Influence of shock propagation on lightning evidence in volcanic ashfall deposits

Abstract

Lightning-induced textures in volcanic ashfall deposits can take several forms, including lightning-induced volcanic spherules (LIVS), spherule aggregates, and pumiceous particles. Here, two newly identified textures are described in the products of current impulse experiments conducted on ash samples spanning a range of bulk compositions from basic (49 wt% SiO2) to intermediate (53–58 wt% SiO2) to acidic (72–76 wt% SiO2), at peak currents of 7 kA, 25 kA, and 100 kA. Due to the location of subjected samples on a solid, flat surface in the experimental apparatus, both hair-like particles (1–10 μm in diameter) and plate-like particles (>100 μm in diameter) are produced. These textures result from not only the high temperatures generated by the discharge (>1500°C), but also by the expansion of a shock front with a radius twice the size of the discharge channel. The pressure behind this shock front ranges from 0.41 MPa for the 7 kA impulse currents to 0.77 MPa for the 100 kA impulse currents and the velocity of the shock ranges from 617 m/s to 846 m/s. The combined high temperatures and shock wave expansion simultaneously melts the volcanic ash particles, fuses them together, and stretches them out, which alters the very small (<32 μm) angular grains into larger hair-like and plate-like shapes. Although LIVS, spherule aggregates, and pumiceous particles have been documented in ashfall samples from explosive volcanic eruptions, these newly identified hair-like and plate-like textures have not. The existence of hair-like and plate-like particles in volcanic ashfall samples expands the variability of lightning evidence and may indicate the occurrence of cloud-to-ground lightning discharges during explosive eruptions.