Constraining the magmatic processes that control how magmas differentiate is essential for understanding reservoir dynamics before and during eruptions. Crystallisation and mixing are the two primary processes governing the evolution of magma reservoirs. However, the influence of crystallisation on the physical and chemical mixing of magmas remains poorly constrained, limiting our understanding of textural and chemical evolution of eruptible magma prior to eruptions. Here, we present an experimental study investigating the simultaneous occurrence of crystallisation and dynamic magma mixing using basaltic and dacitic end members at sub-liquidus conditions. Our experiment directly captures the interaction of crystallisation and magma mixing under dynamic conditions, revealing how these processes produce enclave disaggregation, filaments, and compositional gradients in the melt. Our experiment reproduces the interaction of mafic and felsic magmas, and the derived processes of mixing while the crystallisation proceeds. The results indicate that basaltic magmas crystallise rapidly, forming crystal-rich mafic enclaves within a felsic host and producing basaltic andesitic to andesitic melts. Advection promotes stretching and folding, which enhance both chemical exchanges and physical magma mixing effects, leading to enclave disaggregation and the formation of crystal clusters in disequilibrium with the surrounding melt, within a few hours in the investigated experimental setup. We used the parameter sigma 2n (normalized variance) to quantify the mixing efficiency and differential elemental mobilities. This indicates that crystallisation of the mafic magma can promote the evolution of melt compositions that enhances magma mixing efficiency with the more evolved end-member.

Laterally directed blasts are explosive events following a major sector collapse of a volcano, with the potential for devastating areas of several hundred km2, due to powerful dilute and turbulent pyroclastic density currents. The catastrophic flank collapse on 30 March 1956 of Bezymianny (Kamchatka, Russia) was the climactic phase of the first historical magmatic eruption of this volcano, after 1000 years of dormancy. Magma stored in a cryptodome was depressurized by a sector collapse, generating a laterally directed blast immediately followed by pumiceous concentrated pyroclastic density currents. By combining petrological data from Bezymianny plumbing system and temporal constraints from orthopyroxene, magnetite, and amphibole chronometers, we tracked magmatic processes over twelve years prior to the eruption, followed by magma ascent to a shallow reservoir and a heating process at least three months before the eruption. Magma was last stored in a cryptodome at least two months before the climactic phase of the eruption. Evidencing magma dynamics of a few months to a few years before major flank collapses and laterally directed blasts thus represents valuable information for volcanic risk mitigation (as it also occurred at Mt St. Helens).

Volcanic mineral texture and compositional zoning offer crucial insights into magmatic processes and their timing preceding an eruption. Each mineral may capture different aspects of the pre-eruptive magmatic processes. Here we use a multimineral (plagioclase, orthopyroxene, and magnetite) approach to decipher the magma dynamics prior the 2010 magmatic eruption of Kizimen volcano (Kamchatka). The eruption comprised explosive episodes generating pyroclastic density currents followed by the extrusion of a thick lava flow. We combined crystal system analysis with diffusion chronometry on plagioclase and magnetite, together with the orthopyroxene data of Ostorero et al. (3:290, 2022). Plagioclase crystals record up to four different magmatic environments which include two distinct magma mixing events. The first one involved the injection of mafic magma into an initially dacitic reservoir. The magma intrusion led to significant environmental changes within the reservoir which became thermally and compositionally zoned, with remnant dacitic magma at the top and newly created andesitic magma at its base. Both plagioclase and orthopyroxene record the interaction between the dacitic and andesitic magmas during a second mixing event at their interface. This event can be linked to a seismic crisis approximately 1.5 years before the eruption, and is also recorded by Fe–Mg diffusion chronometry in orthopyroxene. Magnetite zoning recorded a final heating event of a few days, potentially marking magma ascent and storage in the lava dome. The compositional zoning plagioclase and magnetite crystals is consistent with the spatio-temporal interpretations made from orthopyroxene crystals zoning and timescales. Plagioclase serves as a reliable yet more complex archive compared to orthopyroxene. Correlating different mineral records enables a more precise reconstruction of magmatic history. Combining petrological and monitoring data provides a more robust understanding of pre-eruptive reactivation.

