Textural dependence of shape evolution during granular flow

Fragmentation during explosive silicic volcanic eruptions produces angular, porous pyroclasts that are subsequently transported within eruption plumes or pyroclastic density currents (PDCs). Within PDCs, particle–particle and particle-substrate interactions substantially modify their size and shape through abrasion and secondary fragmentation, causing, in particular, significant pumice rounding associated with ash generation. The efficiency of these processes is directly linked to the textural properties of the pumice clasts (i.e., pore and crystal characteristics), but this aspect remains poorly constrained to date.

We performed controlled tumbling experiments using pumice clasts from the 13 ka Laacher See (LS, Eifel, Germany) and the 79 AD Vesuvius (VS, Italy) eruptions. Both sample sets are phonolitic in composition but texturally distinct. At different times during tumbling (5, 10, 15, 20, and 60 minutes), the bulk samples were sieved at 2 mm to quantify ash generation. Shape parameters (axial ratio, convexity, form factor, and solidity), and petrophysical properties (volume and porosity) were quantified on a constant subset of 100 clasts (colour impregnated) to constrain the evolution of individual particles. In addition, we analysed the texture (porosity, pore connectivity, crystal content, and pore size distribution) of the starting material and tumbled clasts.

In all experiments, clasts exhibit a continuous but decelerating rate of change in shape and surface roughness, approaching a time-invariant state. This kinetic behaviour, characterized by a fast initial change followed by a progressively slower evolution, is analogous to structural relaxation processes in glasses. Thus, the shape evolution and surface roughness were framed within a structural relaxation framework in terms of relaxation times. The results reveal systematic differences in abrasion behaviour between the two sample sets. LS pumice displays faster shape evolution and higher ash production than VS pumice, consistent with its higher porosity and pore connectivity as well as lower crystal content.

Our findings confirm the major control of pumice texture on abrasion propensity during transport. The continuous ash generation will ‘buffer’ the decrease of ash concentration during PDC transport by sedimentation and elutriation and thus contribute to maintaining PDC mobility high and CO-PDC plume formation. Framing these processes in terms of relaxation times provides a quantitative link between clast texture, shape evolution, ash generation, and the mobility of PDCs.