Sahand Hajimirza , Helge M. Gonnermann, James E. Gardner

Plinian eruptions are characterized by high magma discharge rates and pyroclastic material containing extraordinary large numbers of bubbles. Upon ascent to the surface magma decompresses and volatiles become supersaturated, causing bubbles to nucleate with a rate dependent on the degree of supersaturation. Thus, the conventional view is that the number of bubbles nucleating within the erupting magma depends on decompression rate, which is in turn a function of discharge rate. The interplay between decompression and bubble nucleation rates is of importance for the explosive nature of eruptions and thus provides an incentive for quantifying their dependency. Conventional approaches, however, predict unrealistically high decompression rates for the observed bubble number densities. Moreover, inferred pre-eruptive saturation pressures are often too low to overcome the surface energy barrier for homogeneous nucleation. To resolve these discrepancies, we simulated bubble nucleation in silica-rich magma under Plinian eruptive conditions. We demonstrate that bubble number densities and saturation pressures of Plinian silicic eruptions can be reconciled with heterogeneous nucleation, if facilitated by magnetite nanolites.