The Pyroclastic deposits of Mount Etna volcano,Sicily

Throughout most of its geological evolution Etna has been characterized by the eruption of lava flows of a predominantly hawaiitic composition, but within the stratigraphical record there are four major sequences of pyroclastic materials: the Acireale tephra and lahars (˜100000 B.P.); the ‘lower tephra’ and Milo lahars (both ˜26000 B.P.); the Biancavilla ignimbrites (15–15500 B.P.) and the ‘upper tephra’ (˜5000–6000 B.P.). This paper reports investigations carried out on these deposits in order to determine their stratigraphy, petrology, sedimentology, and likely origins. Whereas the Biancavilla ignimbrites were generated when a more evolved, gas-charged magma (benmoreite) was being produced by the volcano, the other suites of pyroclastic deposits were erupted from hawaiitic magmas—similar to those that have characterized the volcano during historical times. These deposits resulted from two processes: violent strombolian activity producing lapilli-rich. coarse, but well-sorted sediments, and hydrovolcanism when the mixing of water and magma in the conduit, brought about more violently explosive activity, giving rise to highly fragmented, poorly sorted, airfall tephra and lahars. Conditions favouring hydrovolcanism occurred at times in the volcano's history when palaeoenvironment and palaeogeography were conducive to the retention of large amounts of surface and subsurface water. Although climates favouring the retention of water at high levels on the volcano have occurred on many occasions in the history of the volcano, at ˜26.000 and ˜5000-6000 B.P. these occurred in conjunction with a construct of sufficient height and suitable configuration to allow storage of water and give rise to hydrovolcanic activity. The nature of the mechanisms responsible for the emplacement of these hydrovolcanic deposits is considered and it is concluded that airfall is the most probable process. Finally, the implications of this research for the assessment of hazard are reviewed.