Geophysical models of Mount Etna, Sicily : its structural evolution and implications for slope stability

Abstract

Mount Etna, Europe's largest active volcano, shows a long history of changing structure and environment, culminating in the modern volcanic edifice with its observed slope instability and region of past collapse (the Valle del Bove). The structure and evolution of the volcano and, more importantly, the Valle del Bove are poorly understood but have major effects on its present day behaviour. Several geophysical techniques have been used in order to determine new models for the sub-surface structure of the volcano, concentrating on the Eastern Flank and the Valle del Bove. The Valle del Bove is an important feature on the Eastern Flank. It measures 5 by 7 km and is 1200m deep at its maximum. Its formation and evolution are poorly understood and represent the main thrust of this work. The primary work has been gravity and aeromagnetic surveys combined with 2.5-D and 3-D modelling to develop a better understanding of the sub-surface structures of the area. This work has identified several large gravity and magnetic anomalies indicating areas of contrasting geophysical properties. A 16mGal positive gravity anomaly over the Southern Wall of the Valle del Bove is interpreted as a large (volume = 38km3) high density (2950 kg m-3) body, related to the old Trifoglietto centre, possibly representing the ancient feeder system. A second high density body is seen under the present day summit region and is interpreted as an area of shallow level magma storage within the upper flanks of the volcano. A negative gravity anomaly of 10mGal towards the coast, over the Chiancone sedimentary fan deposit, is interpreted as reworked debris flow material, derived from the Valle del Bove. The shape of the anomaly is strongly asymmetrical relative to the geographical extent of the deposit, with the centre of the anomaly sitting over its Northern extent. The resultant model of this material gives a thickness of 700m, with the material being deposited within a fault controlled basin, open to the sea. Such a shape suggests that a sizeable volume of material may be deposited off-shore. Below the Etnean volcanics lie sedimentary layers which slope in an easterly direction, providing a surface over which the Eastern Flank. is free to slide. Finite element modelling of the stress fields within the volcanic edifice and basement shows that the effects of sea-level variations and glaciations may be of a sufficient magnitude to affect the volcanic system, however, the time over which these changes occur may be more significant than the changes themselves. Similarly the Valle del Bove is shown to have had a significant effect on the stress patterns, and has resulted in a self-reinforcing process whereby the tension caused by the removal of mass encourages further collapse. Palaeomagnetic measurements show that rotational failure has not been important in the formation of the Valle del Bove, and combined with the results of the other methods, show the Valle's formation to have been via a series of small piecemeal collapses, possibly relating to oversteepening of the walls following dyke emplacement. These results are combined together to show that the Valle del Bove has been developing over much of the history of Etna, and is not a recent addition to the volcano.