Bezymianny volcano (Kamchatka, Russia) is an andesitic island arc stratovolcano that started to erupt in 1955 after 1000 years of dormancy. On March 30, 1956, the climactic phase of the eruption was preceded by a 4-month-long emplacement of a shallow cryptodome, which triggered a flank collapse violently decompressing the magma into a laterally directed blast followed by an explosive phase emplacing extensive pumice concentrated pyroclastic density currents (pumice C-PDC). Aiming at constraining the plumbing system below Bezymianny volcano prior to the 1956 eruption, we performed a multiphase textural and petrological study using dense to vesiculated clasts of the blast and pumice samples from the post-blast C-PDC deposits. We inferred the pressure and temperature conditions of magma storage using sample vesicularity, amphibole destabilization rims, volatile contents in melt inclusions, microlite textures, and phase compositions (phenocrysts, microlites, and glasses). We propose a three-level magma storage characterized by a deep reservoir (≥ 200–350 MPa, ≥ 840 °C, 4.0–8.0 wt

Pyroclastic density currents (PDCs) are a major volcanic hazard, whose variability of triggering and deposition mechanisms suggests highly complex and different initial states to be considered. Here, we describe block and ash flow deposits from the March 2019 eruption of Bezymianny volcano, Kamchatka. Ash clouds from this eruption extended into the Pacific Ocean, while block and ash flow deposits were found widely across the slopes of the edifice. We use satellite and drone-based photogrammetry to show material dispersal and accumulation during the eruption. We also use these photogrammetric data to obtain basic granulometry, suggesting dominantly 60 cm block dimensions, some exceeding 2 m in scale, embedded in a fine ash matrix. In addition, we sampled the deposit and herein demonstrate how distinct petrographical features can be used to distinguish the type of block and ash flow. Deposit characteristics, density, dimension, and petrography suggest that PDC initiation occurred during an eruptive episode conventionally considered as a “boiling over” event. This activity is characterized by rapid magma volume expansion due to intense gas exsolution which is driving a frothed mass out of the vent leading to the formation of large but highly vesicular juvenile blocks. Such an eruption style is transitional between effusive (lava dome forming) and explosive activity, and we suggest a new term “effervescent fountaining” to replace the term “boiling over” as a more appropriate description of such an eruption. Material dispersal, density of juvenile material, and Fe-Ti mineralogy are useful features to distinguish different types of block and ash flow deposits. These characteristics are also applicable to deposits from eruptions and deposits within the prehistoric geological record, improving our understanding of historic eruption patterns.

For active volcanoes, knowledge of the architecture of the plumbing system and the conditions of magma storage prior to an eruption are highly important, given their influence on the eruptive style and, thus, the management of future volcanic crises. Here, chlorine is used as a geobarometer for potassic alkaline magmas at the Campi Flegrei volcanic complex, revealing the shallowest depth of fluid-melt equilibration with respect to Cl. The results for representative fallout deposits of selected explosive eruptions show the existence of a multi-depth equilibration zone through time, including shallow magma storage. We describe evidence for the shallowest zone located at a depth equivalent to 65 MPa for the Agnano Monte Spina eruption (4482-4625 cal. yrs BP), at similar to 100 MPa for the Pomici Principali (11 915-12 158 cal. yrs BP), and the Astroni 6 (4098-4297 cal. yrs BP) eruptions, and close to 115 MPa for the last explosive eruption of Monte Nuovo (AD 1538). For comparison, the pressure estimated for a possible reservoir feeding the Cretaio eruption of Ischia island (AD 430), the only studied eruption on Ischia, is similar to 140 MPa. The pressure estimates for the two largest magnitude eruptions, the Campanian Ignimbrite (40 ka) and the Neapolitan Yellow Tuff (14.9 ka), are also discussed with respect to available magma withdrawal models. The pressures estimated using the Cl geobarometer for the magma leading to the fallout phases of these two eruptions provide evidence for a low-volume, shallow domain (similar to 40 MPa) for the Plinian phase of the Campanian Ignimbrite eruption and a main, deeper reservoir (similar to 130-165 MPa) for the Neapolitan Yellow Tuff eruption. The inferred shallowest equilibration pressures are interpreted here as corresponding to transitory, short-lived magma apophyses, whose eruption may have been facilitated by optimum tectonic stresses, rheological behavior of the crust, and efficiency of volatile exsolution. Alternatively, these magma apophyses may represent an evolved, crystal-rich ponded magma into which a volatile-rich magma ascending from depth was injected. The transient nature of such very shallow reservoirs is suggested by the short timescales inferred from diffusion modeling on crystals available in the literature for the studied Campi Flegrei eruptions. The influence of sulfur (S) on Cl solubility is assessed through Cl solubility modeling and applied to different eruptions. In addition, the pressure at which magmatic fluids and melts equilibrated with respect to Cl is shallower for the Campi Flegrei volcanic complex than the Somma-Vesuvio volcanic complex, erupting more homogeneous differentiated magma, of trachytic or phonolitic composition. This approach of using Cl to investigate the architecture of the plumbing system can be extended to all alkali-rich magma systems.

