Spatiotemporal sediment transport and deposition processes in experimental dilute pyroclastic density currents

Abstract

Dilute pyroclastic density currents (dilute PDCs) are frequent and highly dangerous flows of hot gas and particles occurring at explosive volcanoes. The study and interpretation of the sedimentary characteristics of their associated deposits is one of the most important approaches to better understand these violent phenomena and to characterise their dynamics, frequency and magnitude in the geological record of volcanoes. Current strategies are based on sediment transport principles developed for fluvial and aeolian systems. How well these analogies capture the sediment transport behaviour of dilute PDCs remains poorly understood, as the hot and hostile conditions of these fast-moving volcanic flows hamper direct measurements.Here we report the results of large-scale experiments that aim to synthesise the behaviour of dilute PDCs in order to investigate the transport and sedimentation processes inside the hot flows. These flows reproduce the spatiotemporal deposit facies variations seen in natural deposits of dilute PDCs. Through measurements of the evolving flow structure (velocity, particle concentration and flow grain size distribution), we show that the lower region of the density stratified current can be subdivided from the aggrading deposit upwards into a dynamic bedload region with particle concentrations of c. 0.5 to several vol%, a transient region with particle concentrations of c. 0.1–1.5 vol%, a dilute, fully turbulent region with formation of mesoscale clusters, and an upper dilute turbulent region absent of mesoscale clusters. We show that the particle feeding mechanisms of the transient region is related to the occurrence of mesoscale clusters. This process has a key role in modifying the sediment transport modes inside the bedload region. These modifications cause a variation of the dynamics of the lower flow boundary, including erosion/deposition events, and variations of the deposition rate over at least three orders of magnitude. We present images and video footage of these processes that include the formation of shifting sandwaves, rolling and saltation of particles inside the experimental dilute PDC, and analyse how these relate to the development of massive, stratified and laminated deposit structures. Through mapping the isochrones of deposition across the flow length and measuring the propagating flow above, we capture a spatiotemporal view of the aggrading deposit, which can guide the interpretation of natural deposits.