Water/magma interaction: some theory and experiments on peperite formation

Abstract

Experiments, using molten thermite as a magma analog, produce peperite when the melt interacts with wet sand. These experiments also show explosive behavior, developing Strombolian- and Surtseyan-like bursts. The results demonstrate that the application of fuel–coolant interaction (FCI) theory is appropriate for interpretation of peperites. The theory described includes discussion of the importance of mass interaction ratios of wet sediment and magma ( R s), which determine thermal equilibrium temperature limits and contact interface dynamics. The dynamics of the interface between magma and wet sediments involves heat transfer over a wide range of rates from passive quenching to explosive fragmentation. A vapor film layer develops at the interface and acts both as an insulating barrier, promoting passive quenching, as well as a potential energy reservoir that can cause magma fragmentation, mingling of the magma with wet sediments, and explosive quenching when the vapor film becomes unstable. An important parameter in determining the behavior of the vapor film is the value of R s, which controls whether heat can be convectively removed from the layer as more is being added from its contact with magma. If R s>1 for fully saturated sediments, there is enough water in the sediments to make convective heat flow effective in quenching the magma, but below that value, there is the potential that the vapor film will be unstable, producing highly dynamic phenomena, including explosive fragmentation. At values of R s<0.1 there is insufficient water to allow the escalation of explosive fragmentation.