Newly discovered submarine flank eruption at Stromboli volcano (Aeolian Islands, Italy)

The NW submarine portion of Stromboli volcano has been investigated by deep-towed sidescan sonar, bathymetric surveys, video camera runs and dredging during two research cruises in 2002 and 2004. The surveys resulted in the identification of an extensive pillow lava field (106-107 m3)at about 2300 m of water depth and 9 km from the shoreline of Stromboli Island. Marine geology data coupled with volcanological observation and volatile content in the glass, demonstrate that this lava flow-field is in place and is related to a recent submarine effusive eruption originating from a nearby eruptive fissure. Anyway bulk rock and glass composition of the lava differs significantly from present day summit products and from those erupted during the historical flank eruptions. Pillow lavas compositions show a high K2O content and a general geochemical pattern more similar to the less evolved K-alkaline products erupted between 13 and 6 ka during the Neostromboli period. However, the silica content is outside of the known compositional range of Neostromboli, hence the pillow lava could represent a not yet documented stage of the structural evolution of the volcano. The compositional and volcanological evolution within the subaerial Neostromboli products is relatively well constrained, including the early stages of the period; on the other hand, the passage between Neostromboli period and early stages of development (6-2 ka) of the Recent Stromboli period (after the Neostromboli collapse) are still poorly known. The unique composition of the pillow lavas is interpreted as the result of the interaction between magmas feeding the new cycle and batches of previous Neostromboli magmas, still present after the emptying of the Neostromboli magma chamber. In this perspective, the investigated flank eruption could be the 'missing link' of early Recent Stromboli evolution by representing the beginning of this new intrusive cycle. In addition to its structural and volcanological significance this eruption opens new scenarios in Stromboli volcanic hazard: while flank eruptions on the subaerial part of the volcano could have an obvious direct impact on inhabited areas, submarine eruptions have the potential to trigger slope instability. Processes of flank inflation, opening of the eruptive fissures and gravitational loading due to the lava flows piling-up, are all able to induce sediment destabilization and possible flank failure.