The effect of magma flow on nucleation of gas bubbles in a volcanic conduit

We solve the dynamics of magma ascent and the kinetics of bubble nucleation and growth simultaneously, which allow us to predict bubble sizes and number densities under ascent conditions. As magma rises toward the surface, the pressure decreases and eventually becomes less than the solubility pressure. When the degree of supersaturation becomes great enough, bubbles nucleate. Nucleation will stop as the concentration of volatiles in the melt decreases due to growth of existing bubbles and hence the degree of supersaturation decreases. We show that a second nucleation event may occur just below the fragmentation level. Near that level, the degree of supersaturatiori continuously increases as the magma is rapidly decompressed. As a result, nucleation will not stop until fragmentation occurs. This second nucleation event should be taken into account when interpreting bubble size distribution measurements made on natural pumices. The bubbles of the second nucleation event have high internal gas pressures up to 2 MPa greater than the liquid pressure, suggesting that the second nucleation event may enhance fragmentation of magma. We apply the model to the calculation protocol defined at the Volcanic eruption mechanism modeling workshop, Durham, 2002. We found that as a result of disequilibrium degassing fragmentation occurs higher in the conduit than under equilibrium degassing. (c) 2005 Elsevier B.V. All rights reserved.