The influence of loading by lavas on the siting of volcanic eruption vents on Mt Etna

Abstract

Until now it has been considered that the positioning of magma conduits, dykes and eruption sites on Mt Etna is controlled by stresses set up by regional tectonism. It is also likely that gravitational stresses have some influence on the subsurface path taken by Etnean magmas prior to eruption. An additional effect discussed here is based mainly on convincing field evidence gathered over a number of years. It is proposed that fracturing of the surface and changes in the stress pattern due to loading by recent lava flows may strongly influence the siting of eruptions. Detailed field observations are presented, showing that cracks and fumaroles have consistently appeared round the edges of recent lava flows, and that these cracks are sometimes later used by erupting lavas. Eruption statistics indicate that since 1970, 50% of new eruptive vents have appeared within 80 m of a recent flow edge, and field measurements show that downward flexure is still occurring at the edges of lava flows more than sixteen years old. Simple theoretical modelling indicates that this effect is strongest at the surface, with gravitational and regional tectonic control of eruption sites dominating at progressively deeper levels.