Introduction

The Terror Rift - an active magmatic rift and the youngest known extent of the West Antarctic rift system1,2,3 - is located in the western Ross Sea embayment (Fig. 1). The region hosts two prominent volcanic fields: Ross Island Volcanic Field and the Terror Rift Volcanic Field4. They are both the expressions of the Late Miocene to present magmatic systems of the Erebus Volcanic Province. Along the Terror Rift, volcanic edifices and fields are arranged en échelon and include from south to north, Beaufort Island, Aurora Bank, Davey Bank, and the Flapjack Field (Fig. 1) with its cluster of flat-top circular edifices that were considered by prior studies to be non-igneous mud volcanoes5,6,7. To the east, a second rift branch includes Franklin Island, surrounded on three sides by Quaternary submarine volcanic edifices (Fig. 1). However, only a few of these seamounts have been sampled and dated, lacking basic rock descriptions in geological context and well-controlled mapping8,9,10, and therefore the origin and the style of volcanism in this region remain largely unresolved.

Fig. 1: Locations and images of samples from targeted sites in the Terror Rift.
Fig. 1: Locations and images of samples from targeted sites in the Terror Rift.
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A Overview map with newly acquired high-resolution multibeam bathymetry data (rainbow colored), dredge sample locations (circles – beige-filled and green-filled, the later signifying dredges that recovered mantle xenoliths), and TowCam imagery dives (stars) (also see Supplementary Fig. S1). Underlying gray-scale bathymetry is GEBCO2024 compilation49 using stereographic projection. The inset B shows high resolution TowCam seafloor imagery of a part of the ledge structure that composes Feather Top seamount nearby Squid Ridge, representing a ledge forming outcrop that consists of a sequence of variably resistant volcaniclastic deposits. The inset C shows Squid Ridge, located at the northwest of likely older flat-topped construct, which is recently formed fissure-driven eruptive seamount with mantle peridotite-cored bombs discovered during our mapping and dredge campaign.

Newly obtained data during Research Vessel/Ice Breaker (RV/IB) Nathanial B. Palmer expedition NBP25-01 in the western Ross Sea conducted from 11th of February to 16th of April, 2025, provides evidence to resolve these gaps. The dataset includes 2233 km2 of high-resolution seafloor bathymetry, 50 dredges from 33 volcanic seamounts, many of which were followed by high-resolution seafloor imagery (Figs. 1–3). Volcanic rocks recovered from seamounts are dominantly mafic alkaline (i.e. basanitic) olivine-rich and frequently host mantle xenoliths (Figs. 1C, 2C and 3D) without crustal lithologies. A novel finding is the ubiquity of the volcanic edifices that have a flat-top morphology with 20 identified during our mapping effort. These edifices are typically 1−4 km in diameter, and 20−100 m in height with steeper eastern flanks and more gradual western slopes. Several flat-tops within the Flapjack Field form doublets (e.g. Pfannkuchen and Baby Pfannkuchen, Fig. 2) and many are coalesced along a north-south trend.

Fig. 2: Characteristics of a flat-top seamount.
Fig. 2: Characteristics of a flat-top seamount.
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A High-resolution bathymetry of an archetypical flat-top seamount mapped during our expedition. Two flat-tops (Pfannkuchen and Baby Pfannkuchen) form a double-stack (“doublet”), which indicate the on-lap of deposits from multiple pulses of monogenetic volcanism in the Flapjack Field. B TowCam image showing exposure of tephra. C Lava fragments recovered in dredge DR35 (A) with phenocrysts of olivine, amphibole and a peridotite xenolith. Amph amphibole, ol olivine, xeno mantle xenolith. D, E cross-sectional profiles reflect post-eruption erosion by glaciers. Layers are based on bathymetry (A) and reflect differential erosion of exposed volcanic lithofacies by glacial advances.

Fig. 3: Characteristics of a young explosive seamount.
Fig. 3: Characteristics of a young explosive seamount.
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A High-resolution bathymetry of Feather Top seamount with locations of dredges NBP25-01_DR04 and NBP25-01_DR34 (purple stars) and TowCam dive track (yellow line). B TowCam image of upper slope composed of coarse ash-lapilli agglomerate confirmed by dredged samples (C). D peridotite-cored bomb with fusiform shape. The peridotite is oxidized (reddish color) and partially coated by coral (white).

