Crater-rim collapse at Stromboli volcano: understanding the mechanisms leading from the failure of hot rocks to the development of glowing avalanches

The failures of volcanic crater-rims frequently lead to the development of avalanches of glowing rocks, a hybrid phenomenon between rock avalanches and pyroclastic density currents, reaching considerable distances from the eruptive centres and therefore be a serious threat for inhabited areas. The collapse conditions have been here taken in to account by means of slope stability analysis using a Limit Equilibrium Method analysis on the crater-rim of Stromboli volcano (Italy). A Stromboli, crater-rim collapses occurred frequently (at least seven events in the last two decades) and were always associated with high-level of magma within the conduits, testified by the increased eruptive activity and ground-deformation. The more frequent/intense eruptive activity produced a greater accumulation of volcaniclastic material, whereas the high level of magma increased magmastatic thrust on the deposits. Volcaniclastic material was modelled by combining the nonlinear failure envelopes as the Generalized Hoek and Brown criterion, with the addition of the failure's envelope of the rockfill-like material described by the Barton-Kjaernsli criterion, taking into consideration the presence of discontinuities within proximal, partially welded, volcaniclastic masses. In addition to the lithological and morphological characteristics of the crater terrace rim and the magmatic thrust, the effects of the explosions in terms of seismic ground acceleration and disturbance factor (D) of the volcaniclastic material were also considered here. While the ground acceleration compatible with the explosive activity of Stromboli has little influence on the stability of the crater terrace rims, the increase in D increases the proneness for failure.