Making pumice: a post-fragmentation process?

Vesicular pyroclasts are the most abundant product of Plinian eruptions. Most Plinian lapilli clasts have a vesicularity of 60-90% and vesicle number density of 10¹⁴-10¹⁶ m^-3, with a wide range of vesicle sizes that often display contiguous heterogeneous textures at the submillimeter scale. Neither experiments nor numerical models have hitherto been able to fully reproduce all these characteristics.

Recent analytical and experimental studies have shown that pyroclastic obsidian can form by sintering volcanic ash along the conduit margins after magma fragmentation. We surmise that this process is not confined to the conduit margins and that fragments of magma colliding immediately after fragmentation can agglomerate, sinter, and evolve texturally also inside the conduit during their ascent towards the surface in the expanding continuous gas phase. During these few tens of seconds of decompression between agglomeration and quenching, the magmatic vapor entrapped in pores between particles can expand, thus modifying the clast vesicularity, micro-texture, and vesicle number and size distributions.

Our hypothesis is supported by both observations of natural samples and experiments. First, scanning electron and X-ray microscope images of rhyolitic Plinian pumices from Medicine Lake Volcano (California, USA) show that some individual lapilli are made of discrete domains of relatively undeformed vesicular material with different bubble number densities and size distributions, each bounded by thin shear zones/bands. Second, our results from cold seal pressure vessel experiments in which <43 um dry rhyolitic particles are rapidly hydrated and decompressed from 20-60 MPa down to 0.1 MPa at 0.01-1 MPa/s and 850 °C exhibit textures with vesicularity, vesicle number density and size distribution quantitively similar to the pumice clasts from Medicine Lake and other natural pumices of comparable composition.

We thus suggest that the texture of some porous pyroclasts is the result of post-fragmentation agglomeration, sintering, and expansion of numerous magma fragments rather than inherited from the brittle breakage of a porous magma that did not evolve much texturally after its fragmentation.