Together with “ground truths” from dredge samples and seafloor imagery, the seafloor characterization of the western Ross Sea encompasses the widespread occurrence of aligned volcanic edifices, which is observed at multiple scales (Fig. 1). Volcanic structures on the eastern shoulder of the Terror Rift, such as Davey and Aurora Banks, and the Flapjack Field, along with the Coco Channel Mounds and edifices north of Franklin Island, all share the dominant N-S alignment parallel to previous mapped major rift structures1. Aligned seamounts include both monogenetic and polygenetic volcanic edifices that are likely controlled by a combination of differential stress and variation in magma production rate11. Based on seafloor topography illuminated by our mapping effort, we identify monogenetic edifices as small volume and low relief seamounts likely formed from a single vent or fissure and include those in the Flapjack Field and Coco Channel Mounds (Fig. 2 and Supplementary Fig. S1A). Polygenetic edifices such as Davey, Aurora and Beaufort Banks, and Franklin and Beaufort Islands are larger in volume with high relief and were likely constructed from multiple eruptions and eruptive centers (Fig. 1).

Strong evidence for widespread explosive volcanism on today’s Ross Sea seafloor comes from dredged volcaniclastic samples and imaged seafloor deposits (Figs. 1B, 2B and 3B), which include ash-lapilli-sized agglutinate (i.e. deposits formed by partial fusion of hot clasts) (Fig. 3C) and peridotite-cored fusiform bombs (Fig. 3D). To drive magmatic fragmentation at pressures imposed by the current water column (300–700 m) or equivalent past ice thickness, basaltic compositions require elevated volatile contents (i.e. 0.5–1 wt.% H2O or greater12). In addition, the occurrence of amphibole-bearing basalts support magma as being volatile rich. The assemblage of dredged rock types provides a unique perspective on the source and magmatic architecture for volcanism in this region (Fig. 4). The common and widespread occurrence of mantle xenoliths (Figs. 1, 2C and 3D) implies volatile-charged, rapid magma ascent rates from mantle depths13 and strongly suggests that primary magmas are produced by small degree partial melts of a hydrated and or carbonated (i.e. metasomatized) mantle source (Fig. 4). Mantle xenoliths are more prevalent in smaller monogenetic seamounts, suggesting that individual batches of magma rose rapidly from mantle depths. In contrast, rocks collected from larger polygenetic edifices (e.g., Davey Bank) are more differentiated, containing plagioclase phenocrysts indicative of a mature magmatic system composed of multi-level magma reservoirs (Fig. 4).

Fig. 4: A conceptual model for magmatism and volcano-glacial interaction in the Terror Rift.
Fig. 4: A conceptual model for magmatism and volcano-glacial interaction in the Terror Rift.
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A Melt generated from metasomatized mantle sources near the asthenosphere-lithosphere boundary. Volatile-charged melts rise rapidly to the surface to erupt as xenolith-bearing basalts forming monogenetic edifices (e.g., Flapjack Field and Coco Channel Mounds) while other melts stall at the crust-mantle boundary (i.e. Moho seismic discontinuity) to form reservoirs that supply magmas to longer-lived polygenetic edifices (e.g., Davey and Aurora Banks and Franklin Island). B Progression of volcano-ice sheet interaction. Top: monogenetic volcano explosively erupts beneath grounded ice to melt subglacial cavities and deposit volcanoclastic materials. Middle: multiple advances of ice sheets erode weakly consolidated deposits to produce flat top morphology. Bottom: monogenetic volcano explosively erupts after ice retreats in a submarine setting.

A remarkable finding in the style of Ross Sea seafloor volcanism is the predominance of glassy-blocky ash (Supplementary Fig. S2) and lapilli. These materials not only indicate the explosive volcanism but also magma-water interaction (i.e. phreatomagmatic process14), which is anticipated in a submarine environment, but could also be the result of magma coming into contact with an overlying ice sheet.

Results and discussion

Volcanoes shaped by interaction with ice

Interaction between ice and volcanism is an important process on Earth with broader planetary implications, as also documented on Mars15. While glaciovolcanism has been extensively studied on land in higher latitudes of polar regions12, it has never been identified in situ within submarine settings and on such a broad, spatiotemporal scale. Magma-ice interactions produce distinctive lithofacies and volcanic morphologies, the latter as a result of melting beneath glaciers which provides a unique water-confined environment. The western part of the Ross Sea offers a rare and compelling record of these magma-ice interactions which have been extensively documented for terrestrial deposits in this region16,17. In the Terror Rift, seafloor volcanism is contemporary with ice sheet advances, groundings, and retreats over the past few millions of years18,19,20,21,22,23,24 and thus predisposed to interact with ice.

Unique to the Terror Rift and the greater West Antarctic rift system is the wide-spread appearance of seamounts with flat-topped morphology formed by monogenetic explosive activity (Figs. 1, 2 and Supplementary Fig. S1). Dredged rock samples confirm that this morphology is volcanic in origin, thus settling the previous debate on whether they might be non-igneous and establishing a new benchmark of understanding for volcanic interactions with cryosphere-hydrosphere (Fig. 2 and Supplementary Fig. S1).

