have lateral boundary conditions. We have further emphasised this in the\nmanuscript. In the vertically-averaged equations the surface and bottom boundary\nconditions from the 3D equations are incorporated within the equations (this is how eta\nand z_b enter into the equations).\nThis is given is lines 550-554: “The lateral boundary condition for the volcaniclastic density\ncurrent (the lower layer) is a solid boundary (no flux over the boundary) but the boundary of\nthe model is sufficiently far from the volcano that the density current does not reach it. The\nsea water (upper layer) is solved with a radiating boundary condition at the edge of the\ndomain, which allows for waves to propagate out of the domain and not reflect back in.”\nLines 480-481: I do not understand what is meant by this sentence. Are the authors talking\nabout the upper layer of their model? The water mixed with particles in the lower layer?\nWhat is a radiating boundary condition?\nAs detailed above, we have reworded this to make it clearer that we are referring to the\nupper sea water layer and to explain a radiating boundary condition.\nLines 482-487: Thank you for providing initial conditions here. This makes sense for the\nbottom layer. What about the initial conditions for the upper layer? Did the authors run\nmodels for more than one set of initial conditions, however? It would seem so based on your\nconclusion that the flow had to initialize inside the caldera. Perhaps a table that lists the\nconditions for each model run would be useful. Then you could refer to how specific model\nruns (as listed in the table) allowed you to draw specific conclusions.\nThe upper layer is just initialised as a flat surface above the density current. This has\nnow been specified in the manuscript. While other initial conditions could affect the\nwaves generated, they are unlikely to have a significant effect on the density current\nunderneath.\nThis is given in lines 560-561: “The upper sea water layer is initialised as a flat surface\n(η=0) which then evolves according to the movement of the gravity current beneath it.”\nLines 496-499: Thank you for this information. It is helpful.\nLines 499-500: I am somewhat confused. The model cannot erode or deposit material except\nin hollows? How does this work? Can you clarify? What is the model doing to allow for\ndeposition only in certain areas?\nThe model does not include erosion or deposition at all. The dense fluid will eventually\ncome to a stop when it ends up in a depression. We have rephrased this to make it clear\nthat we are not referring to deposition here but where the fluid might end up. This is in\nlines 627-639: “It does not erode sediment as it flows down the flanks of the edifice and\nneither does sediment deposit when the speed decreases, it just flows to the lowest point in\nthe bathymetry. Thus, this model cannot predict where the density current will finally stop,\nexcept in depressions or hollows. If the volcaniclastic density current is created by a column\ncollapsing back into the ocean and mixing with the water, then this model will also\nunderestimate the waves generated as it does not consider the effect of the volcanic material\nfalling back into the water. It can, however, capture the motion of the gravity current as it\ncascades down the sides of the edifice and the wave generation process in the upper layer that\noccurs due to this (see Supplemental Video 2).”\nLines 500-501: I do not understand this sentence. What is meant by “waves generated by this\nmechanism?”\nThe motion of the density current creates waves in the overlying sea water layer. I have\nre-written this to try and make it clearer. See lines 633-638: “If the volcaniclastic density\ncurrent is created by a column collapsing back into the ocean and mixing with the water, then\nthis model will also underestimate the waves generated as it does not consider the effect of\nthe volcanic material falling back into the water.”\nLines 503: What is the wave generation process that the model is simulating? How does it\nassess wave generation? That is, what you do examine in the model to make conclusions\nabout wave generation? How do you define wave generation?\nThis refers to the waves that are generated in the seawater by the movement of the\nPDC underneath the seawater. The waves can be simply seen when looking at the\nsurface of the sea water. We have provided an additional supplemental video (see\nSupplemental Video 2) to illustrate what we are talking about here.\nLines 511-512: What are the “outputs” of the model that allow you to draw conclusions about\nchannelization. Could you be clear about model outputs (in addition to the initial conditions\nyou listed previously).\nThese conclusions are mostly based on the thickness of the density current which is\nchannelled down the erosion scars compared with the rest of the edifice. This is\nconsidered in comparison to the observations from the voyage, notably the difference\nmapping and evidence of chutes, erosional scars, and channelisation in the bathymetry.\nWe have clarified this section as such (lines 650-655): “This initialisation shows that the\nstrong channelisation of the flow on the flanks of the Hunga edifice that match with the\nchutes, erosional scars, and channelisation observed in the bathymetric data from the voyage\ndoes not occur when the volcaniclastic density current originates outside the caldera (see Fig.\nS7 top left panel). Because this channelisation was clearly observed, that is evidence that the\nvolcaniclastic density current must have originated within the caldera.”