Lightning Effects on the Grain Size Distribution of Volcanic Ash

Abstract

Natural volcanic ashfall samples were examined, and high‐current (~100 kA) electrical impulse experiments were conducted to reveal the changes in grain size that can occur during lightning discharge. Experiments on pseudo ash samples manufactured from volcanic deposits of both rhyolitic and basaltic composition show that aggregates of very fine grained ash particles (<32 μm) melt and degas to form vesiculated pumice fragments >100 μm in size. In some cases, bubbles <5 μm in diameter expand and detach from the outer surface of the pumice to form hollow spheres of glass, one type of lightning‐induced volcanic spherule, while other bubbles fragment. Volcanic ashfall from the 2009 Redoubt eruption and the 2016 Pavlof eruption contains both pumiceous grains and individual spherules. Results of this study reveal that volcanic lightning will alter the grain size distribution of ash through melting, vesiculation, and fragmentation of individual particles or ash aggregates.

Investigation of geomechanical properties of tephra relevant to roof loading for application in vulnerability analyses

“…Scollo et al 2017) and atmospheric conditions (e.g. Genareau et al 2019;Poulidis et al 2021). Our results reflect this complexity as eruptions of similar compositions show a wide range of near-source GSDs, while there is a large overlap between samples with different compositions and from eruptions of different sizes.…”

Evaluating the state-of-the-art in remote volcanic eruption characterization Part I: Raikoke volcano, Kuril Islands

“…Previous volcanic lightning studies at Eyjafjallajokull (Aplin et al, 2014;Arason et al, 2011;Woodhouse and Behnke, 2014), Redoubt (Behnke et al, 2013;Genareau et al, 2019;Hoblitt, 1994), Sakurajima (Aizawa et al, 2010;Cimarelli et al, 2016;Miura et al, 2002;Smith et al, 2018a), Bogoslof (Haney et al, 2018;Smith et al, 2018b;Van Eaton et al, 2020), Kelud (Hargie et al, 2019), andCalbuco (Van Eaton et al, 2016) observe ash coincident with electrical activity and lightning generation. These observations suggest volcanic lightning may be a valuable, early indicator of an explosive, ashbearing eruption, particularly for remote volcanoes lacking dedicated monitoring instruments or in regions where meteorological lightning is rare, such as high latitudes (McNutt and Williams, 2010).…”

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“…These experiments produce textures in pseudo-ash samples like those found in ashfall deposits (Genareau et al 2020 ) and show that particles proximal to the lightning channel may be disassociated, melted, and fused together (Genareau et al 2017 ). Although the radial zone of the channel suitable for melting is only 10% of the entire channel volume (Genareau et al 2017 ), these effects will destroy or fuse together fine ash particles (Genareau et al 2019a ) and potentially modify portions of larger pyroclasts in the plume. In the experiments, melted particles tend to lack microlites and display mingled heterogeneous glass compositions with zones of element enrichment (Woods et al 2021 ).…”

“…Laboratory modification of volcanic ash has been achieved through high-current (7–100 kA) electrical impulse experiments, which recreate the temperatures (~ 20,000 ℃) (Genareau et al 2017 ) and magnetic fields generated in the plasma channel of natural lightning exceeding by orders of magnitude the strength of the Earth’s magnetic field. These experiments produce textures in pseudo-ash samples like those found in ashfall deposits (Genareau et al 2020 ) and show that particles proximal to the lightning channel may be disassociated, melted, and fused together (Genareau et al 2017 ). Although the radial zone of the channel suitable for melting is only 10% of the entire channel volume (Genareau et al 2017 ), these effects will destroy or fuse together fine ash particles (Genareau et al 2019a ) and potentially modify portions of larger pyroclasts in the plume.…”