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The electrification of volcanic plumes has been described intermittently since at least the time of Pliny the Younger and the 79 AD eruption of Vesuvius. Although sometimes disregarded in the past as secondary effects, recent work suggests that the electrical properties of volcanic plumes reveal intrinsic and otherwise inaccessible parameters of explosive eruptions. An increasing number of volcanic lightning studies across the last decade have shown that electrification is ubiquitous in volcanic plumes. Technological advances in engineering and numerical modelling, paired with close observation of recent eruptions and dedicated laboratory studies (shock-tube and current impulse experiments), show that charge generation and electrical activity are related to the physical, chemical, and dynamic processes underpinning the eruption itself. Refining our understanding of volcanic plume electrification will continue advancing the fundamental understanding of eruptive processes to improve volcano monitoring. Realizing this goal, however, requires an interdisciplinary approach at the intersection of volcanology, atmospheric science, atmospheric electricity, and engineering. Our paper summarizes the rapid and steady progress achieved in recent volcanic lightning research and provides a vision for future developments in this growing field. [ABSTRACT FROM AUTHOR]

The electrification of volcanic ash plumes and the occurrence of volcanic lightning are now known to be common phenomena during explosive volcanic eruptions. This knowledge stems from centuries of anecdotal observations, and in recent decades, from improved instrumentation and media attention. Following a summary of previous reviews, this contribution will detail the most recent findings concerning electrification mechanisms of eruption columns/plumes (triboelectrification, fracto-electrification) and how hydrometeor charging contributes to this electrification depending upon the eruption style and abundance of external H 2 O. Field measurements to determine the charge structure of volcanic ash and gas plumes reveal wide variability both spatially and temporally, indicating the influence of these different charging mechanisms. The charge structure and resulting lightning characteristics have been provided by a suite of both ground-based and satellite-based lightning detection methods and the various characteristics of each are summarized. As these detection methods have revealed, the electrical properties of ash plumes can provide insight into their physical dynamics throughout the course of an eruption. Lightning may therefore provide a means to track changing eruption conditions and the associated hazards, providing another tool for monitoring efforts. Volcanic lightning also leaves physical evidence in associated ashfall deposits. These lightning-induced textures have been documented and are summarized here, in addition to the different experiments that have reproduced such textures. Lightning simulation experiments provide information on changes to ash grain size, size distribution, chemical, and magnetic properties of ash. Lightning discharge and the lightning-induced changes to ash grains potentially impact not only the hazards induced by ashfall, but also changes in atmospheric chemistry relevant to biologic activity, the fluid dynamics of eruption columns/plumes, and ash dispersion. Additionally, shock-tube experiments provide insight on the microphysical dynamics and environmental variables that influence electrification of dusty gas mixtures. Finally, this review summarizes the challenges to volcanic lightning research and the future efforts that can aid in addressing the unanswered questions regarding this phenomenon. • Mechanisms of volcanic electrification and volcanic lightning are reviewed. • Recent advances provided by field and laboratory techniques are summarized. • Future challenges and directions for volcanic lightning research are highlighted. [ABSTRACT FROM AUTHOR]

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% SiO 2) to intermediate (53–58 wt% SiO 2) to acidic (72–76 wt% SiO 2), 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. • Lightning-induced textures in volcanic ash include plate-like and hair-like particles. • Current impulse experiments permit calculation of shock velocities and pressures. • The shock front expands faster than the speed of sound, elongating melted ash aggregates. [ABSTRACT FROM AUTHOR]