Like meteorites, lightning bolts can reshape rocks
Around the globe, lightning flashes 44 times a second. Contrast that with meteorite impacts, which occur just five to 10 times per year.
“It’s like if someone pushes you, you rearrange your body to be comfortable,” says Gieré, professor and chair of the Department of Earth and Environmental Science in the School of Arts & Sciences. “The mineral does the same thing.”
Gieré, who embarked on the study with researchers from Germany and Australia, discovered the similarity in a sample of quartz from a rock outcropping in southern France. The rock had been struck by lightning and fractured. The researchers could see that the area in and around the fracture was covered in a distinctive black coating.
“When I first looked at it, I thought it almost looked charred or like there was algae growing on it,” Gieré says. “But under a magnifying glass, you can see that it is shiny, like a ceramic glaze.”
Using various types of powerful microscopes, including one that gives a resolution on the atomic level, Gieré and colleagues examined thin slices of the rock. They found that the surface black layer was a glass formed from the heat of the lightning bolt, which effectively vaporized the outer layer of the rock.
What really surprised the researchers, though, was what they found just beneath that layer of glass. Placing a thin slice of the sample under a transmission electron microscope, they caught a glimpse of what are known as shock lamellae: even, parallel lines of atoms with a distinct orientation, which until now have only been seen in minerals at the sites of meteorite impacts.
“To see that lightning literally melts the surface of a rock and changes crystal structures, to me, is fascinating,” Gieré says.
Through further studies of such samples, Gieré and his colleagues hope to gain a clearer understanding of the physics and chemistry of lightning—and a greater appreciation for what it means for a material to be struck by a bolt.
“And to think, some people survive a lightning strike,” says Gieré.