A model for gravity changes induced by lava fountaining at Mt Etna

Lava fountains represent a common eruptive phenomenon at basaltic volcanoes, which consist of jets of fluid lava ejected into the atmosphere from active vents or fissures. They are driven by rapid formation and expansion of gas bubbles during magma ascent. The dynamics of lava fountains is thought to be controlled by the gas accumulation in the foam layer at the top of a shallow magmatic reservoir, which eventually collapses triggering the lava fountaining. Gravity measurements taken from a location close to summit of Mt. Etna during the 2011 lava fountain episodes showed a pre-fountaining decrease of the gravity signal. The interplay between gas accumulation in the foam layer and its subsequent exsolution in the conduit has been interpreted as the mechanism producing the gravity decrease and eventually leading to the foam collapse and onset of the lava fountaining activity. Gravity measurements have proved helpful in recording the earliest phases anticipating the lava fountain episodes and inferring the amount of gas involved. However, more accurate estimates of the accumulating and ascending gas volume and total magma mass require considering the possible effect of non-spherical magma chamber geometries and magma compressibility.

Under task 4.4 of the H2020 NEWTON-g project, we are accomplishing a detailed study aimed to simulate the gravity signal produced in the stage prior to a lava fountain episode, through a magma chamber - conduit model. We use a prolate ellipsoidal chamber matching the inferred shape of the shallow chamber active at Mt. Etna during the lava fountain episodes, and calculate the surface gravity changes induced by inflow of new magma into the chamber-conduit system. We use a two-phase magma with fixed amount of gas mass fraction and account for magma compressibility. We find that a realistic chamber shape and magma compressibility play a key role and must be considered to produce realistic gravity changes simulations. We combine our physical model with empirical distributions of recurrence time and eruption size of the past lava fountains at Mt. Etna to stochastically simulate realistic time series of gravity changes. The final goal of this study is to develop a prediction model for the amount of magma and duration of lava fountains at Mt. Etna.