Very fast canyon formation and evolution along active volcanic flanks: A case from Stromboli Island

Active insular volcanoes are generally characterized by their rapid morphological evolution, as a close interplay exists between eruptive activity and erosive-depositional processes in such volcanoes. The Sciara del Fuoco depression, a sector collapse scar on the NW flank of the Stromboli volcano (Italy), is considered a natural laboratory for studying the response of a volcanic slope to such a dynamic evolution. In this study, we report the very fast morphological evolution that affected the subaerial and submarine Sciara del Fuoco slope from May 2022 to May 2023, a period of time marked by the occurrence of two crater rim failures, pyroclastic density currents and multiple lava flows. The analysis of repeated topo-bathymetric surveys demonstrates that a narrow (100 m wide at maximum) and steep-sided canyon, tens of meters deep, formed in the central part of the Sciara del Fuoco. The canyon was mainly related to the erosive activity of the pyroclastic density currents, which led to the remobilization of (at least) 3.7 × 10⁶ m³ volcanic material, mainly in the subaerial slope. The canyon was initially formed by retrogressive erosion upslope, starting from an initial submarine/coastal landslide. It then evolved through a progressive widening of its flanks through small-scale landslides. The study shows that landslide location, as well as the final canyon shape, were strongly controlled by the lithological limits of previous lava flows, highlighting the importance of inherited morpho-stratigraphy and lateral heterogeneities in slope stability. Since its formation, the canyon has acted as a main conduit for lava flows and volcaniclastic materials supplied on their way to the sea. About 1 × 10⁶ m³ of material filled the subaerial canyon floor through time, and another 1 × 10⁶ m³ of slope accretion was estimated for the submarine part of the Sciara del Fuoco, down to 400 m below sea level. Comparing the volumes associated with slope erosion and accretion, it is evident that a large part of the remobilized material bypassed the Sciara del Fuoco shallow-water sector and was emplaced at greater depths. This study highlights the relevance of an integrated system for monitoring the submarine and subaerial morphological evolution of insular volcanic flanks, contributing to an improved geohazard assessment during eruptive crises.