Structural and geothermal changes mapped by drone-based photogrammetry and thermal infrared during the 2021 volcanic crisis of Vulcano Island, Sicily

Vulcano is an active volcanic island located in the south-central sector of the Aeolian Archipelago (Tyrrhenian Sea, Italy). The most recent active edifice is the La Fossa crater located in the center of the island, neighboring the main settlement Vulcano Porto. Its eruptive history is characterized by frequent transitions from phreatomagmatic to minor magmatic activity. The last eruption occurred in 1888–90 with strong phreatic (“Vulcanian”) eruption pulses. During the last decades, it has undergone several periods of volcanic unrest, accompanied by increasing degassing, rising fumarole temperatures, changing gas compositions, or increasing groundwater- and soil temperatures. Major unrest periods were reported in the 1920s, 1940s, and 1990s. Here we report on the ongoing crisis that initiated in September 2021. Rapidly increasing degassing levels and fumarole temperatures, accompanied by seismic activity and surface deformation were detected and monitored by the monitoring network (INGV bulletin reports). The fast evolution and dynamics of the crisis caused authorities to raise the alert level to orange and led to temporary evacuations in Vulcano Porto. We monitored this crisis from the beginning by monthly drone-based optical and thermal infrared overflights. The drone data was processed by using the Structure-from-Motion approach, allowing to generate spatially dense optical and thermal infrared maps. This way we captured the response of the hydrothermal system at the surface in great detail, were able to monitor the spatio-temporal evolution of the high-temperature fumarole field but also associated mean and low-temperature anomalies of diffuse degassing areas. We compared observations to a previous study considering in detail the structure and thermal expression of the La Fossa fumarole field, and the hydrothermal alteration associated (Müller et al., 2021, JVGR). Major aspects of changes observed at the surface during the crisis that could be constrained are (i) an increase of fumarole temperatures, (ii) the development of new fumarole vents, (iii) the evolution of a thermal aureole surrounding the major fumarole field at a distance, and (iv) the formation of a net-shaped thermal anomaly network. Changes are presented on a spatial and temporal scale and highlight the dynamics of degassing systems at the surface with implications for volcanic monitoring and hydrothermal alteration research and suggest that unrest is detectable at fumaroles but also at diffuse degassing zones elsewhere affecting a larger region of the La Fossa cone.