Ricardo S. Ramalho

Gravitational volcanic flank collapses are recurrent processes producing large-scale landslides, commonly associated with secondary instabilities and destructive tsunamis. Ocean island volcanoes are particularly prone to such failures, yet the dynamics, emplacement mechanisms, and tsunamigenic potential of gravitational collapses remain poorly constrained due to the scarcity of direct observations. The ~ 68 ka Monte Amarelo flank collapse at Fogo, Cabo Verde, is one of the prominent examples of tsunamigenic gravitational collapses globally. But key parameters, including the collapse volume and the number of collapse stages, remain debated. We identify an extensive (914 km ² ) debris avalanche deposit associated with the Monte Amarelo volcanic flank collapse using high-resolution multibeam bathymetry and multichannel seismic reflection data. In addition, we find a smaller, previously unknown debris avalanche deposit superimposed on the northern half of the main deposit. Our results indicate that the Monte Amarelo collapse involved up to ~ 98 km ³ of material—the first volume estimate constrained by direct imaging of the submarine debris avalanche deposit through seismic data. The local morphology and internal structure of the main deposit suggest rapid emplacement and high tsunamigenic potential. The presence of a second deposit indicates multiple stages of collapse. Our results provide new constraints on the dynamics, volume, and sequence of events of one of the most recent gravitational collapses of a volcanic island in the Atlantic. These findings have important implications for tsunami generation and modelling and contribute to an improved understanding of hazards associated with gravitational collapses of volcanic islands.

Situated in the Cabo Verde Archipelago, Fogo is among the most active oceanic volcanoes in the Atlantic, hosting frequent eruptions some of which were highly explosive and at least one gravitational flank collapse in the last 100 kyr. This study presents new volcanic glass shard geochemical data with high spatial distribution from 54 sites comprising samples from both pre- and post-collapse times. The analyzed glasses comprise basanites, tephrites, and foidites with a subset extending into the phonolite field. The glass compositions complement bulk rock data particularly in the range between 6 and 1 wt.% MgO, thus enabling more detailed inter-dataset comparison. Major and trace element data reveal five geochemical Groups characterized by incompatible element contents partly delineating rock series. Compositional diversity is primarily controlled by fractional crystallization, and to lesser extents by mantle-source heterogeneity and degrees of partial melting. A geochemical framework is established for investigating provenance of ash and volcaniclastics in the Cabo Verde region and beyond by integrating new glass data with published bulk rock and tephra records. We validate the classification scheme through comparison with primary marine tephra deposits, demonstrating robust sample assignment to the observed compositional Groups. The new provenance data go beyond simple attribution to Fogo by resolving distinct compositional Groups, enabling improved discrimination within Fogo-derived material. Group assignment and characterization of differentiation trends provide a useful first-order indication of pre- versus post-collapse affinity. The glass dataset serves as a reference for future provenance and source correlation studies across the Cabo Verde Archipelago.

Coastlines of new lava deltas change rapidly after their emplacement if they are exposed to energetic waves and where they are internally weak. Those rapid changes potentially make them easy to study with modern remote‐sensing data. We use 0.5‐m‐resolution Pléiades satellite images to characterize how coastlines of two lava deltas created during the 2021 La Palma eruption evolved on daily and monthly intervals over 14 months, providing young lava‐coastline data of high temporal resolution for the first time. The coasts are classified into pocket beaches and rock promontories. The pocket beaches were formed from eroded volcanic materials. Fluctuations in their shoreline positions correlate strongly with tidal water level. Within the initial few days, coastal promontories retreated by up to 2 m/day, then decreased with time. We attribute this change to the presence of friable materials on the lava fronts (e.g., clinker), which were easily eroded by waves, in contrast with later slow retreat due to more resistant lithologies within flow interiors. Attenuation of waves crossing a growing submarine platform in front of the deltas and protection from clastic materials formed by collapsed sea cliffs may have also slowed coastal retreat. In addition, the coastline of the northern delta became more crenellated in plan‐view and retreated quickly, in contrast with a more rounded coastline of the southerly delta, which changed minimally. These two deltas experienced markedly different histories despite lying less than ~1 km apart, which can be explained by different patterns of lava flow emplacement and internal structure. The north delta has followed a similar retreat history to Heimaey (Iceland), which may therefore provide a possible template for future cliff retreat.

