THE STRUCTURE OF BASALTIC SCORIA AND RETICULITE AND INFERENCES FOR VESICULATION, FOAM FORMATION, AND FRAGMENTATION IN LAVA FOUNTAINS

In this investigation pyroclast structures are used to constrain degas sing in basaltic lava fountains. Vesicle size, shape, number density, interconnectedness and packing character are quantified and related to (1) the kinetics of bubble nucleation and growth, (2) the structural evolution of magmatic foams and (3) the influence of vesiculation rate on magma fragmentation. Measurements made on a diverse suite of pyroc lasts from Kilauea volcano indicate that basaltic foams evolve through an initially disordered, closed-celled, spherical state to a well-ord ered, open-celled, polyhedral state as the vesicularity rises from sim ilar to 75 to 98%. The structural changes occur rapidly (<10 s) in the conduit and fountain in response to an intense vesiculation burst. Ve sicle size distribution systematics indicate bubble nucleation rates ( similar to 2x10(4) events cm(-3) s(-1)) that are approximately three o rders of magnitude greater than those found for effusive eruptive acti vity, Bubble growth rates (similar to 9x10(-4) cm/s) exceed effusive e stimates by a factor of 3. The observed ''runaway'' rate of bubble pro duction indicates strong supersaturations at the onset of nucleation. We speculate that the rise speed of the magma, as it reaches the level where significant volatile exsolution begins, determines the intensit y of the vesiculation burst, and hence the vigor of the eruption. Rapi d expansion and acceleration of the magma under these conditions may p rovide the impetus for fragmentation.