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[dinosaur] Shock quartz generated by lightning as well as impact events




Ben Creisler
bcreisler@gmail.com





John Bois suggested this recent research might be of interest, so here's what I found:




Jiangzhi Chen, Chiara Elmi, David Goldsby & Reto Gieré (2017)
Generation of shock lamellae and melting in rocks by lightning-induced shock waves and electrical heating.
Geophysical Research Letters 44(17): 8757–8768
DOI: 10.1002/2017GL073843  
http://onlinelibrary.wiley.com/doi/10.1002/2017GL073843/full

The very rapid energy release from impact events, such as those resulting from lightning strikes or meteorites, can drive a variety of physical and chemical processes which alter rocks and result in the formation of natural glasses (i.e., fulgurites and tektites). Fulgurite is the vitrified soil, sand, or rock resulting from lightning strikes. A thunderbolt is associated with air temperatures of up to 105 K, which can heat rocks to >2000 K within tens of microseconds. The rapid fusing and subsequent quenching of the surface of the rock leaves a distinctive, thin, garbled coating composed of a glassy to fine-grained porous material. Previous studies on rock fulgurites found planar deformation features in quartz crystals within the target rock substrate, evidence of strong shock waves during fulgurite formation. In this paper, we simulated the shock pressure and temperature caused by an idealized lightning impact on rocks and compared the model results with observations on rock fulgurites from the literature. Our model results indicate that a lightning strike can cause >7 GPa pressure on the rock surface, generate a layer of fulgurite (of radius ~9 cm), and leave a burned region (of radius ~11 cm). The fulgurites found on rock surfaces share many features with sand fulgurites, but their spatial distribution is completely different, as sand fulgurites are hollow tube structures. Our study on rock fulgurites provides an indirect constraint on the energy of a lightning event and also demonstrates that the presence of shock features in rocks cannot be taken as unequivocal evidence for impact events.

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Also, free pdf:

H. J. Melosh (2017)
Impact geologists, beware!
Geophysical Research Letters 44(17): 8873–8874 
DOI: 10.1002/2017GL074840
http://onlinelibrary.wiley.com/doi/10.1002/2017GL074840/full


Impact geologists have long assumed that shock metamorphic features, such as planar fractures and Planar Deformation Features (PDFs) in quartz are reliable indicators of an extraterrestrial impact. A new paper by Chen et al. (2017) now shows that such features might arise in terrestrial lightning strikes, thus raising the bar for identification of impact sites.

News:

http://www.sciencemag.org/news/2017/07/lightning-can-beat-rocks-asteroid-strike-casting-doubt-past-impacts

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