| Popis: |
Vulcano geological history displays several changes of eruption magnitude, eruption styles and composition of magmas through time. Various volcanic edifices and two intersecting multi-stage calderas resulted from such evolution. The collection for many years of detailed geological data on this island (as well as on those nearby) have made it possible to carry out a detailed lithostratigraphy together with the identification of lithosomes and unconformity-bounded units (UBU) based on the main volcano-tectonic events and/or shifting of volcanic centres. Together with the critical use of available radiometric ages and compositional data as well as the knowledge of regional tephrochronology, this has finally provided the ability to synthesize in a map - at 1:10,000 scale - a large number of information, ranging from the duration of the volcanic activity to the hazard(s) assessment on the island. In particular, this geological map includes a thematic map on volcanic hazard derived from an even more detailed stratigraphy and geological survey performed on La Fossa Cone, where the most recent volcanic activity occurred. The historical activity of this tuff cone is characterized by several eruptions, with the latest eruption occurred during 1888–1890 AD and representing the tail of a two millennia long period of activity. As it is deduced from its stratigraphic record, most of the eruptions were characterized by pulsatory explosive events mainly driven by phreatomagmatic processes, with a minor role for magmatic fragmentation. Both the fragmentation mechanisms involved compositionally distinct magmas (from shoshonites to rhyolites), often mutually interacting (mingling, mixing). They led to the formation of hundreds of meters thick, dilute and turbulent pyroclastic density currents (PDCs), of base surges type, formed in rapid succession and moving down the volcano slope at velocities exceeding 50 m/s. The dispersion of these PDCs affected the whole Vulcano Porto and Vulcanello areas and for one eruption (Grotta dei Palizzi 2 Formation, 1.6 ka) also overrode the La Fossa Caldera rims, spreading radially over further Vulcano areas. Fallout of ballistic blocks and bread-crust bombs on intermediate-distal area was limited. With the aim to provide for a possible reactivation of La Fossa cone, a short-term eruptive scenario (tens to hundreds of years) has been drawn. Going into detail, the PDCs related to the Grotta dei Palizzi 2 activity are expected to be the most hazardous events in future eruptions and the corresponding deposits are used as the best reference for this hazard scenario. In order to quantitatively express the expected impact of these density currents, the dynamic pressure and particle volumetric concentration were calculated. It’s important to stress that these parameters and the impact energy of ballistic blocks are derived through the physical stratigraphic study of volcanic products (definition of individual depositional units and their thickness, sedimentary structures, granulometry and clast morphology of the Grotta dei Palizzi 2 pyroclastics). In order to depict even a long-term eruptive scenario with similar physical parameters, also older deposits (the Upper Brown Tuffs) produced by recurrent hydromagmatic eruptions from the multiphase La Fossa Caldera were studied in detail. |
