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

The deposits from explosive volcanic eruptions (those eruptions that release mechanical energy over a short time span1) are characterized by an abundance of volcanic ash2,3. This ash is produced by fragmentation of the magma driving the eruption and by fragmenting and ejecting parts of the pre-existing crust (host rocks). Interactions between rising magma and the hydrosphere (oceans, lakes, and ground water) play an important role in explosive volcanism4,5, because of the unique thermodynamic properties of water that allow it to very effectively convert thermal into mechanical energy. Although the relative proportion of magma to host-rock fragments is well preserved in the pyroclastic rocks deposited by such eruptions, it has remained difficult to quantitatively assess the interaction of magma with liquid water from the analysis of pyroclastic deposits2,3,4,5. Here we report the results of a study of natural pyroclastic sequences combined with scaled laboratory experiments. We find that surface features of ash grains can be used to 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.