\nLine 514-515: How did you determine what was sufficient force to cause the observed cable\ndamage? What do you consider to be the observed cable damage? From my understanding,\nour understanding of how the cables were damaged is quite vague (they may have been\nburied, broken, had their sheaths damaged, or some combination) (Clare et al., in review).\nHow did you fine tune your model to this condition? That, are there other runs you aren’t\nsharing with the reader. Did those runs have other results. If so, I think more justification is\nneeded for showing only one (or two) selected model runs.\nFor smaller initial conditions of density current volume the density current is not able\nto reach over the underwater passes to the south of the Hunga Volcano and reach where\nthe international cable lies. The density current is strongly steered by the bathymetry,\nso as long as the initial volume is sufficiently large it will reach the international cable\nwith sufficient thickness and speed to bury it or drag it. In one location the cable was\nmoved about 5 km towards the Hunga Volcano. (Clare et al., 2023) – our model also\nshows the density current sweeping up in this direction.\nPDC Modeling Animation Video: This video is neat! Could you please put units on your\ntimescale, x and y axes, and scale bar.\nWe have added units and a scale bar as requested.\nSupplemental Material:\nTable S1: This is a terrific data set! I am skeptical, however, of the interpretation of ashfall\nvs. dilute flows. Could you please explain here or elsewhere the text how PDC deposits were\ndistinguished from ashfall and why you interpret these as ashfall rather than something like\ndilute PDCs? The 90/23 sample, 80 km from the Caldera, has ashfall thickness 2-3 times\ngreater than those from Tongatapu (max thickness about 3 cm) and Tongataup is less than 80\nkm from the Caldera, I believe.\nWe could see the ashfall layer clearly from the CT scans, we have tried to illustrate\nthese separations in the diagrams provided in Figure S1 and S2, with Figure S1\ndetailing the descriptions of the ashfall, which differ from the descriptions of the\nvolcaniclastic deposit. Whereas the volcaniclastic density current deposit has\nlaminations of volcaniclastic sands, this layer was finely laminated volcanic glass, a\ntypical ashfall deposit. You are correct that it is intriguing that some of the locations\nfurther from Hunga Volcano had more ashfall than those closer, we credit this to the\ntransport of the ash by atmospheric and ocean currents. Indeed, since the eruption we\nhave heard from others in the community about significant ashfall deposits observed\nsignificant distances away from Hunga Volcano which appear linked to the eruption.\nWe think that this is really interesting, particularly when coupled with the observations\nof volcanic glass still potentially settling out in some portions of the water column.\nWe think that this is really interesting, particularly when coupled with the observations\nof volcanic glass still potentially settling out in some portions of the water column.\nPlease remove highlighted text from the supplement.\nWe have removed this highlighted text\nLine 110 (supplement): Add figure number\nWe have added the figure number\nFigures S6 and S7: Please include units in figure axes, scale bars, and descriptions.\nWe have added these\nFigures S6 and S7: I don’t understand the conclusion that the PDC couldn’t have initiated\noutside the caldera based on comparisons between these figures. The figures look broadly\nsimilar. I do not see what is different between the two sets of figures.\nWe also further explain this in the newly provided Table S2, and have modified Figures\nS6 and S7 to zoom in on the differences between the channelisation when the\nvolcaniclastic density current initiates outside or inside the caldera to enable better\ncomparison. Comparing the top left images in S6 and S7, in S6 (where the density\ncurrent originates inside the caldera, the thickest parts of the current can still be seen\nflowing down the channels where the erosion was seen. In the image in S7 the density\ncurrent quickly flows equally down all the flanks of the edifice and in not steered into\nthe channels. Thus, this shows that if the density current forms outside the caldera it is\nnot steered into the channels and couldn’t cause the erosion that is seen from the\nexpedition.\nFigs S9 - S11: These are super neat! I am glad they are included. Could you please include\nscale bars in these figures.\nThank you! Unfortunately it is not possible to include scale bars in these, as some of the\nimages do not have the lasers apparent for us to scale, and each image is a slightly\ndifferent scale (as you can see in Fig 3). We provide them just to give a general look at\nthe seafloor in these regions, and note that in the description of the DTIS we do specify\nthat it flies about 1 – 1.5 m above the seafloor, if that helps give approximate scaling of\nthe images. We can at request though provide raw copies of the images if there are some\nof interest.\nCongrats again on your ms. I am extremely excited about this work. Although some of my\ncomments on the modeling section may appear critical, they are primarily aimed at\nunderstanding your work.\nThank you very much! We really appreciate your feedback and your time in discussions\nwith us to help make sure this manuscript is as good as possible!\n\nREVIEWERS' COMMENTS\n\nReviewer #3 (Remarks to the Author):\nI want to congratulate the authors again on a very excifing a nicely wriften paper. I especially enjoyed the\nintroducfion.\nOverall, the modeling descripfions are very much improved. I think that this ms is ready for publicafion,\nespecially after addressing several minor comments below.\nHere I summarize my main comments. More specifics are given below.