Abstract Understanding magma differentiation and formation of eruptible magmas is one of the key issues in Earth sciences. Many studies have either focused on mixing or crystallization, but none have studied these two processes simultaneously. Here, we conduct an innovative experimental study investigating the simultaneous occurrence of crystallization and dynamic mixing, using basaltic and dacitic end members at sub-liquidus conditions. We reproduce the injection of mafic magma into felsic magma and their mixing while crystallization occurs. Our results indicate that crystallization of basaltic magmas occurs faster than mixing between basalt and dacite leading to the formation of crystal-rich mafic enclaves within a felsic magma and the development of basaltic andesitic to andesitic melts. Then, convection promotes stretching and folding that favor chemical and physical magma mixing, disaggregation of enclaves and formation of clusters of crystals in disequilibrium with the surrounding melt. Magma mixing is the predominant process after the initial crystallization event. Our results provide insights into pre-eruptive dynamics, which is crucial for improving volcanic hazard assessment.

Abstract For active volcanoes, knowledge of the architecture of the plumbing system and the conditions of magma storage prior to an eruption are highly important, given their influence on the eruptive style and, thus, the management of future volcanic crises. Here, chlorine is used as a geobarometer for potassic alkaline magmas at the Campi Flegrei volcanic complex, revealing the shallowest depth of fluid-melt equilibration with respect to Cl. The results for representative fallout deposits of selected explosive eruptions show the existence of a multi-depth equilibration zone through time, including shallow magma storage. We describe evidence for the shallowest zone located at a depth equivalent to 65 MPa for the Agnano Monte Spina eruption (4482–4625 cal. yrs BP), at ~100 MPa for the Pomici Principali (11 915–12 158 cal. yrs BP), and the Astroni 6 (4098–4297 cal. yrs BP) eruptions, and close to 115 MPa for the last explosive eruption of Monte Nuovo (AD 1538). For comparison, the pressure estimated for a possible reservoir feeding the Cretaio eruption of Ischia island (AD 430), the only studied eruption on Ischia, is ~140 MPa. The pressure estimates for the two largest magnitude eruptions, the Campanian Ignimbrite (40 ka) and the Neapolitan Yellow Tuff (14.9 ka), are also discussed with respect to available magma withdrawal models. The pressures estimated using the Cl geobarometer for the magma leading to the fallout phases of these two eruptions provide evidence for a low-volume, shallow domain (~40 MPa) for the Plinian phase of the Campanian Ignimbrite eruption and a main, deeper reservoir (~130–165 MPa) for the Neapolitan Yellow Tuff eruption. The inferred shallowest equilibration pressures are interpreted here as corresponding to transitory, short-lived magma apophyses, whose eruption may have been facilitated by optimum tectonic stresses, rheological behavior of the crust, and efficiency of volatile exsolution. Alternatively, these magma apophyses may represent an evolved, crystal-rich ponded magma into which a volatile-rich magma ascending from depth was injected. The transient nature of such very shallow reservoirs is suggested by the short timescales inferred from diffusion modeling on crystals available in the literature for the studied Campi Flegrei eruptions. The influence of sulfur (S) on Cl solubility is assessed through Cl solubility modeling and applied to different eruptions. In addition, the pressure at which magmatic fluids and melts equilibrated with respect to Cl is shallower for the Campi Flegrei volcanic complex than the Somma-Vesuvio volcanic complex, erupting more homogeneous differentiated magma, of trachytic or phonolitic composition. This approach of using Cl to investigate the architecture of the plumbing system can be extended to all alkali-rich magma systems.

