Volcanic Lightning Evidence in Ashfall Deposits: Intra-Cloud or Cloud-to-Ground Discharges?
Abstract
During explosive volcanic eruptions, both intra-cloud (IC) and cloud-to-ground (CG) lightning is possible, which may induce particular textures in exposed ash. Lightning-induced volcanic spherules (LIVS), spherule aggregates, and pumiceous particles (differentiated from primary pumice fragments by smooth and glassy surfaces) have been observed in ashfall samples from Alaskan volcanoes (e.g., Redoubt and Pavlof) and also in products from high-current (100 kA) impulse experiments. In addition to these previously documented textures, lightning simulation experiments also create particles similar to those produced during other high-temperature volcanic and anthropogenic processes, as described below. Here, we present two newly identified textures in very fine (<32 μm) volcanic ash subjected to current impulses of 100 kA, 25 kA, and 7 kA. One is comparable to Pele's Hair, produced during fountaining of mafic magmas, and the other is similar to trinitite, produced during nuclear bomb detonation. In the former case, aggregates of ash are melted and elongated into thin threads of glass 1-10 μm in diameter. In the latter case, aggregates of ash are melted into a 'plate' >100 μm in diameter with a smooth, glassy exterior and a vesiculated interior. These particular textures result from not only the high temperatures within the lightning plasma channel (600-30,000 oC), but also the shock wave generated by rapid expansion of the heated ambient air. Calculations reveal that the shock wave radius is twice the size of the discharge channel radius, and the pressure behind the shock front reaches up to ~0.5 MPa for the lowest impulse currents and may exceed 1 MPa for the highest. The radially expanding shock wave simultaneously melts, flattens, and elongates aggregates of exposed ash particles. Although LIVS, spherule aggregates, and pumiceous particles can result from both IC and CG discharges, formation of hair-like and plate-like textures will more likely result from CG discharges striking an ashfall deposit in a stationary position on a solid surface. These CG lightning-induced textures have not yet been observed in ashfall deposits from explosive volcanic eruptions, as their small size and fragile structure may limit preservation, or they may be misinterpreted as products of primary magma fragmentation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V13E0220G
- Keywords:
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- 8428 Explosive volcanism;
- VOLCANOLOGY;
- 8434 Magma migration and fragmentation;
- VOLCANOLOGY;
- 8445 Experimental volcanism;
- VOLCANOLOGY;
- 8486 Field relationships;
- VOLCANOLOGY