Monday, 16 March 2015

The formation of glassy spherules in volcanic ash clouds struck by lightning.


Glassy spherules are found in many geologic strata, and are commonly taken as evidence of meteorite strikes. They can be formed by the melting of mineral grains in the atmosphere at temperatures in excess of 3000 ˚C, and meteorites typically strike the Earth’s atmosphere at very high speeds due to their orbital momentum, resulting in friction with the atmosphere that can heat them to over 10 000 ˚C. However such spherules are also known to form under some circumstances in volcanic rocks, and as a result of lighting striking the ground.

In a paper published in the journal Geology on 27 February 2015, Kimberly Genareau of the Department of Geological Sciences at the University of Alabama, John Wardman and Thomas Wilson of the Department of Geological Sciences at the University of Canterbury, Stephen McNutt of the School of Geosciences at the University of South Florida and Pavel Izbekov of the Geophysical Institute at the University of Alaska Fairbanks, discuss the formation of glassy spherules due to lighting strikes on clouds of volcanic ash.

Genareau et al. were carrying out experiments into the effects of volcanic ash on electric power systems, and in particular ash-induced insulator flashover, where the conductive properties of the ash allow for discharge from high voltage insulators. They found that such discharges commonly resulted in the formation of glassy spherules similar to naturally occurring spherules.

Scanning electron microscope images of pseudo-ash samples collected from tracks of electrical discharge on insulators used in high-voltage flashover experiments. Larger particles (A) display fluidal morphologies indicative of partial melting, but are coated with smaller particles (B–D) with spherical morphologies similar to those found in natural ash-fall samples. Genareau et al. (2015).

Genareau et al. compared these spherules to spherules from ash falls associated with the 2009 eruption of Mount Redoubt, Alaska, and the 2010 eruption of Eyjafjallajökull, Iceland, two volcanic events where numerous lighting strikes on ash columns were documented (such strikes are common, ash columns can build up considerable charge variations leading tofrequent electrical discharges), finding spherules within these deposits similar to those created artificially. Since lightning can generate temperatures in excess of 30 000 ˚C, this is a plausible method for the formation of such spherules.

Secondary electron images of lightning-induced volcanic spherules observed in samples from the A.D. 2009 eruption of Mount Redoubt, Alaska, USA (A–B) and the A.D. 2010 eruption of Eyjafjallajökull, Iceland (C). Textures range from almost perfect spheres to spherules displaying surface cracks and holes to aggregates of numerous spherules fused together. Genareau et al. (2015).

A slightly larger range of spherule types was found in the natural samples, with slightly larger spherules present, aggregates of spherules and spherules with a distinctive cracked surface morphology. The cracked morphology is thought to be due to the vaporization of volatiles (probably water) within mineral grains as they are melted, the fused spherules could either have been adhered to one-another by the action of the lightning or drawn together subsequently by electrostatic attraction, whereas the overall size of the particles is probably directly related to the energy released in the lightning burst; the artificial samples produced larger grains showing signs of partial melting, suggesting that a more powerful discharge could have melted them completely (the experiment was intended to reproduce energy levels in power lines not lighting bursts).

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