The recent eruptive history of Montagne Pele ' e volcano was dominated by a period of vigorous basaltic andesitic magma production (36 -25 ka) followed by a long period of lower activity (i.e., with less frequent and less voluminous eruptions) and a renewal of felsic magma production in the last 10 ka. The temporal succession of volcanic events that occurred during the 25 -10 ka period and the timing of felsic magma production are currently poorly constrained. This study focuses on the stratigraphy and eruptive dynamics of the pyroclastic deposits emplaced immediately after 25 ka. New on-land stratigraphic correlations and radiocarbon dating measurements allow us identifying six major explosive eruptions. We use field data on tephra dispersal, thickness and grain-size distribution together with physical models of explosive volcanic plumes to estimate the eruption source parameters. Our results show that these events are VEI 4 eruptions with intermediate magnitudes (from M = 4.2 to M = 5.1) and intensities (from I = 10.6 to I = 11.6). These eruptions share several characteristics with the most recent Plinian eruptions of Montagne Pele ' e volcano (i.e., mass eruption rate, maximum column height, runout of pyroclastic density currents, glass composition). The tephra succession documents two phases of magma production rates. The first phase from 25 to 14 ka corresponds to a period of low activity with a magma production rate of 0.04 km3 kyr-1. The second phase from 14 to 10 ka is characterized by a significant increase of the volcanic activity with a magma production rate of 0.4 km3 kyr-1, consistent with previous estimates.

Volatiles are an essential aspect of subduction zones and constraining their cycling through subduction zones is of prime importance to better understand the genesis, transport, storage and eruption of arc magmas. Here we performed an along-arc investigation of the chemical composition of melt inclusions trapped in minerals representative of ten volcanic centers and 23 key explosive eruptions along the presently active Lesser Antilles arc, from Montserrat in the North to St. Vincent in the South. We use the melt inclusion compositions to reconstruct pre-eruptive conditions, especially pre-eruptive magma storage and degassing levels that highlight how the magma plumbing system is organized and works and to discuss magma source characteristics. All major and selected trace elements and volatiles (H2O, CO2, S, halogens (F, Cl, Br)) have been measured on the same melt inclusions when possible. Eruptions dominantly involved andesitic to dacitic magmas (Montserrat, Guadeloupe, Dominica, Martinique, St. Lucia) and basaltic andesite magmas from St. Vincent. Melt inclusions have been used as pressure probes for magmas, for inferring crustal equilibration pressures. We shed light on the systematic occurrence and lateral complexity of a vertical transcrustal magmatic systems feeding active volcanoes. The geochemical view of the architecture of the plumbing system and in particular the Moho's depth is more variable than the view obtained by seismic data along the Lesser Antilles arc. We propose that the discontinuity between the upper and the middle crust is a major magma ponding zone beneath most of the Lesser Antilles islands and that the crustal structure thus imparts a control on the geochemical signature of arc lavas. Melt inclusions are highly differentiated (dacitic to rhyolitic in composition), so they are distant in composition from the primary, mantle-derived magmas, but they provide indirect information about the magmatic sources. The along-arc variability in Y and heavy rare earth element contents of melt inclusions is consistent with the presence of garnet in the mantle source. Our results also indicate an important contribution of oxidized and saline slab-derived fluids to the magma source predominantly in the southern Lesser Antilles that may have implication on the accretionary system behavior. In addition, a high geographical gradient between sediment melt and slab-derived fluid contributions, illustrating high variability in magmas erupted in close spatial association is reported for some islands, such as Martinique and Dominica. Volatile contents are variable for MIs across the arc: the highest H2O (<8 wt%), Cl (up to 3800 ppm) and Br (up to 20 ppm) concentrations occur in MIs from Dominica. However, there is no systematic correlation between MI volatile content and position along the arc. Halogen Cl/F and Cl/Br ratios vary from one island to another, even between the different eruptions, but without any along arc zoning, indicating that halogen fractionation occurred by fluid transfer (variable assimilation rate of fluids derived from seawater) or by heterogeneities of mantle origin inherited from the initial differentiation of the mantle.

