When lightning strikes sand in the desert, it instantly melts the grains, creating complex structures that geologists call fulgurites. These structures of partially melted minerals can also be found in lightning-struck rocks. But that's just the beginning: While investigating this phenomenon on an outcropping in southern France, researchers discovered that the changes can go even further — down to the atomic level.
Lightning can warp quartz crystals in rock to form distinct structures typically found in meteorites, reports Elizabeth Goldbaum for LiveScience. So-called shock lamellae are minute parallel lines that run through the quartz and indicate the rock was hit with an intense force.
"It's like if someone pushes you, you rearrange your body to be comfortable," researcher Reto Gieré of the University of Pennsylvania says in a press statement by Katherine Unger Baillie. "The mineral does the same thing." Lightning's push, however, is equivalent to a force that's 20 million times greater than a boxer's punch, as Baillie writes.
Goldbaum reports for LiveScience:
After looking at very thin, almost transparent, slices of the fulgurites under a powerful microscope, the researchers noticed that the black fulgurite looked glossy, "almost like a ceramic glaze," Gieré said. The fulgurite was also porous, similar to foam; the researchers suspect it got that way when the sizzling lightning vaporized the rock's surface.
Chemical traces of sulfur dioxide and phosphorous pentoxide were all that remained of what the researcher believe was once lichen growing on the rock’s surface. Under the foamy, glassy fulgurite layer, shock lamellae were hidden, only visible with a transmission electron microscope. There, a thin layer of what were once quartz crystals had melted and deformed so much that the crystal structure was destroyed. The team published their findings in American Mineralogist.
Once he knew what to look for, Gieré started seeing a wet, darkened look to rocks that indicated the presence of fulgurites in many places. He suggests that hikers should stay on the lookout for such rocks — especially when trekking across exposed rocky faces or scrambling to the crowns of mountaintops. Fulgurites could warn of an area prone to lightning strikes and are a good reason to keep an eye on the clouds.