THE SECCHE-DI-LAZZARO PYROCLASTICS OF STROMBOLI VOLCANO - A PHREATOMAGMATIC ERUPTION RELATED TO THE SCIARA DEL FUOCO SECTOR COLLAPSE

Widespread phreatomagmatic ash deposits (Secche di Lazzaro pyroclastic s) overlying the lavas of the Neostromboli period (13 000 to ca. 5000 years B.P.) have been identified at Stromboli volcano. The deposits co nsist of accretionary lapilli-rich ash-fall layers and debris flows. A sh-fall layers originated by phreatomagmatic explosive activity from a vent probably located in the uppermost part of the sector-collapse sc arp (Sciara del Fuoco) which formed at the end of Neostromboli period. Stratigraphic position, composition of fresh lapilli identical to the Neostromboli lavas, and an unusually large scale of explosions sugges t that the Secche di Lazzaro pyroclastics were erupted in connection w ith the Sciara del Fuoco slope failure. Characteristics of the deposit s are consistent with phreatomagmatic/hydrothermal explosivity related to catastrophic decompression of the plumbing/hydrothermal system. Oc currence of multiple ash-fall episodes, coupled with variation of natu re of lithics lithology, is suggestive of repeated explosions eventual ly triggered by multiple, backward-migrating block detachments.