THE AVELLINO PLINIAN ERUPTION OF SOMMA-VESUVIUS (3760 Y BP) - THE PROGRESSIVE EVOLUTION FROM MAGMATIC TO HYDROMAGMATIC STYLE

A detailed stratigraphic analysis of the Avellino plinian deposit of the Somma-Vesuvius volcano shows a complicated eruptive sequence controlled by a combination of magmatic and hydromagmatic processes. The role of external water on the eruptive dynamics was most relevant in the very early phase of the eruption when the groundwater explosively interacted with a rising, gas-exolving magma body creating the first conduit. This phase generated pyroclastic surge and phreatoplinian deposits followed by a rapidly increasing discharge of a gas-rich, pure magmatic phase which erupted as the most violent plinian episode. This continuing plinian phase tapped the magma chamber, generating about 2.9 km(3) of reverse-graded fallout pumice, more differentiated at the base and more primitive at the top (white and gray pumice). A giant, plinian column, rapidly grew up reaching a maximum height of 36 km. The progressive magma evacuation at a maximum discharge rate of 10(8) kg/s that accompanied a decrease of magmatic volatile content in the lower primitive magma allowed external water to enter the magma chamber, resulting in a drastic change in the eruptive style and deposit type. Early wet hydromagmatic events were followed by dry ones and only a few, subordinated magmatic phases. A thick, impressive sequence of pyroclastic surge bedsets of over 430 km(2) in area with a total volume of about 1 km(3) is the visible result of this hydromagmatic phase.