The structure of basaltic scoria and reticulite and inferences for vesiculation, foam formation, and fragmentation in lava fountains

Abstract

In this investigation pyroclast structures are used to constrain degassing 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 pyroclasts from Kilauea volcano indicate that basaltic foams evolve through an initially disordered, closed-celled, spherical state to a well-ordered, open-celled, polyhedral state as the vesicularity rises from ~ 75 to 98%. The structural changes occur rapidly (< 10 s) in the conduit and fountain in response to an intense vesiculation burst. Vesicle size distribution systematics indicate bubble nucleation rates (~ 2 × 10 4 events cm −3s −1) that are approximately three orders of magnitude greater than those found for effusive eruptive activity. Bubble growth rates (~ 9 × 10 −4 cm/s) exceed effusive estimates by a factor of 3. The observed “runaway” rate of bubble production 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 intensity of the vesiculation burst, and hence the vigor of the eruption. Rapid expansion and acceleration of the magma under these conditions may provide the impetus for fragmentation.