Submarine gravity-driven sliding of sediments are common processes in the vicinity of volcanic islands. In the Lesser Antilles arc, the Montagne Pelée volcano on Martinique Island underwent several flank-collapse events during its long-term eruptive history, resulting in debris avalanches. When the debris avalanches entered into the seawater, they were emplaced over the unstable slope of the volcano, triggering a seafloor sediment failure and massive landslides downslope. Using a laboratory modeling approach, we simulated the gravity-driven sliding of a sand layer lying above a silicone layer. The experiments were performed using various slope geometries (slope lengths and number of slope breaks separating the slopes with different angles), under both dry and aqueous conditions, and while varying the amount of additional sand inputs upslope. The resulting deformations were characterized in each experiment in order to compare the obtained structures with those shown by the seismic lines offshore to the west of Martinique Island. During all the experiments, a compressional frontal deformation zone made of several reverse faults formed downslope, often near the slope breaks. Downslope, a portion of the sediments was mostly displaced and poorly deformed in a damping zone, while an extensional deformation zone formed upslope. The displacements of the surficial markers were measured through time to characterize the sliding dynamics. Our study demonstrates that the slope geometry and additional sand inputs primarily favor and increase the sliding deformation, whereas the hydrostatic pressure plays a secondary catalytic role over time. These results provide new constraints on the driving factors and their consequences on gravity-driven sliding in terms of deformations and runout distance over time. This may have a significant impact on the associated hazard assessment related to offshore infrastructures, in a region known for its seismic and volcanic risks.

The increase in number and intensity of earthquakes during a pre-eruptive crisis is the main basis of seismic volcano monitoring. However, a strong understanding of how these seismic signals relate to magmatic processes in the magma plumbing systems prior to volcanic eruptions is crucial for these efforts. Here we compare the characteristics of a seismo-volcanic crisis prior to the 2010–2013 explosive-extrusive eruption of Kizimen volcano, Kamchatka with the timescales of processes in the magma plumbing system. These timescales are inferred from the numerical modelling of iron-magnesium intracrystalline interdiffusion in 88 zoned orthopyroxene crystals from dacites and silica-rich andesites collected after the eruption. We find that the eruptible magmas were assembled rapidly during a magma mixing process beginning around 1.5 years before the eruption, which is well correlated with the onset of the seismic crisis. We conclude that the observed seismic re-activation marked the onset of magma mixing and led to destabilization of the reservoir, followed by the eruption.

Basaltic eruptions are commonly associated with lava emissions and relatively weak explosive activities, but they can sometimes produce strong explosive eruptive phases. In April and November 2005, two paroxysmal eruptive events occurred within the summit crater of Karthala basaltic shield volcano (Grande Comore Island, Comoros), which hosted a water lake before each of these events. Both 2005 ash plumes spread across the Comoros Archipelago and heavily impacted the whole Grande Comore Island. Associated deposits on the volcano summit are extremely fine-grained (up to 50 wt% of fine ash < 63 mu m for some analyzed layers) and rich in millimeter-sized rounded accretionary lapilli aggregates. Field observations, as well as textural and chemical analyses performed on both coarse-and fine-grained pyroclasts permit to identify juvenile and non-juvenile components and quantify their peculiar characteristics. Coarse ash (710-1000 mu m) mainly consists of juvenile pumice particles (vesicle number density N-V = 4.5 10(4) mm(-3) and gas to melt ratio V-G/V-L = 1.5, on average), characterized by glassy groundmasses and representative of magma portions ascending quickly within the eruptive conduits (up to 10 m s(-1)). A relatively low amount of juvenile scoria particles are also observed in the coarse ash fractions, which are characterized by magma degassing (N-V = 4.7 10(4) mm(-3) and V-G/V-L = 0.5 on average) and associated crystallization (occurrence of dendritic microlites). Non-juvenile fragments (from blocks to coarse ash) are dense lava or intrusive fragments. Their amount decreases exponentially towards the fine ash fractions, which are mainly composed of juvenile, blocky, dense and glassy particles that are characterized by unambiguous textural signs of brittle fragmentation (hackle lines, stepped features and cracks). We support that Molten Fuel-Coolant Interactions between highly porous fast ascending basaltic magmas and external waters occurred during the paroxysmal phases of the studied eruptions, leading to a brittle-dominant and efficient regime of magma fragmentation. Variable but large amount of fine ash grains through the stratigraphic depth of the deposits can be ascribed to the brittle failure of the vesicle walls of the initial porous magma. Concurrently, thermohydraulic explosions caused the host rock fragmentation at shallow level, generating the relatively coarse non-juvenile particles. A short-lived episode of intense lava fountaining associated with steam explosions eventually occurred at the end of the November 2005 paroxysm, forming the last and relatively coarse tephra layer at the top of the studied eruptive sequence. Each paroxysmal phase lasted about a day as each associated water lake and shallow water table progressively vaporized and dried away. Both eruptions ended with lava pond and weak lava fountaining activities confined within the summit crater. We conclude that the contributions of both magmatic processes and phreatomagmatic interaction mechanisms ultimately generated the grain size, grain component and grain texture variabilities observed within the paroxysmal deposits. This work contributes to a better understanding of the generation of unusual fine ash from basaltic explosions as well as their eruptive dynamics and associated mechanisms, from magma ascent in the conduit to the fragmentation level and the interaction with intra-crateric lake waters.

