Ongoing geological processes on the flanks of active volcanoes. A multi-scale geomorphological approach applied to the La Fossa Caldera (Vulcano Island, Italy)

The integrated analysis of successive multibeam bathymetry surveys and seafloor videos acquired from 2005 to 2023 provides crucial insights into the recent morphological evolution of the submerged part of La Fossa Caldera (Vulcano Island). The caldera floor is carved by a network of gullies and channels that often incise a coastal platform and submarine depositional terrace. Gullies are short (similar to 15-430 m) and steep (similar to 30 degrees) erosional features characterized by their V-shaped cross-sections. In contrast, channels are relatively long (similar to 350-1180 m) and flat-bottomed features. Channels show average slope gradients of similar to 15 degrees and often host upper-flow regime bedforms along their thalweg, most of which have wavelengths of similar to 10-80 and heights of similar to 0.5-2 m. Despite the geologically-active setting of the study area, repeated multibeam surveys shows only minor morphological changes on the seafloor over the last 20 years. Seafloor erosion is dominant and often associated with small-scale retrogressive slope failures at the channel head, likely triggered by earthquakes or storm-waves that frequently hit the area. Seafloor erosion due to sediment-laden flows is observed offshore the Rio Grande creek, where coarse-grained deposits mixed with accumulation of plant debris are common. Small-scale rock-falls dominate the evolution of a steep escarpment that bounds the coastal platform, as testified by accumulation of squared metric blocks at their base. This study highlights the importance of multi-temporal and multi-scale geomorphological approaches to understand erosional processes that shape submarine volcanic flanks. Our results have key implications for geohazard assessments in such areas.