Inferring Volcanic Degassing Processes From Vesicle Size Distributions

Both power law and exponential vesicle size distributions (VSDs) have been observed in many different types of volcanic rocks. We present results of laboratory analogue experiments, of the type pioneered by Brad Sturtevant, which reproduce these observations, and use experimental results and computer simulations to show that the distributions can be interpreted as the product of continuous bubble nucleation resulting from non-equilibrium degassing. This ongoing nucleation causes the bubbles to evolve through an exponential size distribution into a power law size distribution as nucleation and growth progress. The process of continuous nucleation is a mechanism whereby the volcanic system maintains near-equilibrium in the case of rapid depressurisation and slow volatile diffusion.