The effect of microlite and permeability on the Plinian eruption of basaltic magma

We study the effect of microlites and permeability on the Plinian style eruption of basaltic magma at Mt. Tarawera, New Zealand (1886 CE) and Mt. Etna, Italy (122 BCE). The syneruptive nucleation and growth of microlites will increase magma viscosity and affect bubble growth, as well as bubble coalescence. Increased viscous resistance to bubble growth may result in the build-up of bubble overpressure, whereas permeability may allow pressurized magmatic volatiles to escape from the magma, thereby reducing bubble overpressure. Thus, it is important to understand the competing effect of these two parameters on the bubble and conduit dynamics during these volcanic eruptions. We measured and analyzed the permeability of 29 pyroclasts from these two basaltic Plinian eruptions. We obtained Darcian permeabilities in the range of approximately 10-12 to 10-11 m2 over a range in porosity spanning 0.47 - 0.78. In order to investigate the effect of permeability and microlites, we modeled coupled bubble growth and flow of magma in a volcanic conduit, where we accounted for the nucleation and growth of microlites, by using the Avrami equation, and for the development of permeability, using the empirical relationship between permeability and porosity for each eruption. We find that increased viscosity of the basaltic melt due to the presence of microlites overrides the effect of permeable outgassing, such that, it generates sufficient bubble overpressure required for magma fragmentation during these basaltic Plinian eruptions.