Drone and field-based monitoring of the degassing and alteration structures of the La Fossa fumarole field (Vulcano Island), before and during the 2021 volcanic crisis

Abstract

Fumarole fields and hydrothermal alteration are prominent signs of volcanic degassing at many volcanoes, and their monitoring is an essential part of the assessment of volcanic unrest. Yet, our knowledge about the detailed structure of fumarole fields and the spatiotemporal processes in their complexity is still poor, owing to limited accessibility. By using modern drone and sensor technologies we now are able to provide high-resolution data that allows us to analyze fumarole fields at cm-scales. From 2018 to 2022, we conducted repeated drone surveys at the fumaroles of La Fossa volcano on Vulcano Island (Italy). Drones equipped with a 20 MP camera and a radiometric thermal infrared sensor allowed the close-range acquisition of optical and thermal infrared images. By means of Structure from Motion (SfM) processing, the generation of high-resolution ortho- and infrared mosaic data was achieved. Applying Principal Component Analysis and image classification to the orthomosaic data, we detected and classified areas affected by degassing and hydrothermal alteration covering more than 60.000 sqm. By analyzing their spectral characteristics, we defined 4 surface types, of which type 1 and 2 are largely coincident with the thermally active surface. Type 3 is an altered low-temperature surface and type 4 is an unaltered surface. To evaluate these surface types, samples were analyzed in the lab for their mineralogical and geochemical composition by X-ray diffraction and fluorescence analysis, showing significant variability in composition. Further, we analyzed the spatial variability of the surface degassing activity using a portable multi-gas device. The combination of these methods allows us to constrain factors that are controlling the observed surface pattern of the degassing system, and to better understand the structural setup of the fumarole field and broader field of activity. We find that the actual high-temperature fumarole sites only account for <10% of the active surface. Besides, large thermally active areas, thermal aureoles for instance, display a rather diffuse activity. During the 2021 volcanic crisis, next to the high-temperature fumaroles, especially those diffuse features showed a response to the increased gas flux, emphasizing their structural importance. We summarize spatiotemporal variations during the crisis, and indicate possible widespread effects of the long-term gas-rock interaction like surface sealing, and forced lateral gas migration, affecting large parts of the fumarole field. The results suggest that detailed structural studies of fumarole fields by means of drone-based remote sensing in combination with in-situ measurements can contribute to a better understanding of degassing and alteration effects, with relevance for degassing sites elsewhere and during volcanic crises.