The Holocene syneruptive volcaniclastic debris fl ows in the Vesuvian area: Geological data as a guide for hazard assessment

Abstract

Volcaniclastic debris fl ows generated in drainage basins of the Apennine mountains of southern Campania in response to pyroclastic fall deposition from four Holocene eruptions of Somma-Vesuvius: Avellino (3.8 ka), A.D. 79, A.D. 472, and A.D. 1631. These syneruptive debris fl ows are lithologically homogeneous and contain more than 90% of material from the parental eruption. They differ from inter-eruptive debris fl ows recognized in the area, which contain mixed lithologies of juvenile material (i.e., volcanic material from different eruptions). Diffuse rill erosion generated fi nesrich volcaniclastic fl ows (mudfl ows), whereas partial saturation of coarse ash and lapilli generated coarser-grained debris fl ows. Lithofacies analysis shows that debris fl ows predominate versus hyperconcentrated fl ows and normal stream-fl ow deposits. Debris-fl ow deposits are massive, matrix supported, and have a gravelly-sandy texture. Large blocks are scarce due to their absence in the pyroclastic source material. Lithofacies association indicates that volcaniclastic debris-fl ow deposits aggraded rapidly by superimposition of different surges that spontaneously developed within the fl ow. Bulk-fl ow density ranges from 1840 to 2260 kg/m3 (mean 2035 ± 207 kg/m3). Geological data supported some considerations of hazard assessment in the study area and indicate that the syneruptive volcaniclastic fl ows stopped distally on active alluvial fans.