On the Ross Sea seafloor, these seamounts with flat-top morphology occur in clusters (e.g., Flapjack Field, Figs. 1 and 2); but also, as isolated flat-top structures among seamounts with conical-shaped, archetypical structures (e.g. Mademoiselle seamount, Coco Channel Mounds, Fig. 1 and Supplementary Fig. S1A). Their ellipse-to-circular, often “doublet” and on-lap morphology (e.g. Supplementary Fig. 2) suggests an eruption environment that restricts the dispersal of erupted materials in all directions such as in a subglacial melt cavity. Their flat-top morphology, in contrast to the adjacent seamounts that displays more elevated, steeper and angular morphologies and, in many cases, accompany far-reaching lava flows (i.e. non-restricted eruptive environment). Rock samples collected from eight of the flat-top seamounts consist of hyaloclastite breccia and hyalotuff (i.e. glassy volcanoclastic deposits), lava fragments, ash and agglutinated ash-lapilli. Glassy, juvenile clasts in these deposits are mostly non-vesicular and blocky (Supplementary Fig. S2), suggesting fragmentation by rapid cooling (i.e. magma-ice interaction) while vesicular ash is evidence for explosive eruptions driven by magmatic fragmentation (Supplementary Fig. S2). Based on coordinates given by Lee et al.10, one flat-top seamount within the Flapjack Field (Fig. 1) was dredged but not described and the samples recovered are reported as basanite and dated at 0.46 and 0.41 (+/−0.03) Ma.

The denuded morphology and relatively uniform water depth at which the flat-top seamounts in the

Flapjack Field were identified, along with their mostly steep southeastern side and more gently sloping northwestern side (Fig. 2), strongly suggests that these volcanic edifices were sculpted by grounded glaciers that advanced from the northwest to the southeast20,21,22,23. Bathymetry and seafloor images show that this erosion has exposed ledges and layering (Fig. 2), a result of variably resistant volcanoclastic lithofacies as evidenced by the variable consistency of the dredged samples.

In the western Ross Sea seafloor, the widely distributed flat-top seamounts represent a unique glaciovolcanic system formed by eruption beneath grounded ice followed by glacial erosion (Fig. 4B). Lawver et al.7,25 first hypothesized that subglacial eruptions formed these flat-top features and represent tuyas or tindars cf. ref. 26. But unlike classical tuyas none of the flat-tops have capping lava (although possibly eroded) and there is no evidence for a basal pillow lava pile. If the eruptions were predominately explosive, as indicated by recovered lithologies, then a tephra cone will be constructed by magmatic and phreatomagmatic fragmentation (e.g., type IX26). Further hypothesis for their subglacial origin is that the age of the dated flat-top13 places its activity during a time of grounded ice dominated conditions based on results from a nearby marine sedimentary core (ANDRILL-1B18,27).

Active (Holocene) volcanism

Newly documented seafloor edifices, including two in the Flapjack Field, exhibit definitive evidence of very recent submarine volcanism. Compared to other volcanic edifices in the extensively mapped western Ross Sea, which exhibit a flattened appearance due to glacial erosion, Feather Top (Fig. 3) and Squid Ridge (Fig. 1) retain much steeper slopes, reaching 28° and 23°, respectively—well above the global average for seamounts of 15–18°28. In addition, several sinuous and anastomosing lava flows that branch out in multiple directions are clearly observed on the flanks of both Feather Top and Squid Ridge, which extend approximately 3 km downslope from their summits towards the north (e.g., Fig. 1C). This morphology suggests that volcanic activity was unobstructed by ice and, at least in the case of Feather Top, was not subjected to glacial erosion. This is also corroborated through seafloor imagery by the lack of sediment cover on the summit of Feather Top where fissures and recent erosional scarps expose interbedded lapilli-ash beds and agglomerate (Figs. 1B and 3B). Dredged samples (Fig. 3C) ground-truth the presence of fresh ash (Supplementary Fig. S2) and mantle peridotite-cored bombs (Fig. 3D), implying recent explosive activity. Furthermore, exposures of red, mineralized, fluidal-shaped deposits found at the base of the Feather Top edifice suggest recent hydrothermal fluid-rock interaction (Supplementary Fig. S3).