Understanding failed volcanic eruptions is key to mapping magma plumbing and forecasting hazards. Faults and fractures guide magma, but their mechanisms remain unclear due to the lack of precise earthquake locations and limited 3-D fault mapping in volcanic regions. The triple-junction setting of the Azores Archipelago, where volcanic systems and seismogenic faults coexist, offers a natural laboratory to study fault–magma interactions. We analysed ~18,000 earthquakes relocated to high precision using onshore and ocean-bottom seismometers, combined with geodetic data and seismic autocorrelation imaging, during a failed 2022 eruption on São Jorge Island. A magmatic dike ascended rapidly and mostly aseismically from the upper mantle, intruding a crustal fault before stalling ~1,600 m below the surface. Seismicity indicates that magma branching and lateral fluid escape along the fault triggered an intense, months-long swarm with rotated focal mechanisms. This study demonstrates the dual role of faults in facilitating and arresting magma ascent.

Water-laden sedimentary archives, such as marine and lacustrine sequences, have revolutionised the reconstruction of eruptive histories, as they usually hold a richer and more continuous tephra record when compared to subaerial environments. Here, we explore peatlands as another sedimentary archive from which highly detailed tephrostratigraphies can be obtained. Peat sequences have the advantage of being logistically easier and cheaper to access than other water-laden sequences, and more readily amenable to radiocarbon dating than terrestrial sequences. Pico Island in the Azores Archipelago provides an ideal laboratory to test the potential of peatlands as high-resolution sedimentary archives. The island is characterised by numerous basaltic monogenetic cones, yet its tephrostratigraphy remains poorly constrained, as such eruptions typically generate small tephra dispersals, and the resulting deposits are difficult to date. Taking advantage of the ubiquitous peatlands found in the Pico central uplands (above ~600 m altitude), a coring campaign was carried out in July 2025. Eight peatlands were cored using a Russian corer and a UWITEC® piston corer installed on a platform raft. Peatland basins were surveyed using a DJI Mavic 2 drone to produce high-resolution georeferenced digital surface models. Recovered cores were opened and logged at the University of the Azores, where peat, lacustrine, and volcanic facies (tephra horizons) were identified. Loss on ignition (LOI) was determined throughout the sedimentary sequences, and their bases were radiocarbon dated. Here, we present the stratigraphic sequences of the four main peatlands: Caiado and Barreira cone craters, and Peixinho and Lavandeira inter-cone depressions. All four stratigraphic sequences contain numerous tephra horizons, ranging in thickness from less than 1 mm up to several tens of centimetres. The thickest tephra horizons are found at the sites located in the eastern sector (Peixinho and Caiado), whereas thinner horizons predominate at the western sites (Lavandeira and Barreira). Most tephra horizons correspond to primary fall deposits, with only a minor portion of reworked materials. Radiocarbon dating reveals maximum sequence ages of 8608-8514 cal yr BP (Caiado), 6558-6399 cal yr BP (Lavandeira), 5588-5474 cal yr BP (Barreira), and 2181-2046 cal yr BP (Peixinho). The bases of the Barreira and Caiado sequences consist of weathered subaerial scoria deposits, interpreted as pre-lacustrine substrate. Lower LOI values, typically found in the lower part of the sequences, suggest initial lacustrine conditions, whereas higher LOI values in the upper part of the records indicate the transition to peatland. Ongoing work will focus on systematic radiocarbon dating below primary tephra horizons and geochemical characterisation of volcanic glass shards, enabling a high-resolution temporal and spatial reconstruction of Pico’s Holocene eruptive history. This work was supported by Fundação para a Ciência e a Tecnologia (FCT) through project ExTRAP (https://doi.org/10.54499/2023.12382.PEX).

