Size distributions of fractures, dykes, and eruptions on Etna, Italy: Implications for magma-chamber volume and eruption potential.

The main magma source for eruptions on Etna (Italy) is poorly constrained. Here we use data on the size distributions of volcanic fissures/feeder-dykes, crater cones, dyke thicknesses, and lava flows to estimate the average magma volume flowing out of the chamber during eruptions and the volume of the chamber. For the past four centuries the average magma volume leaving the chamber during each eruption is estimated at 0.064 km. From the theory of poroelasticity the estimated chamber volume is then between 69 and 206 km. For comparison, a sill-like, circular chamber (an oblate ellipsoid) 1 km thick and 14 km in diameter would have a volume of about 154 km. The elastic strain energy stored in the host rock during inflation of such a chamber is about 2.8 × 10 J. Estimating the surface energy of a typical dyke-fracture as about 10 J m, the results suggest that the stored strain energy is sufficient to generate a dyke-fracture with an area of about 28 km. The average strike-dimension of volcanic fissures/feeder-dykes in Etna is about 2.7 km. It follows that the estimated strain energy is sufficient to generate a feeder-dyke with a strike-dimension of 2-3 km and with a dip-dimension as great as 10 km, agreeing with the maximum estimated depth of the magma chamber.