Influence of the fragmentation process on the dynamics of Vulcanian eruptions: An experimental approach

The dynamics of magma fragmentation is a controlling factor in the behavior of explosive volcanic eruptions. If porous magma is sufficiently decompressed, a fragmentation front develops and travels through the magma at a certain speed (fragmentation speed) while the resulting particles are being ejected. To investigate the influence of the fragmentation process on eruption dynamics, we have performed fragmentation experiments in a shock-tube apparatus using natural volcanic samples with diverse porosities and different applied pressures (4–20 MPa). For each experiment, we simultaneously measured the fragmentation speed and ejection velocities. The results are consistent with a theoretical model based on a 1-D shock-tube theory considering the conservation laws across the fragmentation front. Our results show that a certain pressure threshold has to be exceeded for fragmentation and ejection of the particles to take place and that the fragmentation speed determines the initial conditions of the expansion of the gas–particle mixture. The fragmentation process has a controlling influence on the velocity, density and mass discharge rate per unit area of the gas–particle mixture, all factors which can affect eruption dynamics significantly. The model presented herein may help describe the dynamics of Vulcanian eruptions and improve hazard assessment.