Abstract The increase of number and intensity of earthquakes during a pre-eruptive crisis is the main basis of seismic volcano monitoring. However, the exact relationship between the seismic activity and the volcano-magmatic processes remains unclear. Here we present a direct comparison between characteristics of a seismo-volcanic crisis recorded prior to the 2010-2013 eruption of Kizimen volcano (Kamchatka, Russia) and the timescales of processes in the magma plumbing system. These timescales are inferred from the modelling of Fe-Mg intracrystalline interdiffusion in 88 zoned orthopyroxene crystals from dacites and silica-rich andesites samples collected after the eruption. We show that the eruptible magmas were assembled rapidly during a magma mixing episode ~1.5 years before the eruption, which is well correlated with the onset of a seismic crisis. We conclude that the observed seismic re-activation marks the onset of magma mixing leading to destabilization of the reservoir followed by the eruption after ~1.5 years.

Explosive eruptions inject a great amount of gases in the atmosphere through a plume that can reach altitudes as high as several tens of kilometers. Such events affect the stratosphere chemistry, in particular its ozone content, and can thus impact the climate. Halogen compounds are known to be highly reactive but fluxes of volcanogenic halogen elements are only sparsely monitored. It results in an unawareness of the volume of halogen elements that can reach the stratosphere and their contribution to the impact of volcanoes on climate. We studied several plinian eruptions from volcanoes of various geographic origins : Kizimen and Bezymianny (Kamchatka, Russia), Phlegrean Fields, Vesuvius and Etna (Italy), Morne Trois Pitons-Micotrin (Dominica, Lesser Antilles), Montagne Pelée (Martinique, Lesser Antilles) and Huaynaputina (Peru). We present herein eruptive fluxes towards the atmosphere for F, Cl, Br and S during these eruptions, calculated by the difference in concentration between pre-eruptive concentration in magmas (measured in melt inclusions) and post-eruptive concentrations (measured in residual glasses). All the concentrations provided for melt inclusions were obtained by direct measurement and not through ratio estimates, including for bromine. These fluxes are linked to plume and chemistry models to estimate

