Volcaniclastic debris flows 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 flows are lithologically homogeneous and contain more than 90% of material from the parental eruption. They differ from inter-eruptive debris flows recognized in the area, which contain mixed lithologies of juvenile material (i.e., volcanic material from different eruptions). Diffuse rill erosion generated fines-rich volcaniclastic flows (mudflows), whereas partial saturation of coarse ash and lapilli generated coarser-grained debris flows. Lithofacies analysis shows that debris flows predominate versus hyperconcentrated flows and normal stream-flow deposits. Debris-flow 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-flow deposits aggraded rapidly by superimposition of different surges that spontaneously developed within the flow. Bulk-flow 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 flows stopped distally on active alluvial fans.