Since May 2018 till the end of 2021, Mayotte island has been the locus of a major submarine volcanic eruption characterized by the offshore emission of more than 6.5 km\(^{3}\) of basanitic magma. ...The eruption occurred along a WNW–ESE trending submarine ridge on the east flank of the island where, in addition, several seemingly recent phonolitic bodies were also identified close to the island. To define realistic scenarios of magma ascent and potentially predict the style of an upcoming event, it is crucial to have a precise understanding on the plumbing system operating below volcanoes. The putative relationships between basanites emitted by the new volcano and these recent phonolites have been experimentally explored by performing crystallization experiments on a representative basanite over a large range of pressures (up to 400 MPa). The results show that the crystallization of basanite at crustal levels (\({\le }\)12–15 km) yields a phonolitic residual liquid containing up to 3–4 wt% \(\mathrm{H}_{2}\mathrm{O}\) after \({\ge }\)65 wt% of an assemblage of olivine\(+\)plagioclase\(+\)amphibole\(+\)clinopyroxene\(+\)biotite\(+\)magnetite\(+\)ilmenite\(+\)apatite. The final iron content of the residual phonolitic liquids is strongly controlled by the depth/pressure of fractionation. Fe-rich phonolites from the submarine ridge are produced at 6–8 km depth, while a shallower differentiation (\({\le }\)4–5 km) results in the production of liquids with trachyte–benmoreite affinities. If the fractionation process occurs at depths higher than 8 km, the resulting phonolitic melts are progressively enriched in \(\mathrm{SiO}_{2}\)–\(\mathrm{Al}_{2}\mathrm{O}_{3}\) but depleted in FeO*, ie unlike those erupted. We therefore conclude that phonolitic magma production and storage at Mayotte is a rather shallow process.
Flank-collapse events are now recognized as common processes of destruction of volcanoes. They may occur several times on a volcanic edifice pulling out varying volumes of material from km3 to ...thousands of km3. In the Lesser Antilles Arc, a large number of flank-collapse events were identified. Here, we show that some of the largest events are correlated to significant variations in erupted magma compositions and eruptive styles.
On Montagne Pelée (Martinique), magma production rate has been sustained during several thousand years following a 32ka old flank-collapse event. Basic and dense magmas were emitted through open-vent eruptions that generated abundant scoria flows while significantly more acidic magmas were produced before the flank collapse. The rapid building of a new cone increased the load on magma bodies at depth and the density threshold. Magma production rate decreased and composition of the erupted products changed to more acidic compared to the preceding period of activity. These low density magma generated plinian and dome-forming eruptions up to the Present. In contrast at Soufrière Volcanic Centre of St. Lucia and at Pitons du Carbet in Martinique, the flank-collapses have an opposite effect: in both cases, the acidic magmas erupted immediately after the flank-collapses. These magmas are highly porphyritic (up to 60% phenocrysts) and much more viscous than the magmas erupted before the flank-collapses. They have been generally emplaced as voluminous and uptight lava domes (called “the Pitons”). Such magmas could not ascent without a significant decrease of the threshold effect produced by the volcanic edifice loading before the flank-collapse.
•Flank-collapse effect on magma plumbing system in the Lesser Antilles arc•Threshold effect of volcanic edifice on magma emission•By-pass of the threshold due to the unload by flank-collapse•By-pass of the threshold by magma differentiation•Combining on-land and marine tephrochronology data on Montagne Pelée
Since 2018, the submarine east flank of Mayotte Island (Comoros archipelago) is the site of a major eruption located at 3.5 km depth bsl on a WNW-ESE volcanic ridge. Samples brought by oceanographic ...cruises carried out to monitor this seismo-volcanic crisis indicate that this volcanic ridge is built by a bimodal sodic alkaline magmatic series that includes basanites and phonolites. A petrological study of dredged samples allowed us to image the magmatic system feeding the volcanic ridge and to determine the link between basanitic and phonolitic magmas. The magmatic system feeding the volcanic ridge comprises multiple levels of magma storage. Basanitic magmas generated at 80–100 km mantle depth are stored in two or more deep reservoirs (≥ 37 km) and then in shallower basanitic and phonolitic lenses located close to the Moho interface before rising the surface. This study identifies three possible scenarios: (1) the deep basanitic magma rises directly and quickly to the surface from the deep mantle reservoir (as is currently happening 60 km offshore), (2) the basanitic magma stalls in a shallower reservoir near the Moho before resuming its ascent toward the surface and erupting as porphyritic basanite, (3) the basanitic magma stops and evolves to phonolite in these sub-crustal reservoirs. The phonolitic lavas are produced by approximately 80% fractional crystallization (34% clinopyroxene, 30% anorthoclase feldspar, 15.5% magnetite, 12.5% olivine, 5% apatite and 4% ilmenite) of a hydrous basanitic magma at mantle depths (
P
> 0.6 GPa) under reduced oxygen fugacity (~ FMQ-1). In this third scenario, the phonolitic magma might be reactivated by the arrival of a new batch of deeper basanitic magma.
