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Lightning Strikes May Leave Traces Like Those of Meteorites

From Research Spotlights

Scientists have long interpreted shocked quartz as definitive evidence of a past meteorite impact, but the shock wave caused by lightning striking granite also produces this distinctive feature.

When a meteorite strikes a rock, it triggers rapid changes in pressure and temperature that alter the rock’s structure. Traditionally, scientists have treated microscopic planar deformation features in quartz crystals as a telltale sign of past meteorite impacts. However, recent research has demonstrated that lightning strikes can also leave similar signatures of shock.

In a new paper, Chen et al. mathematically simulate a lightning strike on a granite surface. They demonstrate that the resulting changes in the rock are a fingerprint of the energy and intensity levels of the lightning that caused them. More specifically, they demonstrate that shock features in quartz are created by the intense shock wave associated with the lightning strike. The results suggest that shocked quartz should not be interpreted as certain evidence of past meteorite impacts.

Scientists have known for decades that lightning can rapidly heat rock to over 2,000 kelvins near the strike point. Organic material on the surface burns off, and part of the rock itself melts almost instantaneously, later cooling to form a glassy surface layer called a fulgurite. It wasn’t until 2015 that researchers discovered shocked quartz in the granite substrate of a fulgurite.

In the new study, the research team developed a mathematical model to estimate the pressure exerted by a lightning strike on a granite surface, as well as the rapid heating and cooling of the rock. The model incorporated physical characteristics of lightning and granite, such as the typical temperature of lightning, the melting temperature of granite, and the temperature at which organic material on the granite surface would likely burn.

The simulations showed that a lightning strike can impart more than 7 gigapascals of pressure on the granite surface, enough to trigger the formation of shocked quartz. The strike creates a roughly circular layer of fulgurite about 18 centimeters across within a slightly wider region of burned organic material about 22 centimeters across.

These results are consistent with observations of fulgurite samples collected from field sites. For example, fulgurites collected from Mount Mottarone in Italy have regions of burned organic matter that are of similar size, roughly 20 centimeters across. Fulgurites from Les Pradals in France feature shocked quartz in a surficial layer less than 3 micrometers thick, consistent with the pressure calculations in the lightning strike model.

With this discovery, additional evidence will likely now be needed to convince impact geologists that shocked quartz indicates a past meteorite impact. Furthermore, these findings could help explain confusing occurrences of higher than expected impact rates, according to evidence for shocked quartz, in some regions.

Impact Geologists, Beware!—Commentary in Geophysical Research Letters 

For decades now, impact crater geologists have relied on a seemingly infallible test for the pedigree of a suspected impact structure:  If you can find shock-metamorphosed minerals, especially quartz, then the structure was made by a forceful extraterrestrial impact onto our surface because no other known process could achieve the pressures necessary to alter quartz into one of several high-pressure forms, commonly referred to as “shocked” quartz.  This idea has continued to hold sway in the field in spite of a few cases where such quartz is found, but an impact origin seems unlikely [Haines et al., 2001; Schultz et al., 2004].  Now, a group at the University of Pennsylvania [Chen et al., 2017] has challenged this canon by demonstrating that, under the right circumstances, ordinary storm-generated lightning may produce shock lamellae in quartz. More…

By H. J. Melosh, Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN

-- Sarah Stanley, Freelance Writer,


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