Identifying magma-water interaction from the surface features of ash particles

The deposits from explosive volcanic eruptions (those eruptions that releas e mechanical energy over a short time span(1)) are characterized by an abun dance of volcanic ash(2,3). This ash is produced by fragmentation of the ma gma driving the eruption and by fragmenting and ejecting parts of the pre-e xisting crust (host rocks). Interactions between rising magma and the hydro sphere (oceans, lakes, and ground water) play an important role in explosiv e volcanism(4,5), because of the unique thermodynamic properties of water t hat allow it to very effectively convert thermal into mechanical energy, Al though the relative proportion of magma to host-rock fragments is well pres erved in the pyroclastic rocks deposited by such eruptions, it has remained difficult to quantitatively assess the interaction of magma with liquid wa ter from the analysis of pyroclastic deposits(2-5). Here we report the resu lts of a study of natural pyroclastic sequences combined with scaled labora tory experiments. We find that surface features of ash grains can be used t o identify the dynamic contact of magma with liquid water, The abundance of such ash grains can then be related to the water/magma mass ratios during their interaction.