Magmatic and phreatomagmatic contributions on the ash-dominated basaltic eruptions: Insights from the April and November–December 2005 paroxysmal events at Karthala volcano, Comoros

Basaltic eruptions are commonly associated with lava emissions and relatively weak explosive activities, but they can sometimes produce strong eruptive phases. In April November 2005, two paroxysmal events occurred within the summit crater of Karthala basaltic shield volcano (Grande Comore Island, Comoros), which hosted a water lake before each these events. Both 2005 ash plumes spread across Comoros Archipelago heavily impacted whole Grande Island. Associated deposits on extremely fine-grained (up to 50 wt% fine <63 μm for some analyzed layers) rich in millimeter-sized rounded accretionary lapilli aggregates. Field observations, as well textural chemical analyses performed both coarse- pyroclasts permit identify juvenile non-juvenile components quantify their peculiar characteristics. Coarse (710–1000 μm) mainly consists pumice particles (vesicle number density NV = 4.5 104 mm−3 gas melt ratio VG/VL 1.5, average), characterized by glassy groundmasses representative magma portions ascending quickly conduits 10 m s−1). A low amount scoria also observed coarse fractions, degassing (NV 4.7 0.5 average) crystallization (occurrence dendritic microlites). Non-juvenile fragments (from blocks ash) dense or intrusive fragments. Their decreases exponentially towards composed juvenile, blocky, that unambiguous signs brittle fragmentation (hackle lines, stepped features cracks). We support Molten Fuel-Coolant Interactions between highly porous fast magmas external waters during phases studied eruptions, leading brittle-dominant efficient regime fragmentation. Variable large grains through stratigraphic depth be ascribed failure vesicle walls initial magma. Concurrently, thermohydraulic explosions caused host rock at shallow level, generating particles. short-lived episode intense fountaining steam eventually end paroxysm, forming last tephra layer top sequence. Each phase lasted about day table progressively vaporized dried away. ended pond activities confined crater. conclude contributions magmatic processes phreatomagmatic interaction mechanisms ultimately generated grain size, component texture variabilities deposits. This work contributes better understanding generation unusual from dynamics mechanisms, ascent conduit level intra-crateric waters.