THE OTTAVIANO ERUPTION OF SOMMA-VESUVIO (8000 Y BP) - A MAGMATIC ALTERNATING FALL AND FLOW-FORMING ERUPTION

The Ottaviano eruption occurred in the late neolithic (8000 y B.P.). 2.40 km(3) of phonolitic pyroclastic material (0.61 km(3) DRE) were emplaced as pyroclastic flow, surge and fall deposits. The eruption began with a fall phase, with a model column height of 14 km, producing a pumice fall deposit (LA). This phase ended with short-lived weak explosive activity, giving rise to a fine-grained deposit (L1), passing to pumice fall deposits as the result of an increasing column height and mass discharge rate. The subsequent two fall phases (producing LB and LC deposits), had model column heights of 20 and 22 km with eruption rates of 2.5 X 10(7) and 2.8 1 X 10(7) kg/s, respectively. These phases ended with the deposition of ash layers (L2 and L3), related to a decreasing, pulsing explosive activity. The values of dynamic parameters calculated for the eruption classify it as a sub-plinian event. Each fall phase was characterized by variations in the eruptive intensity, and several pyroclastic flows were emplaced (F1 to F3). Alternating pumice and ash fall beds record the waning of the eruption. Finally, owing to the collapse of a eruptive column: of low gas content, the last pyroclastic flow (F4) was emplaced.