Experimental observations of the effect of crystals and pre-existing bubbles on the dynamics and fragmentation of vesiculating flows

The effect of crystals and pre-existing gas phases on the dynamic degassing and fragmentation of magmas in a volcanic conduit have been simulated by analogue experiments. A series of decompression experiments of viscous solutions of gum rosin and acetone with and without the incorporation of particles (silicon carbide, glass beads, organic seeds) or pre-existing air bubbles have been performed in a transparent shock-tube. The addition of internal bubbles or solid particles in the starting solution shifts the fragmentation conditions towards lower acetone (volatile) contents and lower decompression ratios. The effect of solid particles depends on their number density and spatial distribution in the starting solution. A small number of sinking particles has little impact on the flow dynamics. However, a floating layer of particles provides a high concentration of surface bubbles that leads to an early fragmentation pulse. The observations also imply that the presence of crystals, if able to generate a high number density of bubbles, will lead to an earlier and deeper fragmentation of the magma in the volcanic conduit.