ANALYTICAL MODELS FOR BUBBLE-GROWTH DURING DECOMPRESSION OF HIGH-VISCOSITY MAGMAS

Analytical models for decompressional bubble growth in a viscous magma are developed to establish the influence of high magma viscosity on v esiculation and to assess the time-scales on which bubbles respond to decompression. Instantaneous decompression of individual bubbles, anal ogous to a sudden release of pressure (e.g. sector collapse), is consi dered for two end-member cases. The infinite melt model considers the growth of an isolated bubble before significant bubble interaction occ urs. The shell model considers the growth of a bubble surrounded by a thin shell and is analogous to bubble growth in a highly vesicular mag matic foam. Results from the shell model show that magmas less viscous than approximate to 10(9) Pa s can freely expand without developing s trong overpressures. The timescales for pressure re-equilibration are shortened by increased ratios of bubble radius to shell thickness and by larger decompression. Time-scales for isolated bubbles in rhyolitic melts (infinite melt model) are significantly longer, implying that s uch bubbles could experience internal pressures greater than the ambie nt pressure for at least a few hours following a sudden release of pre ssure. The shell model is developed to assess bubble growth during the linear decompression of a magma body of constant viscosity. For the r ange of decompression rates and viscosities associated with actual vol canic eruptions, bubble growth continues at approximately the equilibr ium rate, with no attendant excess of internal pressure. The results i mply that viscosity does not have any significant role in preventing t he explosive expansion of high viscosity foams. However, for viscositi es of >10(9) Pa s there is the potential for a 'viscosity quench' unde r the extreme decompression rates of an explosive eruption. It is prop osed that the typical vesicularities of pumice of 0.7-0.8 are a conseq uence of the viscosity of the degassing magmas becoming sufficiently h igh to inhibit bubble expansion over the characteristic time-scale of eruption. For fully degassed silicic lavas with viscosities in the ran ge 10(10) to 10(12) Pa s time-scales for decompression of isolated bub bles can be hours to many months.