DISCOVERING NEW CATASTROPHIC EFFECTS OF A FAMOUS ERUPTION: PYROCLASTIC DEBRIS FLOWS OCCURRED IN THE LATTARI MOUNTAINS SOON AFTER THE 79 AD PLINIAN ERUPTION OF VESUVIUS VOLCANO (SOUTHERN ITALY)

During the 79 AD Vesuvius' eruption, the Lattari Mountains (limestone relief about 20 km south of Vesuvius) received up to 2m of pumiceous fallout plus several decimetres of final ash rain. To reconstruct the earliest downwasting events of that cover, we studied several sites where the correlative deposits crop out. They were found along both the northern and southern flanks of the ridge, mostly as bodies (up to 50m thick) aggrading the bottom of V-shaped valleys downstream of steep catchment heads. In the case of incisions descending toward the Sarno coastal plain, the aggradational events spread down -with reduced thickness- on the piedmont alluvial fans. In the peninsular part of the ridge, the rapid post-eruption downwaste promoted also the growth of fan-deltaic bodies that pushed the coastline some hundreds of metres ahead.

The first and largest events of downwasting were wet debris flows mixing together ash and pumice fragments plus calcareous clasts entrained from pre-existing veneers. They form stacked sets of grey, matrix supported massive conglomerates up to tens of metres thick. A period of frequent flash-floods followed that aggraded the terminal fans with sandy and pebbly deposits from hyper-concentrated flood-flows and minor debris flows.

As stratigraphical and archaeological data demonstrate, the debris flows occurred soon after the eruption or even during it, triggered by eruption-induced rainstorms. The following phase of prevailing sheet-flood deposition ended during the 2nd Century.

The study demonstrates that Plinian eruptions from Vesuvius may very well be hazardous for the closest mountains of the Apennine Chain. Beside the distal fallout, just-post-eruption events of rapid downwasting must be considered, especially for valley-floor and alluvial-fan settlements.