Pyroclastic deposits as a guide for reconstructing the multi-stage evolution of the Somma-Vesuvius Caldera

The evolution of the Somma-Vesuvius caldera has been reconstructed based on geomorphic observations, detailed stratigraphic studies, and the distribut ion and facies variations of pyroclastic and epiclastic deposits produced b y the past 20,000 years of volcanic activity. The present caldera is a mult icyclic, nested structure related to the emptying of large, shallow reservo irs during Plinian eruptions. The caldera cuts a stratovolcano whose origin al summit was at 1600-1900 m elevation, approximately 500 m north of the pr esent crater. Four caldera-forming events have been recognized, each occurr ing during major Plinian eruptions (18,300 BP "Pomici di Base", 8000 BP "Me rcato Pumice", 3400 BP "Avellino Pumice" and AD 79 "Pompeii Pumice"). The t iming of each caldera collapse is defined by peculiar "collapse-marking" de posits, characterized by large amounts of lithic clasts from the outer marg ins of the magma chamber and its apophysis as well as from the shallow volc anic and sedimentary units. In proximal sites the deposits consist of coars e breccias resulting from emplacement of either dense pyroclastic flows (Po mici di Base and Pompeii eruptions) or fall layers (Avellino eruption). Dur ing each caldera collapse, the destabilization of the shallow magmatic syst em induced decompression of hydrothermal-magmatic and hydrothermal fluids h osted in the wall rocks. This process, and the magma-ground water interacti on triggered by the fracturing of the thick Mesozoic carbonate basement hos ting the aquifer system, strongly enhanced the explosivity of the eruptions .