Buoyancy-controlled eruption of magmas at Mt Etna

Abstract

Buoyancy controls the ability of magma to rise, its ascent rate and the style of the eruptions. Geophysical, geological and petrological data have been integrated to evaluate the buoyancy of magmas at Mt Etna. The density difference between host rocks and magmas is mainly related to the amount of H2O dissolved in the magma and to the bubble-liquid separation processes. In the depth interval 22 km b.s.l. highly hydrated (H2O) basaltic magmas or mixtures of bubbles + liquid have positive buoyancy and rise rapidly. Conversely, bubbledepleted liquids, with an intermediate H2O content (1.5), having neutral buoyancy, will spread out and form magmatic reservoirs at different depths until cooling crystallization further modify composition and density. These different processes account for the magma compositions, location of magmatic reservoirs as determined by geophysical methods, and the complex eruptive cycles (slow effusions, fire fountains and Plinian eruptions) that have been observed in the history of the volcano.