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1. Seismic Tomography Reveals a Shared Lower-Crustal Magma Source for Proximal Santorini and Kolumbo Volcanoes

   Autumn, Kaisa R; Hooft, Emilie EE; Toomey, Douglas R (December 2023, 2023 Fall Meeting, American Geophysical Union)

   Santorini volcano in the South Aegean Volcanic Arc has a detailed history of ongoing volcanic and seismic activity, making it a prime location for studying magma storage and transport at arc volcanoes. The shallow magmatic system (<5 km depth) is well constrained by geophysical studies, but the deeper crustal structure is not. Located 15 km NE of Santorini, the Kolumbo seamount is also an active edifice, with consistently more seismicity and hydrothermal venting than Santorini. Geochemical studies indicate that Santorini and Kolumbo are fed by separate mantle and crustal magma sources, but prior seismic studies suggest otherwise (Dimitriadis et al, 2010; McVey et al, 2020). This study addresses the nature of lower-crustal magma structure beneath arc volcanoes and whether evolved volcanoes and nearby vents are connected through their plumbing. Tomographic inversion of P-wave Moho reflection (PmP) and turning P-wave (Pg) traveltimes is used to create 3-D models of Moho depth and P-wave velocity (Vp) down to depths of ~25 km. The PROTEUS experiment provides an exceptionally dense and large aperture traveltime dataset from an amphibious array of ~150 seismometers and ~14,000 active marine sources. The data are ~33,000 manually picked PmP arrivals and ~256,000 Pg arrivals from existing studies. Results show a low Vp anomaly extending from the Moho to the surface. This anomaly starts at the base of the crust under the NW Santorini caldera and extends up to the east. It is most pronounced at 10-15 km depth, where it is offset from both Santorini and Kolumbo. Limited resolution prevents imaging of a connection between this mid-crustal anomaly and the known shallow magma storage region under the Santorini caldera. A high-velocity core beneath Santorini is not found, a feature interpreted at other volcanoes as a cooled intrusive complex. Because no additional low Vp anomalies are found in the lower crust, we infer that a common mantle source and mid-crustal plumbing system is actively feeding both Santorini and Kolumbo. The spatial offset and elongated nature of magma storage implies a complex relationship between evolving magmatic structures and tectonics. 

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2. Exploring Mid‐to‐Lower Crustal Magma Plumbing of Santorini and Kolumbo Volcanoes Using PmP Tomography

   https://doi.org/10.1029/2025GC012170  

   Autumn, Kaisa R; Hooft, Emilie_E E; Toomey, Douglas R (April 2025, Geochemistry, Geophysics, Geosystems)

   Deep‐crustal magma plumbing at arc volcanoes controls the volume, frequency, and composition of magma being transported to and stored in the upper crust. However, the mid‐to‐lower crust remains a challenging region to image. We explore the mid‐to‐lower crustal velocity structure beneath the Christiana‐Santorini‐Kolumbo Volcanic Field (CSKVF) to better understand how an established stratovolcano and flanking volcano (Santorini and Kolumbo) are fed through the mid‐to‐lower crust. We use active‐source seismic data to obtain a P‐wave velocity model of the crust below the CSKVF. We invert direct and reflected P phases to cover the entire depth extent of the crust and solve for the Moho interface depth. Our model requires a curved Moho interface representative of crustal thickening via underplating. Results show a highV_panomaly in the lower crust under Santorini and a mid‐crustal lowV_panomaly offset from both Santorini and Kolumbo. We find that accumulation of magma is located under the local extensional basin in the upper mid‐crust (<10 km) but is offset at deeper depths. We find evidence for melt storage at 11–13 km depth feeding volcanism at the Kolumbo volcanic chain. This melt is also a plausible source for the 2025 seismic swarm and dike intrusion. Resolution is limited in the mid‐crust below the Santorini caldera, leaving Santorini's mid‐crustal magma plumbing unconstrained. We think it likely that Santorini and Kolumbo have entirely separate crustal plumbing systems and mantle sources, but allow the possibility of a connection in the mid or lower crust. 

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3. Stacked Magma Lenses Beneath Mid‐Ocean Ridges: Insights From New Seismic Observations and Synthesis With Prior Geophysical and Geologic Findings

   https://doi.org/10.1029/2020JB021434  

   Carbotte, Suzanne M.; Marjanović, Milena; Arnulf, Adrien F.; Nedimović, Mladen R.; Canales, Juan Pablo; Arnoux, Gillean M. (April 2021, Journal of Geophysical Research: Solid Earth)

   Abstract Recent multi‐channel seismic studies of fast spreading and hot‐spot influenced mid‐ocean ridges reveal magma bodies located beneath the mid‐crustal Axial Magma Lens (AML), embedded within the underlying crustal mush zone. We here present new seismic images from the Juan de Fuca Ridge that show reflections interpreted to be from vertically stacked magma lenses in a number of locations beneath this intermediate‐spreading ridge. The brightest reflections are beneath Northern Symmetric segment, from ∼46°42′‐52′N and Split Seamount, where a small magma body at local Moho depths is also detected, inferred to be a source reservoir for the stacked magma lenses in the crust above. The imaged magma bodies are sub‐horizontal, extend continuously for along‐axis lengths of ∼1–8 km, with the shallowest located at depths of ∼100–1,200 m below the AML, and are similar to sub‐AML bodies found at the East Pacific Rise. At both ridges, stacked sill‐like lenses are detected beneath only a small fraction of the ridge length examined and are inferred to mark local sites of higher melt flux and active replenishment from depth. The imaged magma lenses are focused in the upper part of the lower crust, which coincides with the most melt rich part of the crystal mush zone detected in other geophysical studies and where sub‐vertical fabrics are observed in geologic exposures of oceanic crust. We infer that the multi‐level magma accumulations are ephemeral and may result from porous flow and mush compaction, and that they can be tapped and drained during dike intrusion and eruption events. 

