Hydrothermal alteration and mechanical weakening identified at La Fossa, Vulcano island (Italy) by combining remote sensing and in situ strength measurements

Volcanic flank instability poses a significant multi-hazard risk, encompassing caldera collapses, landslides, rock avalanches, and potential tsunami generation in active and dormant volcanoes. The mechanical strength and regions of hydrothermal alteration may play a fundamental role in locating and scaling volcano instability. Therefore, investigating hydrothermal alteration, which consequently alters the physicochemical properties of volcanic rocks, is crucial to better understanding the processes that lead to volcanic flank instability and collapse.

Here, we use the southernmost exposure of the Aeolian volcanic archipelago, La Fossa of Vulcano Island (Italy), as our focus site. La Fossa's history of mass wasting, regions of hydrothermal alteration, and episodic fumarole activity make it an ideal natural laboratory for our investigation. Here, we used high-resolution drone remote sensing techniques coupled with in-situ uni-axial compressive strength measurements to identify regions of hydrothermal alteration and assess their associated compressive strength properties. In summary, our results show (1) a heterogeneous distribution of alteration types and intensities, (2) a relationship between increasing alteration intensity and decreasing rock strength, (3) a correlation between regions with the weakest rock strength and the most intensely altered areas, and finally (4) a spatial association of alteration and deep scars resulting from erosion and landslides. Our combined approach allows us to explore the association between rock strength and hydrothermal alteration, enabling us to understand volcanic flank instability better and help us improve future hazard assessment.