Abstract
Glass spherules have been documented in many geologic deposits and are formed during high-temperature processes that include cloud-to-ground lightning strikes, volcanic eruptions of low-viscosity magmas, and meteorite impacts. This study reviews the known glass spherule– forming processes and proposes, for the first time, a mechanism induced through the heat generated by volcanic lightning in eruptive columns and plumes (laterally spreading clouds) during explosive eruptions. Ash-fall samples were collected from two eruptions where volcanic lightning was extensively documented: the A.D. 2009 eruption of Mount Redoubt, Alaska (USA), and the 2010 eruption of Eyjafjallajokull, Iceland. These samples reveal individual glass spherules ~50 mm in average diameter that compose <5% of the examined portion of the deposit. Textures include smooth, hollow, or cracked spherules, as well as aggregates, which suggest melting of ash particles as a result of proximity to the electrical discharge channel and subsequent re-solidification of the particles into spherical morphologies. The natural ash-fall samples are compared with pseudo-ash samples collected from high-voltage insulator experiments in order to test our hypothesis that volcanic ash particles can be transformed into glass spherules through the heat generated by electrical discharge. We refer to this new morphological classification of ash grains as lightning-induced volcanic spherules and hypothesize that this texture not only provides direct physical evidence of lightning occurrence during explosive eruptions, but will also increase settling velocities and reduce aggregation of these particles, affecting ash transport dynamics.
Highlights
The effects of volcanic ash deposition on electric power systems have been documented following several notable explosive eruptions, and, of the known impacts, ash-induced insulator flashover is the most common
In the study presented here, we compare textures within the products of high voltage (HV) flashover experiments to those observed in natural ash-fall samples in order to determine whether volcanic lightning can act as a mechanism for the generation of glass spherules in geologic deposits
We provide a review of other processes known to create these particular textures under the hypothesis that, in many cases, spherules formed as a result of lightning discharge may be overlooked, despite the evidence they provide of lightning occurrence in eruptive columns and plumes and the effect they have on ash transport and deposition by increasing settling rates while inhibiting particle aggregation
Summary
The effects of volcanic ash deposition on electric power systems have been documented following several notable explosive eruptions (see Wardman et al, 2012a), and, of the known impacts, ash-induced insulator flashover (unintended electrical discharge) is the most common. The occurrence of this problem during ash falls created the need for analogue experiments to systematically examine the role of ash properties in inducing flashover across high voltage (HV) insulators (Wardman et al, 2012b, 2014). We provide a review of other processes known to create these particular textures under the hypothesis that, in many cases, spherules formed as a result of lightning discharge may be overlooked, despite the evidence they provide of lightning occurrence in eruptive columns and plumes and the effect they have on ash transport and deposition by increasing settling rates while inhibiting particle aggregation
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