In-conduit capture of sub-micron volcanic ash particles via turbophoresis and sintering

Ash emission in explosive silicic eruptions can have widespread impacts for human health, agriculture, infrastructure, and aviation. Estimates of the total grainsize distribution (TGSD) generated during explosive magma fragmentation underpins eruption models and ash dispersal forecasts. Conventionally, the TGSD constrained via erupted deposits is assumed to match the TGSD produced at explosive fragmentation. Here we present observations from within the vent of a recent rhyolitic eruption (Cordon Caulle, Chile, 2011-2012), demonstrating that fine (<63 mu m diameter) and ultra-fine (<2.5 mu m diameter) ash particles are captured and sintered to fracture surfaces, and thus sequestered in the shallow subsurface, rather than emitted. We establish a conceptual model-uniquely contextualised through a combination of syn-eruptive observations and detailed post-eruption field investigation-in which turbophoresis (particle migration towards zones of lower turbulence) and rapid sintering create an inverse relationship between particle size and the probability of its subsurface capture. Such size-dependent capture efficiency preferentially removes submicron-diameter ash from the erupted componentry, decoupling the erupted size distribution from magmatic source conditions and potentially playing an important role in modulating eruption dynamics. Here the authors document evidence of ultrafine ash captured within ash-venting nozzles at Cordon Caulle volcano (Chile). This decouples eruptive processes from the emitted products, as explained by a new model of in-vent ash migration and sticking.

Automated summary

Emissions of ash in explosive silicic eruptions can have significant impacts on human health, agriculture, infrastructure, and aviation. This study presents evidence from the Cordon Caulle volcano in Chile, showing that fine and ultra-fine ash particles are captured and stored in the shallow subsurface instead of being emitted. The authors propose a new model of in-vent ash migration and sticking, which decouples eruptive processes from the products emitted.