Dynamics of blast eruptions: insights from the deposits of the 13th century AD Breccia di Commenda phreatic explosion (Island of Vulcano, Italy)

Understanding the dynamics and effects of phreatic eruptions is crucial to the hazard assessment of volcanic and geothermal areas. These eruptions may occur associated with magmatic phases or as isolated events, with the most recent examples being those of Te Maari (Togariro, New Zealand) in 2012 and of Ontake volcano (Japan) in 2014. The active caldera in the northern part of the Island of Vulcano (southern Italy) hosts in its center the cone of La Fossa. During its activity, la Fossa experienced various phreatic eruptions (e.g. in 1444 and in 1727 AD), with the most important event which occurred during the 13th century AD (Breccia di Commenda eruption, BdC). We present a detailed study of the BdC eruption through the integration of the stratigraphic characterization with sedimentological data, which allowed for the reconstruction of both eruption timing and eruptive mechanisms. The BdC event occurred in contemporaneous with the eruption of Rocche Rosse (1230±20 AD) from the neighbouring Island of Lipari, 12 km north of la Fossa; white, rhyolitic ash layers are in fact ubiquitously interfingered with the BdC deposits. The study of about 170 tephra logs revealed that the eruption occurred in three main phases. The opening, north-westerly-dispersed fallout (Phase 1) emplaced grey, altered lithic ash bearing accretionary lapilli. The eruption waxed (Phase 2) with several explosions, producing an asymmetric shower of ballistic blocks and the emplacement of narrowly-dispersed, lithic-rich, stratified pyroclastic density current (PDC) deposits, followed by a radially distributed, topographically-controlled, coarse-grained PDC which represent the main body of the breccia deposit. Finally, the eruption waned with the generation of accretionary lapilli-bearing ash fall deposits (Phase 3). Single PDC units range in volume from 2.1X104 m3 to 2.7X105 m3, with a total dispersal of ~4.5 km2. Sedimentological analyses revealed that the eruption occurred with little or null involvement of fresh magma and that the breccia deposit was mostly composed by lava fragments. This latter observation suggests that the crater area prior to eruption was almost filled by lavas, which possibly helped in gas pressure build up. Deposit analyses also reveal that currently inhabited areas could be severely damaged by PDC and ballistic fallout activity during similar events, posing a severe threat to people and infrastructures.