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4. Seismic Structure of the Mid to Upper Crust at the Santorini‐Kolumbo Magma System From Joint Earthquake and Active Source V _p ‐V _s Tomography

   https://doi.org/10.1029/2024GC012022  

   Hufstetler, R S; Hooft, E_E E; Toomey, D R; VanderBeek, B P; Papazachos, C B; Chatzis, N (April 2025, Geochemistry, Geophysics, Geosystems)

   Santorini volcano has a history of caldera‐forming eruptions, most recently in the Late Bronze Age, at 3.4 kya, and remains volcanically active. The Kolumbo submarine volcano, located 7 km to the northeast of Santorini, erupted in 1650 AD in a deadly phreatomagmatic eruption. Ongoing seismic activity and active hydrothermal venting at Kolumbo indicate this volcano is a significant hazard to the Santorini region. The magma source for Santorini and the Kolumbo edifice are considered separate in the shallow crust, though their deeper magma distribution is not yet constrained. In this study, we improve constraints on the mid‐crustal magma system of Santorini caldera and the nearby Kolumbo volcano using local earthquake tomography. We use 1515 P‐wave and 1435 S‐wave arrival times from 63 local earthquakes with magnitudes from 0.5 to 3.0 that occurred between 5 and 15 km depth together with an existing data set of active source Pg arrivals. The upper crustal magma system beneath Santorini is imaged to at least 6 km depth, and to 12 km depth beneath Kolumbo. We recover a high P‐wave velocity layer (∼6–8 km) under the Kolumbo magma reservoir that we infer is a rheologically strong seismogenic layer. We also recover a mid‐crustal magma body below 8 km depth located to the NE of Santorini and Kolumbo. 

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5. Persistent High-Pressure Magma Storage beneath a Near-Ridge Ocean Island Volcano (Isla Floreana, Galápagos)

   https://doi.org/10.1093/petrology/egaf031  

   Gleeson, Matthew; Wieser, Penny E; DeVitre, Charlotte L; Shi, Sarah C; Millet, Marc-Alban; Muir, Duncan D; Stock, Michael J; Lissenberg, Johan (May 2025, Journal of Petrology)

   Abstract Volcanic evolution in ocean island settings is often controlled by variations in the chemistry and volumetric flux of magma from an underlying mantle plume. In locations such as Hawaiʻi or Réunion, this results in predictable variations in magma chemistry, the rate of volcanic activity, and the depth of magma storage with volcanic age and/or distance from the centre of plume upwelling. These systems, however, represent outliers in global plume volcanism due to their high buoyancy flux, frequent eruptions, and large distance from any plate boundary. Most mantle plumes display clear interaction with nearby plate boundaries, influencing the dynamics of solid plume material in the upper mantle and the distribution of melt across regions of active volcanism. Yet, the influence of plume–ridge interaction and plume–ridge distance on the structure, characteristics, and evolution of magma storage beneath ocean island volcanoes remains under constrained. In this study, we consider the evolution of magmatic systems in the Galápagos Archipelago, a region of mantle plume volcanism located 150–250 km south of the Galápagos Spreading Centre (GSC), focusing on the depth of magma storage during the eastward transport of volcanic systems away from the centre of plume upwelling. Geochemical analysis of gabbro xenoliths from Isla Floreana in the southeastern Galápagos suggest that they formed at ~2–2.5 Ma, when the island was located close to the centre of plume upwelling. These nodules, therefore, provide rare insights into the evolution of volcanic systems in the Galápagos Archipelago, tracking variations in the magma system architecture as the Nazca plate carried Isla Floreana eastwards, away from the plume centre. Mineral thermobarometry, thermodynamic modelling, and CO2 fluid inclusion barometry reveal that Isla Floreana’s plume-proximal stage of volcanic activity—recorded in the gabbro xenoliths—was characterized by the presence of high-pressure magma storage (>25 km), below the base of the crust. In fact, we find no petrological evidence that sustained, crustal-level magma storage ever occurred beneath Isla Floreana. Our results contrast with the characteristics of volcanic systems in the western Galápagos above the current centre of plume upwelling, where mid-crust magma storage has been identified. We propose that this change in magmatic architecture of plume-proximal volcanic centres in the Galápagos—from high-pressure mantle storage at 2.5 Ma to mid-crustal storage at the present day—is controlled by the variations in plume–ridge distance. Owing to the northward migration of the GSC, the distance separating the plume stem and GSC is not constant, and was likely <100 km at 2.5 Ma, significantly less than the current plume–ridge distance of 150–250 km. We propose that smaller plume–ridge distances result in greater diversion of plume-material to the GSC, ‘starving’ the eastern Galápagos islands of magma during their initial formation and restricting the ability for these systems to develop long-lived crustal magma reservoirs. 

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