Formation of rock coatings induced by the acid fumarole plume of the passively degassing volcano of La Fossa (Vulcano Island, Italy)

At La Fossa volcano (Vulcano island) and in the surrounding environment, rock coatings (RC) develop owing to exposure of the ground surface to a volcanic acid plume. This plume is produced by the passive degassing of La Fossa active cone. Proximal and distal (with respect to the high temperature fumarolic field) RC have been identified which differ in mineralogy, texture and chemical composition. Proximal RC are characterised by high silica content, massive texture and strong leaching at the expense of the pyroclastic material. Distal RC are characterised by the presence of silica, jarosite and minor goethite, laminated texture and less developed evidence of leaching. The RC are produced by direct interaction between the acid fumarolic aerosol and rocks and by reaction of dews and rains, acidified by the absorption of acid gases (SO2, HCl, HF) of the fumarole plume, with volcanic material. Proximal RC are produced at conditions of pH<2, distal RC form at pH 2–3, based on the presence of jarosite, although the occurrence of goethite in some samples suggests higher values of pH. The influence of the volcanic plume in the development of these RC is indicated also by the coincidence of the RC areal distribution with the distribution of the volcanic plume, controlled by the local wind field. This is furthermore supported by the presence in RC of chloride and/or sulphur bearing compounds, enriched in volatile elements and metals, commonly carried by volcanic aerosols.