| Popis: |
Stromboli (southern Italy) is a 924 m-high active composite volcano made up of thick successions of scoriaceous products and lava flows subdivided by erosional discontinuities related to five calderas and seven large-scale lateral collapses (Francalanci et al. 2013). Volcanic hazards generated by the present-days intermittent and moderately explosive strombolian activity mostly consist of fallout of scoriae and ballistic blocks affecting the area around the summit craters, together with lava flows flowing down the Sciara del Fuoco collapse. These processes produce a limited hazard for the settled areas, which are only occasionally affected by the fallout of bombs and blocks during higher-energy explosions or paroxysms (at a decade-scale). Moreover, events of gravitational instability at various scales (both in time and volume) have occurred throughout the Stromboli history mostly along the north-western flank of Stromboli in the frame of the Sciara del Fuoco collapse, although minor SE-dipping collapse events are recognized to truncate the Pizzo cone on the summit of Stromboli in late Medieval times. The steep Sciara del Fuoco scar is the place where the historic to Present-day activities have mostly taken place, and is the result of recurrent NW-dipping collapses, at times associated with the development of landslides or debris avalanches giving rise to tsunamis events. A low-probability but remarkable hazard scenario is that a flank collapse triggers a large-scale hydromagmatic explosion (as a consequence of a sudden decompression of the magmatic-hydrothermal system) resulting in the generation of pyroclastic density currents (PDC) that flow down the slopes of the volcano (Bertagnini and Landi 1996; Giordano et al. 2008). Based on new geological and geochemical data, we provide the evidence for three distinct hydromagmatic eruptions occurred during the Holocene evolution of Stromboli at ~12.5 ka (Semaforo Nuovo), ~7 ka (Secche di Lazzaro) and ~4 ka (Semaforo Labronzo). As discussed in the literature, they are more likely related to successive collapse events of the Stromboli volcano. These eruptions have produced dilute PDCs able to reach the lower slopes of the volcano and deposit meters-thick pyroclastic accumulations in the surroundings of the settled areas of San Vincenzo-Stromboli (Semaforo Nuovo), Ginostra (Secche di Lazzaro) and Piscità (Semaforo Labronzo). According to these data, the occurrence of collapse-driven hydromagmatic eruptions and PDCs is an eruptive scenario more frequent than previously thought, thus introducing further (previously undervalued) source of volcanic hazard on Stromboli. |
