Etna’s submarine flank morphology and basement: new insight from microbathymetry and revised structural interpretation

Mount Etna, one of Earth's most active volcanoes, rises to an elevation of 3,400 meters. Its eastern flank extends seaward, descending to approximately 1,500 meters below sea level and creating a total vertical relief of nearly 5,000 meters. While it is known that Etna's offshore flank is highly mobile, the seafloor morphology and associated structures remain poorly understood.

During the 2024 RV METEOR cruise M198, high-resolution microbathymetry data were collected using an Autonomous Underwater Vehicle (AUV), and rock samples were dredged from distinctive morphological features. Using new AUV microbathymetry, we characterise a stiff layer that forms a narrow canyon in the Valle di Archirafi, featuring high relief and rough surfaces exposed by the erosion of overlying marine sediments. This layer is also forming in the upper part of the Amphitheatre, a chain of cliffs overlooking a gentler slope. The layer is characterised by a chaotic, high-amplitude facies in the seismic lines, which can be followed from the Valle di Archirafi to the Amphitheatre. Dredging during the M198 cruise enabled sampling phyric lavas in the upper part of the Amphitheatre and chemical analyses suggest cooling in a subaerial environment. These findings imply more than 600 m of subsidence of the entire area (42 km²). The area is located between 4 and 8 km from the coastline and lies directly beneath the Giarre wedge, which exhibits the highest sliding velocity on the eastern flank. This suggests that the offshore part exerts a strong pulling force on the northern part of Etna’s mobile sector and is thus key to understanding the dynamics of the onshore sector. In line with the onshore block structure inferred by geodetic methods, our new findings support a decoupling of a shallower block riding on top of the larger southeastern mobile flank. Finally, based on existing knowledge of Etna’s edifice, our new offshore interpretation, and existing seafloor morphology constraints, we propose an extended map of the offshore flank thickness. These new data necessitate a revised interpretation of the submarine structural model and challenge existing paradigms regarding the mobile flank.