Alluvial and coastal hazards caused by long-range effects of Plinian eruptions: the case of the Lattari Mts. after the ad 79 eruption of Vesuvius

Abstract

AbstractThe Lattari Mountains (a limestone ridge about 20 km south of Vesuvius) received 1–2.5 m of fallout from the famous Plinian eruption of ad 79. As demonstrated by many residual outcrops of thick volcanoclastic debris-flow and alluvial deposits (referred to here as the Durece unit), the pyroclastic fall was soon followed by rapid erosion and landsliding that produced (1) decametre-scale aggradation of some narrow valley floors; (2) reactivation of alluvial fans; and (3) growth of new fan-deltas (extending as far as 500 m) at the coast. This response was primarily due to the steep topography of the area and the high erodability of the pyroclastic materials (light and cohesionless pumice fragments). Several geo-archaeological data indicate that the accelerated sedimentation had a duration of the order of decades and was followed by rapid dissection of the Durece unit deposits and fast dismantling by wave action of the newly created fan-deltas. This case highlights the need to consider the possibly catastrophic reaction of fluvial and coastal systems to large explosive eruptions, even in non-volcanic terrains at some distance from the volcano.