Over the past 9,150 years, at least 9 flank collapses have been identified in the history of La Soufrière of Guadeloupe volcano. On account of the volcano's current unrest, the possibility of such a ...flank collapse should not be dismissed in assessing hazards for future eruptive magmatic as well as non-magmatic scenarios. We combine morphological and geophysical data to identify seven unstable structures (volumes ranging from 1 × 10
m
to 100 × 10
m
), including one that has a volume compatible with the last recorded flank collapse in 1530 CE. We model their dynamics and emplacement with the SHALTOP numerical model and a simple Coulomb friction law. The best-fit friction coefficient to reproduce the 1530 CE event is tan(7°) = 0.13, suggesting the transformation of the debris avalanche into a debris flow, which is confirmed by the texture of mapped deposits. Various friction angles are tested to investigate less water-rich and less mobile avalanches. The most densely populated areas of Saint-Claude and Basse-Terre, and an area of Gourbeyre south of the Palmiste ridge, are primarily exposed in the case of the more voluminous and mobile flank collapse scenarios considered. However, topography has a prominent role in controlling flow dynamics, with barrier effects and multiple channels. Classical mobility indicators, such as the Heim's ratio, are thus not adequate for a comprehensive hazard analysis.
West off Martinique (Lesser Antilles), the Grenada Basin submarine sediments were affected by the emplacement of Debris Avalanche Deposits (DAD). Montagne Pelée Volcano has experienced three major ...flank collapses during the last ca. 127 kyrs, resulting in a cumulated volume of up to 300 km3 offshore. Using a combination of geophysical and geotechnical data, we investigate the effect of these debris avalanches emplacements on the basin hydrogeology and their relationship with the observed sediment deformation in the seismic and coring data. The geotechnical test carried on IODP‐340 cores samples reveal four sediment types within the basin with distinctive mechanical and hydraulic properties: proximal volcanoclastics, distal volcanoclastics, hemipelagic and ash‐rich sediments. These results, together with margin stratigraphic models obtained from seismic reflection data, were used as inputs for the numerical finite‐element model. This model shows that the coupling of the sediment properties with the mid‐ to low‐sedimentation rates results in the development of low overpressures prior to the first flank collapse at 127 ka. However, the emplacement of the first two DADs, between 127 and 36 ka, developed high overpressures ratios (λ* > 0.9) in the easternmost part of the Grenada Basin. According to the model, the sudden compaction of the pre‐existing sediments due to the DADs load created fluid flow velocities up to 7 times higher than the hydraulic conductivities, which would have thus reduced the sediment bearing capacities and shear strength, favouring their mobilization and deformation. From 127 to 36 ka, the sea‐floor sediments suffered a long‐term deformation driven by the combination of the weight of the emplaced material and the persistence of high overpressure ratios through time. This deformation propagated tens of kilometres away from the DAD’s emplacement and it is possible that still continues today due to the persistence of low overpressure ratios. This long‐term and long‐distance deformation and persisted overpressures are a key factor to take into account in the framework of a geohazards evaluation in areas recurrently affected by earthquakes and volcanic flank collapses.
The Grenada Basin was affected by at least two flank collapse debris avalanches from the Montagne Pelée volcano during the last 130 ka. These mass‐transports combine with their derived overpressures built up may cause the long‐term and long‐distance deformation that can be observed nowadays.
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.
Results from recent marine geophysical surveys offer a new perspective for characterizing the evolution processes of volcanic islands. In 2006, cruises FOREVER and ERODER 1 investigated the submarine ...flanks and the surrounding abyssal plain of La Réunion (Indian Ocean) to obtain for the first time a complete geophysical survey of the area. Combined analyses of these data reveal major differences in the evolution of the two emerged volcanoes, Piton des Neiges and Piton de la Fournaise. We show that debris avalanche deposits extend on the abyssal plain only offshore the active Piton de la Fournaise volcano attesting the occurrence of large flank‐collapse events. The absence of such deposits offshore Piton des Neiges and the presence of compressive structures within the sedimentary unit below the edifice support a mechanism of slow deformation of this volcano, such as sliding or spreading. The slow deformation of Piton des Neiges has led to numerous secondary submarine slope instabilities and favored some unconfined turbidity flows which generated large sediment waves running downward all around the island. This study proposes a new model using the most complete marine data set available: slow deformation controls the evolution of Piton des Neiges whereas Piton de la Fournaise (formed on the flanks of a pre‐existing edifice) experienced catastrophic, large flank‐collapse events.
Key Points
New offshore analysis of La Reunion volcanic complex using a unique data set
Piton des Neiges: controls the geometry of La Reunion, slow deformation
Piton de la Fournaise: large flank‐collapse events
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