\n1) The ability for the model to address flow runout could be further clarified. Specifically, it is stated that\nthe model cannot be used to address runout because (other than pooling in topographic lows), there is\nno physical mechanism in the model that stops the current. The authors then later use runout distance,\nas demonstrated by the model, to make interpretafions about flow dynamics.\n2) The content and inifial condifions used to created Figures S6 and S7 could be clarified. At present, it is\nvery difficult to interpret these.\n3) I disagree with the authors interpretafion of fallout deposits on the seafloor in Table S1. Specifically,\nthe authors do not explain what they use to interpret porfions of the seafloor deposits as fallout. The\nfallout deposits on land are much less thick (up to 3.5 cm) than those they interpret on the seafloor at\nfurther distances from the vent. There is a strong possibility, I think, that these seafloor deposits could\nbe seftling of material suspended by PDCs in the water column or deposits from the dilute and upper\nporfions of currents. I suggest, at minimum, caveafing the interpretafion of these deposits as fallout and\nacknowledging the possibility that other processes could form the upper and more laminar porfions of\ndeposits in their cores.\nI feel that these comments and quesfions are fairly minor but worth addressing. Congrats again on a\ngreat manuscript.\nLine by line comments:\nLine 121: “Below 600 m water depth” is confusing. Could you specify whether “below” refers to more or\nless shallow?\nLine 131: Typo\nLine 545-546: I am sfill confused by the lateral boundary condifion for the volcaniclasfic current. This is\nrelated to my lack of understanding about what causes the model currenet to stop (if it does). With a\nfree slip condifion, I would expect that the current does not stop. That is, I would expect that you do not\nmodel net deposifion (other than in low points where the current may pool). Could you clarify the\nprocesses that would cause the model current to stop or not (i.e., is the fricfion in your model enough to\neventually stop the current?). Assuming that I am correct in that there is not process that would allow\nthe model current to stop, I don’t understand why the current wouldn’t reach the lateral boundary. Do\nyou simply stop the model run before the current could reach the boundary? If so, could you clarify this\npoint and state clearly that your model cannot examine runout distance (since there would be no\nphysical process that allows the current to stop). I now see that you menfion this point about runout on\nline 569. You could also menfion that you therefore cannot determine a minimum (or maximum) runout\ndistance.\nLine 551-552: Could you please add here water the mixture rafio is of seawater to parficulate mafter that\nyou assume for the current?\nLine 591-593: Could you please elaborate on this point? How do you determine what the minimum force\nwould be to cause the observed damage? How do you relate that minimum force to the force of the\nflows (especially since, to my knowledge, there isn’t an process that can extract energy from your model\nflows like there would be in a natural current).\nLine 594 and 595: Can you clarify how you are interprefing runout distance. Earlier you state that you\ncannot specifically examine runout because there isn’t a deposifional process in your model. Here,\nhowever you state that small flows don’t extend to the cables. Is this because of pooling in topographic\nlows? If so, can you state explicitly?\nLine 595 – 598: I do not understand this sentence and how it follows from your previous one.\nTable S1: This table is super important and I appreciate having it here. I would caufion the authors\nagainst pufting interprefive ashfall thicknesses here because: 1) It is not clearly explained in the text how\nthe authors disfinguish seafloor ashfall deposits from low energy dilute flows that could have also made\ndeposits that appear laminar on the scale of a core; 2) The maximum ashfall thickness observed on land\nwas ~3.5 cm (Kelly et al., in review), which is inconsistent with the large ashfall thicknesses (up to 7.5 cm)\ninterpretated in this table. At minimum, I suggest acknowledging that these are possible fall deposits,\nbut that similar deposits could result from seftling of parficles in the water column after the passing of\nlarge flows or low energy dilute currents.\nSupplementary Figures S6 and S7:\nThe content of these figures is sfill confusing and unclear. Could you please put the infifial condifions for\neach set of model runs in the capfion to each figure. Currently, it is very challenging to understand the\ninifial condifions for each set of figures. I am unclear on what “modeled” vs. “fountaining” density\ncurrent means for example.\nI also don’t understand what is going on in the top left panel in both S6 and S7. The scale bar is not\nlabelled and it appears that the currents extend further at 300 s than they do at 2400 s. Could you please\nlabel each subfigure as a, b, and c and explain the condifions for each subfigure in the capfion. Please\nchange only one variable at a fime as you compare subfigures for clarity.\nSupplementary Videos: I like the Density current model video. I can’t find capfions or explanafions for\neither video, however. As a result, they are hard to understand (especially the wave animafion). I don’t\nunderstand what the wave animafion is showing. Maybe I just missed seeing the capfion somewhere.\nThe density current video also seems to contradict your statement in the text about the current not\nhifting the lateral boundary (the current appears to reach the edge of your frame).\nFig. S19 is neat!"}, "author_response": ""}]

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