Montagne Pelee is one of the most active volcanoes of the Lesser Antilles arc, with two to three magmatic eruptions per millennium and an estimated magmatic production rate in the order of 0.7 km(3)/1000 years. Montagne Pelee is also infamous for the very large number of people (30000) killed by an eruptive phenomenon at the onset of the 1902-1905 dome-forming eruption. Active for similar to 550 kyrs, Montagne Pelee has undergone two major flank collapses that influenced its volcanological as well as magmatic evolution. The last one occurred at around 36 ka. Due to changes in the threshold effect following the decrease in load of the volcanic edifice due to flank collapse, there was a switch in emitted magma from generally andesitic to basaltic andesites. After 10 kyrs of intense activity, the load exerted by the new edifice once again prevented dense basaltic andesite magma from reaching the surface, whereas andesitic magmas, similar to the initial ones, continued to be emitted. All the magmas come from a common magma ponding zone at 200 +/- 50 MPa, 875 +/- 25 degrees C, an oxygen fugacity (fO(2)) between 0.4 and 0.8 log unit above the nickel-nickel oxide (NNO) oxygen buffer, and melt H2O contents of 5.3-6.3 wt%. Based on comparative on-land and marine tephrochronological studies, we have reconstructed a detailed eruptive history of the volcano over the last 25 kyrs. The volcano produced a succession of PlinianSubPlinian and dome-forming eruptions, making it a textbook case for studying this duality, which sometimes occurred during a single eruption. We identified more than 55 magmatic eruptions, with a ratio of 2/3 for domeforming vs. Plinian eruptions. An unusual feature of this volcano is that dome-forming eruptions often start with violent, superficial and laterally directed explosions. These generate highly devastating dilute and turbulent pyroclastic density currents on the southwestern and southern flanks of the volcano, as illustrated by the seven events of this type during the first months of the 1902-1905 eruption. On the basis of the past eruptions over the last millennia, a series of scenarios can be proposed in the event of reactivation, including no magmatic eruption, a phreatic event or a magmatic eruption (Plinian or dome-forming eruption, with or without an explosive phase).

Dominica, one of the most magmatically active islands of the Lesser Antilles through its four active volcanoes, is likely host under its central part, below Morne Trois Pitons–Micotrin, to a well-established transcrustal mush system. Pre-eruptive spatiotemporal magma dynamics are examined for five, explosive, pumiceous eruptions of this volcano in the last 24 kyrs through a combined Crystal System Analysis and intracrystalline Fe–Mg interdiffusion timescales modelling approaches. Before all eruptions, two magmatic environments of close compositions have interacted. These interactions began ~ 10–30 years prior to the four smaller of these eruptions, with more sustained mixing in the last decade, accelerated in the last 2 years. This contrasts with the largest pumiceous eruption, involving deeper magmas, with magma interaction starting over roughly a century but with various patterns. This suggests a possibility that increasing reactivation signals could be registered at the surface some years before future eruptions, having significant implications for volcanic risk mitigation.

Montagne Pelée is one of the most active volcanoes of the Lesser Antilles arc, with two to three magmatic eruptions per millennium and an estimated magmatic production rate in the order of 0.7 km3/1000 years. Montagne Pelée is also infamous for the very large number of people (30000) killed by an eruptive phenomenon at the onset of the 1902–1905 dome-forming eruption. Active for ~550 kyrs, Montagne Pelée has undergone two major flank collapses that influenced its volcanological as well as magmatic evolution. The last one occurred at around 36 ka. Due to changes in the threshold effect following the decrease in load of the volcanic edifice due to flank collapse, there was a switch in emitted magma from generally andesitic to basaltic andesites. After 10 kyrs of intense activity, the load exerted by the new edifice once again prevented dense basaltic andesite magma from reaching the surface, whereas andesitic magmas, similar to the initial ones, continued to be emitted. All the magmas come from a common magma ponding zone at 200 ± 50 MPa, 875 ± 25 °C, an oxygen fugacity (fO2) between 0.4 and 0.8 log unit above the nickel‑nickel oxide (NNO) oxygen buffer, and melt H2O contents of 5.3–6.3 wt%. Based on comparative on-land and marine tephrochronological studies, we have reconstructed a detailed eruptive history of the volcano over the last 25 kyrs. The volcano produced a succession of Plinian-SubPlinian and dome-forming eruptions, making it a textbook case for studying this duality, which sometimes occurred during a single eruption. We identified more than 55 magmatic eruptions, with a ratio of 2/3 for dome-forming vs. Plinian eruptions. An unusual feature of this volcano is that dome-forming eruptions often start with violent, superficial and laterally directed explosions. These generate highly devastating dilute and turbulent pyroclastic density currents on the southwestern and southern flanks of the volcano, as illustrated by the seven events of this type during the first months of the 1902–1905 eruption. On the basis of the past eruptions over the last millennia, a series of scenarios can be proposed in the event of reactivation, including no magmatic eruption, a phreatic event or a magmatic eruption (Plinian or dome-forming eruption, with or without an explosive phase).