Volcaniclastic debris flows at La Fossa Volcano (Vulcano Island, southern Italy): Insights for erosion behaviour of loose pyroclastic material on steep slopes

This paper describes a qualitative model of erosion of loose volcaniclastic material on steep slopes of La Fossa cone (Vulcano Island, Italy) that generates small volume debris flows. The loose volcaniclastic cover rests on an almost impermeable substratum, which comprises a fine-grained hydromagmatic tuff. Morphologically, three sectors were distinguished: 1) a water supply zone (slope > 32°) where rain water is enriched in mud and concentrated in the initiation zone; 2) an initiation zone (or source area sensu stricto slope with ∼30°) where debris flows form and; 3) a transport and deposition zone where debris flows deposited levees and terminal lobes (slope ∼30°). Main episodes of erosion of loose volcaniclastic cover and debris flow generation occur in response to thunderstorms. The erosion of tuff from superficial water added a significant amount of fines (mud fraction) to the initiating flows. Debris flow stoppage on steep slopes was interpreted as the result of a loss of mass (estimated between 35% and 53% of total mass) to form levees. From field and laboratory data the density of debris flows was estimated at about 1950 kg/m3. Morphological data, matrix texture, mass number (Nm) and clay content of debris flow deposits suggest that they were deposited by saturated granular slurries.