The porosity of pyroclasts as an indicator of volcanic explosivity

The porosity of pyroclasts is a valuable indicator of the character of a volcanic eruption. Here, we analyse the porosity distribution of deposits of 17 different volcanic eruptions, covering a broad range of inferred magma properties and observed eruption styles. Histograms of porosity versus pyroclast frequency show distinctive features in terms of the mean, variance, modality, and skewness. We present characteristic porosity distributions for five different eruption-style categories: dome-forming, explosive basaltic, cryptodome forming, Subplinian/Plinian/Ultraplinian, and phreatomagmatic. We find that the mean porosity values of the deposits correlate positively with the Volcanic Explosivity Index (VEI) of Newhall and Self (1982), regardless of the degree of complexity of the individual distributions. We then propose causes of the deviations from a general trend in terms of magma viscosity, degassing efficiency, and cap-rock confinement. Finally, a generalised correlation between clast porosity and explosivity of an eruption is developed that should help, together with conventional field techniques, facilitate the interpretation of pyroclastic sequences, even in cases where no direct observation of the eruption exists.