BUBBLE-GROWTH IN HIGHLY VISCOUS MELTS - THEORY, EXPERIMENTS, AND AUTOEXPLOSIVITY OF DOME LAVAS

We examine the physics of growth of water bubbles in highly viscous me lts. During the initial stages, diffusive mass transfer of water into the bubble keeps the internal pressure in the bubbles close to the ini tial pressure at nucleation. Growth is controlled by melt viscosity an d supersaturation pressure and radial growth under constant pressure i s approximately exponential. At later stages, internal pressure falls, radial growth decelerates and follows the square-root of time. At thi s stage it is controlled by diffusion. The time of transition between the two stages is controlled by the decompression, melt viscosity and the Peclet number of the system. The model closely fit experimental da ta of bubble growth in viscous melts with low water content. Close fit is also obtained for new experiments at high supersaturation, high Pe clet numbers, and high, variable viscosity. Near surface, degassed, si licic melts are viscous enough, so that viscosity-controlled growth ma y last for very long times. Using the model, we demonstrate that bubbl es which nucleate shortly before fragmentation cannot grow fast enough to be important during fragmentation. We suggest that tiny bubbles ob served in melt pockets between large bubbles in pumice represent a sec ond nucleation event shortly before or after fragmentation. The presen ce of such bubbles is an indicator of the conditions at fragmentation. The water content of lavas extruded at lava domes is a key factor in their evolution. Melts of low water content (<0.2 wt%) are too viscid and bubbles nucleated in them will not grow to an appreciable size. Bu bbles may grow in melts with similar to 0.4 wt% water. The internal pr essure in such bubbles may be preserved for days and the energy stored in the bubbles may be important during the disintegration of dome roc ks and the formation of pyroclastic flows. (C) 1998 Elsevier Science B .V. All rights reserved.