Based on these observations from high-resolution bathymetry and seafloor imagery, we conclude that glaciers have not eroded Feather Top seamount and only nominally eroded the summit of Squid Ridge. Furthermore, both Feather Top and Squid Ridge have negligible ice rafted debris (IRDs), implying a formation postdating the Last Glacial Maximum (LGM). Models for grounding line retreat since the LGM suggest an age of less than 10,000 years19. The presence of pyroclastic bombs along with agglomerates characterized by having glassy, blocky, mostly non-vesicular ash-lapilli implies the occurrence of both magmatic and phreatomagmatic eruptions. These newly discovered active Antarctic seafloor volcanism (active as defined by Geyer et al.29) within the Terror Rift lie between the on-going volcanic activities of Mount Erebus on Ross Island, including modern day eruptions, ~195 km to the south30 and Mount Melbourne ~165 km to the north31, manifesting widespread Holocene volcanism across the rift. Such active volcanism is also consistent with observed heat flow data32 (115–135 mW/m2).

Unique volcanism in Earth systems

The Cryosphere occupies ~13% of today’s Earth’s surface33 with Antarctica serving as the largest reservoir of ice on this planet34. Antarctic ice sheets play an important role in the interaction among Earth systems across a wide range of temporal and spatial scales35,36,37. Detailed interpretations of unique seafloor glacial landforms in the western Ross Sea reveal the dynamics of ice-ocean-lithosphere interactions since the LGM38 and into the Plio-Pleistocene18,39, advancing our knowledge on the cryospheric evolution of cryosphere-driven climate and its influence on ecosystems40,41. However, until our expedition, no systematic investigation had been conducted on how seafloor volcanism contributes to these interconnected processes.

We show that regional volcanism in the Terror Rift is clearly influenced by both deep-seated tectonic and magmatic processes as well as surficial factors, such as the advancement and retreat cyclicity of ice sheets. This interplay between Earth’s deep interior and surface processes controls magma generation and magma ascent, eruptive dynamics and the resulting edifice and seafloor morphologies. Our comprehensive seafloor surveys and recovered volcanic rocks across the region now provide the means to constrain both the style and age of volcanism, which in turn, delimits glacial history as documented by glaciovolcanism and glacial advances responsible for erosion of these seafloor features. Moreover, the identification of near-primary basalt and their mantle xenolith cargo will be used to test hypotheses on triggers and sources of volcanism, contributing to the ongoing debate surrounding the origins of intraplate magmatism in West Antarctica42. Lastly, deciphering volcanic-ocean-cryosphere interactions in this region (e.g. Fig. 4) offers a unique opportunity to better understand glacio-magmatism coupling in modern Earth systems, potentially informing our understanding of similar processes on icy-planets and moons43,44,45,46,47.

Methodologies

Bathymetry

High resolution bathymetry data were acquired using the Kongsberg EM122 multibeam system aboard the RVIB Nathaniel B. Palmer. Data collection was conducted at an average speed of approximately 4 knots under new-ice conditions. The raw data were processed using MBsystem48 with post-processing performed through the mbeditviz functions. Final gridded bathymetric dataset used in this study has a resolution of 10-m.

Two Cam imagery

Seafloor imagery was obtained using the custom-built MISO TowCam platform maintained by the Multidisciplinary Instrumentation for Science in Oceanography (MISO) facility, supported by the US National Science Foundation. The imaging system includes OIS (Ocean Imaging Systems) Camera, DeepSea Power and Light IP SeaCam, and MISO GoPro Camera. Imagery is complemented by data from the following environmental and positioning sensors: Valeport VA500P Altimeter – altitude and depth; MAT-1 Data Logger – compass heading and temperature; Miniature Autonomous Plume Recorder (MAPR) – light-backscattering, oxidation-reduction potential, temperature and pressure; RBR Concerto CTD – conductivity, pressure, depth and optical dissolved oxygen; and two APS1540 fluxgate magnetometers mounted at 0.5 m vertical separation. When ice conditions permitted, the platform was tracked with Ultra Short Baseline (USBL) using EvoLogics 18/34 system deployed on an over-the-side pole provided by National Deep Submergence Facility (NDSF) Sentry Autonomous Underwater Vehicle (AUV) program. The data from the tows with USBL system confirms that the towed package remains almost vertical down from the stern deployment point consistently throughout the duration of the tow. This USBL configuration for a few tows made it possible to calculate its position simply based on the offset from the vessel’s Global Positioning System (GPS) Seapath 380 when ice condition met. The platform was deployed from the stern of the ship and towed at maximum 0.5 knots using a. 680 fiber optic cable maintaining a typical altitude of 2–5 m above the seafloor. All the sensor and imagery data were time-synchronized with the ship’s GPS clock. For the purpose of this study, we primarily analyzed imagery from the OIS camera system with optical specification of: 800 ISO, 5.6 fstop, 24 megapixel (MP), Nikon D3300 DSLR, Lens: Nikkor 20 mm 1:2.8D.