When lightning with a temperature of at least 1,800°C (3,272°F) strikes a silica-rich patch of ground, the intense heat can melt the silica to form glassy tube-like structures known as fulgurites. Now, researchers have found that when lightning strikes solid rock, it not only melts the surface, but can actually cause the rock to change on an atomic level, creating parallel structures within the rock.
It was originally thought that the only natural event to be able to create such structures, called shock lamellae, were meteorite impacts. As such, when scientists previously observed these rock alterations, they were often assumed to be indicators of meteorite strikes.
“Most geologists are careful; they don’t just use one observation,” says Reto Gieré, who co-authored the paper published in American Mineralogist. “But this is a good reminder to always use multiple observations to draw big conclusions, that there are multiple mechanisms that can result in a similar effect.”
When rock is struck by lightning, it often melts the surface and results in a black shiny “glaze,” similar to the glassy fulgurites created when the ground is hit. But Gieré and his team decided to take samples and then cut thin slices so they could look at them under a transmission electron microscope, enabling them to examine the samples at the atomic level.
They found that due to the lightning’s heat vaporizing the rock's surface, the shiny black layer was actually incredibly porous. Furthermore, this outer layer of the rock lacked any crystalline structure, consistent with it having been melted. But when they then looked under the surface layer, they found something else: a set of straight parallel lines, more normally associated with the shock lamellae created by meteorite strikes.
The formation of the shock lamellae is thought to occur when the crystal structure of quartz is “pushed over,” or deformed, by a large wave of pressure. They reckon that it takes a pressure of more than 10 gigapascals, or as the authors explain, a force roughly 20 million times greater than that of a boxer’s punch, in order to form the lamellae.
“I think the most exciting thing about this study is just to see what lightning can do,” explained Gieré. “To see that lightning literally melts the surface of a rock and changes crystal structures, to me, is fascinating.”
Center image: Lightning strikes create a "glassy" layer on the rock it hits. Credit: University of Pennsylvania.