Pyroclastic density currents (PDCs) are a prominent hazard of volcanic activity; however, fully quantitative observations are lacking and little direct evidence exists to constrain the parameters controlling ash production and runout. Here, we use rotary tumbling experiments to investigate ash generation efficiency and clast morphometrics in the dense basal flow of PDCs. We observe greater ash generation with periodic ash removal and with higher starting mass. By scaling to the bed height and clast diameter we obtain a general description for ash production in all experiments as a function of flow distance that we parameterise in dimensionless space. We also show that ash production correlates with clast shape changes and with the Inertial number for our experiments. This work introduces some of the first systematic and generalizable experimental parameterizations of ash production and clast evolution in PDCs and should advance the ability to understand flow mobility and associated hazards.

Formation of soluble sulfate and halide salts on volcanic ash particles via syn-eruptive interactions between ash surfaces and magmatic gases is a ubiquitous phenomenon in explosive eruptions. Surficial salts may be rapidly mobilized into their depositional environment undermining the quality of drinking water, harming aquatic life, and damaging soil and vegetation. Assessment of the potential for salt formation on ash and related environmental impacts have been based almost exclusively on bulk mineralogical or chemical analyses of ash; similarly, quantification of surficial salts has been made via leachate analysis only. However, it is the ash surface state and salt crystal properties that exert the predominant control on its reactivity, thus in determining their immediate environmental impact. Here, using scanning electron microscope (SEM) images, we present a novel image analysis protocol for the quantitative characterization of surficial salts, together with chemical analyses of resulting leachates. As volcanic ash proxies, we used synthetic rhyolitic glass particles (with systematic variations in FeOT and CaO content) and a crushed obsidian. Using an ash-gas reactor, we artificially surface-loaded samples with CaSO4 and NaCl crystals, the most common crystal phases found on volcanic ash surfaces. Analogous variations were found using both methods: for CaSO4 crystals, higher temperature treatments or increasing FeOT content at the same temperature led to higher concentrations of salt leachate and higher salt volumes; unexpectedly, increasing the CaO content caused only a minor increase in salt formation. In addition to bulk salt formation, morphometric results provided insight into formation processes, nucleation and growth rates, and limiting factors for salt formation. Higher temperatures increased CaSO4 crystal size and surface coverage which we infer to result from higher element mobility in the glasses driving crystal growth. Increasing FeOT content of the glasses yielded increased salt surface coverage and leachate concentrations, but decreased crystal size (i.e., the salt number density increased). This latter effect likely relates to the role of iron as an electron-donor to charge balance salt-forming cation migration to the ash surface, indicating the importance of iron in determining surface reaction site density and, consequently, environmental reactivity. The controlling roles of ash composition and temperature on salt formation observed here can improve estimations for surface salt formation, volatile scavenging, and environmental impact for eruptions producing glass-rich ash. Our characterization protocol can therefore become a useful tool for the investigation of solid–gas reactions for terrestrial and planetary processes, and it also appears to be a powerful complement to research into atmospheric processes mediated by ash surfaces, such as ash aggregation and nucleation of water or ice on ash.

<p>Pyroclastic density currents (PDCs) present perhaps the greatest proximal primary hazard of volcanic activity and produce abundant fine ash that can present a range of health, environment and infrastructure hazards. However, direct, fully quantitative observation of ash production in PDCs is lacking, and little direct evidence exists to constrain the parameters controlling ash generation in PDCs. Here, we use an experimental approach to investigate the effects of starting mass, material density and ash removal on the efficiency of ash generation and concurrent clast rounding in the dense basal flow of PDCs. We employ a rotary drum to tumble pumice and scoria lapilli clasts over multiple transport “distance” steps (from 0.2 to 6 km). We observe increased ash generation rates with the periodic removal of ash during the experiments and with increasing starting mass. By scaling to the bed height and clast diameter we obtain a general description for ash production in all experiments as a function of flow distance, bed height and average clast diameter. We confirm that changes in lapilli shape factors correlate with the ash fraction generated and that the grain size of ash produced decreases with distance. Finally, we estimate shear rate in our experiments and calculate the inertial number, which describes the ratio between clast-scale and flow-scale rearrangement during flow. We show that, under certain conditions, fractional ash production can be calculated accurately for any starting mass solely as a function of the inertial number and the flow distance. This work sheds light on some of the first systematic and generalizable experimental parameterizations of ash production and associated clast evolution in PDCs and should advance our ability to understand flow mobility and associated hazards.</p>