Eruption dynamics of “popping rock” from vesicle morphologies

The textures, elongation parameters and size distribution of vesicles frozen in a section of “popping rock” glass are presented. These show that the vesicles in this particular piece of “popping rock” were deformed during eruption. The degree of vesicle elongation is consistent with a velocity gradient across the section of between 0.8 and 4 s−1. This would suggest that magma rise speeds for “popping rock” were in excess of 1 m s−1. There are examples of vesicles fragmenting and coalescing preserved in the glass. Vesicle fragmentation occurs at comparatively low Capillary number (0.3). Vesicle overpressuring due to rapid decompression is thought to result in low capillary number fragmentation. The vesicle size distribution (the number of vesicles in a given size interval per unit volume of magma) of this section suggests that continuous vesicle nucleation and growth took place in the magma, consistent with two other “popping rock” sections studied previously. Minor vesicle coalescence has occurred in two of the sections. The results from all the sections suggest that these three sections experienced a similar growth rate (G) and growth interval (τ) of Gτ≈0.1 mm and had similar initial bubble densities (≈200 mm−4).