Evolution of the etna volcano: Information from the southern wall of the Valle Del Bove Caldera

Abstract

The greater part of Etna can be regarded as a complex strato-shield volcano constructed from the overlapping products of several centres of trachy-basaltic activity. The Valle del Bove is a horse-shoe-shaped caldera, 8 km long and 5 km wide, cut into the eastern flanks of Etna. The caldera is one of the few areas on the volcano where historic eruptions have not obscured the products of pre-historic centres of activity and these are well exposed in the cliff walls surrounding the caldera. Examination of these older volcanics provides important information on the eruptive style and internal plumbing of the Etna volcano during pre-historic times, and suggests that both were significantly different from the present day. Much of the southern wall of the Valle del Bove represents a surviving portion of the Trifoglietto II volcano, the largest pre-historic centre of activity. A stratigraphy is constructed for the southern wall, the Trifoglietto II lavas and pyroclastics rest unconformably upon the eroded remnants of an older centre, and are themselves overlain by the products of younger centres. All the lavas exposed in the southern wall are of alkalic affinity and comprise a trachybasaltic suite ranging from hawaiite to benmoreite. Variation in the chemistry of the lavas can be explained by their differentiation at high levels in the crust from a more basic magma of alkali olivine-basalt/hawaiite composition. An anomalous trend in the TiO 2 content of the Trifoglietto II lavas may be explained by the fractionation of kaersutite (Ti-rich amphibole). A study has been made of the numerous dykes exposed in the walls of the Valle del Bove, the alignments of which parallel trends which are important on Etna at the present time. It is proposed that the initial opening of the Valle del Bove occurred sometime between 20,000 and 10,000 y. B.P., as a result of a phreatic or phreato-magmatic explosion near the base of the eastern flank of Trifoglietto II. This is visualised as triggering a slope failure and resulting in the destruction of much of the centre by a catastrophic landslide. This mechanism has much in common with the explosive eruptions which produced both the Bandai-san (Japan) caldera in 1888, and the Mount St Helens caldera in May, 1980.