The use of grain-size dependent magnetic susceptibility and gamma-ray measurements for the detailed reconstruction of volcanostratigraphy: The case of La Fossa di Vulcano, S. Italy

Abstract

The tuff cone La Fossa is the active volcanic edifice of Vulcano (Aeolian Archipelago, S. Italy). La Fossa was the site of several eruptions, many of them of explosive nature, in the last 6000 years. The last eruptive activity, which occurred in 1888–1890, is the archetype of Vulcanian eruptions. The stratigraphic sequence of La Fossa is largely composed of pyroclastic deposits that vary widely in thickness and distribution over very short lateral distances. This variability causes some problems in the detailed reconstruction of the volcanostratigraphy, especially in places where marker beds are lacking. In some instances, classical sedimentological and geochemical methods are not sufficient to unequivocally determine the stratigraphic position of layers cropping out in distal zones where the sequence of deposits is not completely preserved. Because of the volcanic hazard of Vulcano, a full and detailed knowledge of the stratigraphy is important for the assessment of a likely scenario of future eruptions, and the complete dispersal pattern of deposits related to each of the most recent eruptions is needed. In this paper, gamma-ray measurements and measurements of the grain-size dependent magnetic susceptibility are used to unravel some uncertainties of the volcanostratigraphy of the La Fossa deposits. First, measurements of gamma-ray and grain-size dependent magnetic susceptibility were performed in outcrops of known stratigraphy to test the ability of these methods to discriminate among deposits of different eruptive sequences. Then both methods were applied to outcrops, where the use of the classical sedimentological techniques alone were not sufficient to unequivocally establish the stratigraphic position of the various layers. Results of this study show that the combined use of both classical and simple geophysical methods suggest a re-interpretation of the dispersal of some deposits of the La Fossa cone. In particular, the dispersal area of deposits that were previously thought to occur only within the Caldera of La Fossa is discussed. As a consequence of these analyses the eruption and energy models must be reviewed in some cases. The geophysical methods applied on Vulcano will also be useful in other areas of volcanic activity, solving various stratigraphical problems, and particularly for hazard assessment.