Making dust: The easy way of generating (a lot of) fine ash during tumbling experiments

Pyroclastic density currents (PDCs) are mixtures of volcanic particles and gas that flow down the flanks of volcanoes, guided to some degree by the morphology. They are the deadliest and most destructive volcanic phenomena, primarily due to their mobility and unpredictability. Mechanical interaction of clasts during transport produces fines through abrasion and comminution. The ash content is believed to have a positive influence on mobility, however, the in-situ production of ash in PDCs is still poorly quantified.

 

Three different types of experiments (T1, T2, T3B), each starting with 2 kg angular pumice lapilli from the Laacher See (Eifel, Germany) eruption at 12,900 a BP, were conducted to gain a better understanding of ash production rates and related lapilli clast shape changes (Figueiredo et al., 2025). Every set of experiments eventually tumbled the lapilli for 120 minutes. At five time increments (15’, 30’, 45’, 60’, 120’) the drum load was dry sieved at 2 mm. For T1 experiments, ash and lapilli were returned to the drum after each time step. In experiments T2 and T3B, the ash was stored separately, and only the lapilli fraction was returned to the drum. In experiment T3B, steel balls (220 g each) were added to simulate dense blocks.

 

The amount of ash produced analysed after each tumbling step was plotted as weight fraction of the starting load. To understand fine generation better, the ash was analysed by dry sieving at half-φ and laser diffraction analysis. For all three experiments, ash generation efficiency is negatively correlated with tumbling time, with T1 producing the smallest and T3B the highest amount of ash (as high as 47 wt.%). Noteworthy is the production of up to 18,20 wt.% of fine ash (<63 µm) and 2,83 wt.% of PM10 (≤10 μm) relative to the initial starting weight. These numbers are surprisingly high given the comparatively short and low-energy experiments. Accordingly, uninterrupted abrasion and comminution during PDC transport is a quasi-infinite source of ash supply, influencing PDC flow conditions and mobility and should be considered in future PDC runout and health impact models.

Reference: Figueiredo, C., Kueppers, U., Pereira, L. et al. Shape evolution of pumice during granular flow. Commun Earth Environ 6, 941 (2025). https://doi.org/10.1038/s43247-025-02936-4