Geology and tectonics of the southwestern boundary of the unstable sector of Mt. Etna (Italy)

The Etna volcano is known to be unstable and gradually moving downslope toward east and south. The study of the local geology and structure of these unstable flanks and of their boundaries is important to understand the evolution of the flank instability and the stress field driving the spreading of the edifice.In this paper we present the geological evolution of the lower south-western sector of Mount Etna base on a detailed field survey at 1:10,000 scale. This area located at the boundary of the unstable south-eastern flank of the volcano is affected by the active Ragalna Fault and by distinctive eruptive fissures (Monte Calvario formation, 18-15. ka). The Monte Calvario formation is made of viscous lavas often associated to breccia and autoclastic lava facies fed by a set of eruptive fissures NE-SW oriented. Their emplacement occurred immediately before the Ellitico caldera collapse, marking a previously poorly constrained shift in the feeding system of the volcano. The orientation of these eruptive fissures and lineaments, and the geometry of the sedimentary substratum modeled from subsurface data, indicate that the area was dominated by a local stress field consistent with the lateral spreading of Mount Etna since at least from the final Ellittico phase (about 18. ka) with a nearly constant mean displacement rate. These data give new insights into the geology and structure of the flank instability at Etna.The main geologic hazard of the studied area is not related to the opening of a new eruptive fissures, but it is posed by the presence into Il Calvario quarry of fluoro-edenite, an asbestiform mineral that caused an high mortality rate for respiratory system cancer in Biancavilla town. In fact the Monte Calvario formation groups other eruptive fissures with similar volcanological and geochemical characteristics, where hydrothermal alteration occurred with the likely formation of health-threatening minerals. © 2010 Elsevier B.V.