Abstract
Lightning strikes are known to morphologically alter and chemically reduce geologic formations and deposits, forming fulgurites. A similar process occurs as the result of volcanic lightning discharge, when airborne volcanic ash is transformed into lightning-induced volcanic spherules (LIVS). Here, we adapt the calculations used in previous studies of lightning-induced damage to infrastructure materials to determine the effects on pseudo-ash samples of simplified composition. Using laboratory high-current impulse experiments, this research shows that within the lightning discharge channel there is an ideal melting zone that represents roughly 10% or less of the total channel radius at which temperatures are sufficient to melt the ash, regardless of peak current. The melted ash is simultaneously expelled from the channel by the heated, expanding air, permitting particles to cool during atmospheric transport before coming to rest in ash fall deposits. The limited size of this ideal melting zone explains the low number of LIVS typically observed in volcanic ash despite the frequent occurrence of lightning during explosive eruptions.
Highlights
Volcanic ash produced during explosive eruptions with documented lightning contain rounded, glassy particles due to melting and re-solidification of the material into lightning-induced volcanic spherules (LIVS)[1]
This would suggest that millions of ash particles should be morphologically transformed by the occurrence of lightning discharge, but numerical models indicate that both the timescale of lightning discharge and the size of exposed ash particles determine the likelihood of forming LIVS36
The study presented here addresses the location within the discharge channel where volcanic ash will be affected and the resulting proportion of volcanic ash within an explosive eruption column that will be morphologically altered when subjected to lightning discharge, revealing why textural evidence of volcanic lightning is scarce in ash fall deposits
Summary
Volcanic ash produced during explosive eruptions with documented lightning contain rounded, glassy particles due to melting and re-solidification of the material into lightning-induced volcanic spherules (LIVS)[1]. Some previous analyses of fulgurites found throughout the world have revealed that the geologic materials are chemically reduced as the result of lightning discharge[16,20,21] This potential for morphological and chemical change implies that the ordered structure and composition of volcanic ash exposed to lightning discharge will be fundamentally altered. Volcanic lightning occurs in regions of the eruptive column and plume that have widely variable ash particle concentrations, ranging from 104 to 108 particles per m3 for the grain sizes (1–10 μm)[34,35] utilised in the experiments presented here This would suggest that millions of ash particles should be morphologically transformed by the occurrence of lightning discharge, but numerical models indicate that both the timescale of lightning discharge and the size of exposed ash particles determine the likelihood of forming LIVS36. SiO2 is the primary component of silicate magmas and Fe2O3 was used because there may be an observable colour change between the more oxidised form, which is red, and a reduced form (FeO or Fe), which can be black in colour, potentially enabling identification of chemical reduction following lightning discharge
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