Semi-analytical studies have demonstrated that water compressibility, seabed elasticity, and gravitational potential modify tsunami phase speed and can explain systematic arrival-time deviations observed in farfield measurements [1]. However, operational and research tsunami models remain based on incompressible formulations, preventing explicit simulation of acoustic modes and limiting investigation of gravity–acoustic coupling in large-scale free-surface flows.We present the derivation and numerical implementation of a compressible set of water-wave evolution equations compatible with the widely used finite-volume tsunami modelling frameworks. Starting from the compressible Euler equations, the formulation retains weak compressibility and acoustic propagation while preserving the long-wave structure required for basin scale simulations. Particular attention is given to the pressure closure, dispersion relation, and numerical consistency with existing solvers.The equations are being implemented within an open-source solver and validated against analytical limits and controlled numerical benchmarks. Preliminary results demonstrate stable coexistence of surface-gravity and acoustic modes, recovery of expected dispersion behaviour, and improved consistency of wavefront propagation speed relative to incompressible formulations. Synthetic impulsive source experiments of landslides illustrate the generation and radiation of coupled hydroacoustic–surface wave fields and their sensitivity to compressibility effects.The proposed framework provides a physically consistent pathway for extending dispersion based corrections into fully time-dependent numerical models, which enables systematic investigation of gravity–acoustic coupling, compressibility effects, and wave–acoustic energy partitioning in long-wave ocean dynamics. The formulation also establishes a foundation for coupling numerical wave physics with hydroacoustic observations in future integrated modelling studies.Reference[1] A. Abdolali, U. Kadri, & J. Kirby, 2019. Effect of Water Compressibility, Sea-floor Elasticity, and Field Gravitational Potential on Tsunami Phase Speed. Scientific Reports, 9 (1), 1-8.

Volcanic flank collapses represent one of the most hazardous mass-wasting processes at ocean-island volcanoes due to their potential to generate large tsunamis. Fogo volcano, located within the Cabo Verde Archipelago off the coast of West Africa, is a highly active volcanic system associated with multiple geohazards, including explosive eruptions, seismicity, landslides, and tsunamis caused by flank collapses. These processes result in the widespread submarine deposition of volcaniclastic material. However, the source mechanisms, emplacement dynamics and spatial distribution of these deposits remain poorly understood.Here we present 12 sediment cores, recovered proximal to distal from Fogo during research cruise M155 aboard RV Meteor, which contain multiple volcanogenic event layers. These layers provide archives for reconstructing mass-wasting activity and offer potential for stratigraphic correlation in marine records.This study unravels the mechanisms responsible for the formation of volcanogenic turbidites and establishes diagnostic criteria for distinguishing different types of volcaniclastic mass-transport deposits. Thereby emphasis is placed on identifying characteristic sedimentological and compositional features associated with individual event layers. We use a combined approach of structural analyses on sediment epoxy peals, quantitative sediment petrography coupled with glass-chemistry and age-modelling. This unique approach allows assigning the compositional and structural variations of volcaniclastic material to primary deposits related to flank collapses, landslides, and pyroclastic density currents from Fogo, and secondary deposits generated by other mass-wasting events and tsunami backwash in the region. Furthermore, these findings contribute to a better understanding of submarine volcanic processes and associated geohazards at ocean-island volcanoes.

Rocky coastlines of newly formed lava deltas change rapidly after their emplacement if they are exposed to energetic waves and where they are internally weak. Such rapid changes can be studied in detail using time-lapse remote sensing data from recent eruptions. Two lava deltas were created during the 2021 La Palma eruption. We use 0.5-m-resolution Pléiades satellite images to characterize their evolution on daily and monthly intervals over 14 months, providing young lava-coastline change data of high temporal resolution for the first time. Coasts of these two deltas are classified into pocket beaches and rock promontories. The pocket beaches were formed from eroded volcanic materials. Fluctuations in their positions correlate strongly with tidal water level. Within the first 39 days, annualised coastal retreat rates of the promontories reached 712 m/yr, the fastest retreat rates found so far in young lava coasts, before decreasing with time. We interpret this change as due to the presence of friable materials on the lava fronts (e.g., clinker), which were easily eroded by waves initially, in contrast with later slow retreat due to more resistant lithologies within lava flow interiors. Slowed coastal retreat may also be due to attenuation of waves crossing a growing submarine platform in front of the deltas and protection from clastic materials formed by collapsed sea cliffs. In addition, the coastline of the northern delta became more crenelated in plan-view and retreated quickly. That retreat followed a similar systematic slowing found in other new rocky coasts formed by historical eruptions. In contrast, coastline of the southerly delta was more rounded and changed minimally. These deltas experienced two markedly different histories despite lying less than ~1 km apart, which can be explained by different patterns of lava flow emplacement and internal structure.