Evolution of the Fossa Cone, Vulcano

In decreasing order of abundance, the principal emplacement mechanisms for deposits of the Fossa edifice of Vulcano include: dry-surge, wet-surge, pyroclastic-fall, lahar, lava-flow, pyroclastic-flow and epiclastic processes. Most samples from pyroclastic beds on Fossa contain two grain-size populations. The coarser mode at 0.5 - 1.5 φ resulted from modified ballistic transport whereas the finer mode at 2.5 - 3.5 φ is related to surge transport. Reconstruction of the entire stratigraphy of the cone permits the beds to be grouped into five principal eruptive cycles, all of which postdate the lower Pilato ash of Lipari (dated at 11,000 to 8,500 y.b.p): Pte. Nere cycle, Palizzi cycle, Commenda, cycle, Pietre Cotte cycle, and the modern cycle. Additional cycles may be present, but exposures on the cone are insufficient for their adequate definition. All cycles follow a similar stochastic pattern starting with surge eruptions and ending with effusion of lava from the crater rim. The frequency of pyroclastic-fall events increases with time throughout each cycle. This progressive decrease in the efficiency of water/melt interaction through the duration of each cycle may be due to a gradual rise of each new magma column coupled with an increase in magmatic volatiles. Tectonism and magma mixing both play a role in triggering eruptions on Fossa. The present crater lies at the intersection of two prominent tectonic trends. A NNW fracture system parallels the main eruptive centers and an ENE set aligns with the migration of minor vents. The silicic lavas of Fossa contain a mixture of two components. Xenocrysts of plagioclase, augite, and olivine, surrounded by a dark glass are intimately dispersed into an evolved rhyolitic matrix.