The Influence of Grain Size Distribution on Laboratory-Generated Volcanic Lightning
C. Springsklee, B. Scheu, M. Manga, V. Cigala, C. Cimarelli, D. B. Dingwell
Journal of Geophysical Research: Solid Earth https://doi.org/10.1029/2022JB024390
Over the last decades, remote observation tools and models have been developed to improve the forecasting of ash-rich volcanic plumes. One challenge in these forecasts is knowing properties at the vent including the mass eruption rate and grain size distribution. Volcanic lightning is a common feature of explosive eruptions with high mass eruption rates of fine particles. The grain size distribution (GSD) is expected to play a major role in generating lightning in the gas thrust region via triboelectrification. Here, we experimentally investigate the electrical discharges of volcanic ash as a function of varying GSD. We employ two natural materials, a phonolitic pumice and a tholeiitic basalt, and one synthetic material (soda-lime glass beads). For each of the three materials, coarse and fine grain size fractions with known GSDs are mixed, and the particle mixture is subjected to rapid decompression. The experiments are observed using a high-speed camera to track particle-gas dispersion dynamics during the experiments. A Faraday cage is used to count the number and measure the magnitude of electrical discharge events. Although quite different in chemical composition, tholeiitic basalt and soda-lime glass beads show similar vent dynamics and lightning properties. The phonolitic pumice displays significantly different ejection dynamics and a significant reduction in lightning generation. We conclude that particle-gas coupling during an eruption, which in turn depends on the GSD and bulk density, plays a major role in defining the generation of lightning. The presence of fines, a broad grain size distribution, and dense particles all promote lightning.