Stroke of Good Fortune: A wealth of data from petrified lightning

The lumps of glass created when lightning strikes sandy ground can preserve information about ancient climate, new research indicates.

BOLT FROM THE BLUE. When lightning strikes the ground, it fuses sand in the soil into tubular masses of glass called fulgurites (top). The gases trapped in bubbles in that glass (bottom) yield clues to ancient soil and atmospheric chemistry and climate. L. Carion/Carion Minerals, Paris; Navarro-González

Worldwide, lightning flashes occur about 65 times per second. Each bolt releases as much energy as is stored in a quarter-ton of TNT. The flash heats the air to about 30,000°C, about five times the temperature of the surface of the sun. If that electrical discharge strikes sandy ground, it can melt and then fuse sand and other materials into masses of glass called fulgurites, says Rafael Navarro-González, a geochemist at the National Autonomous University of Mexico in Mexico City. Those masses take their name from fulgur, the Latin word for lightning.

Although thunderstorms are common in many parts of the world, they’re rare in the desert of southwestern Egypt. “Satellite data gathered between 1998 and 2005 detected little, if any, lightning in that area,” says Navarro-González. However, the lumps and tubes of glass that litter the region’s shifting dunes are proof that lightning, the only source of fulgurites, frequently touched down there in the past.

Studying samples of a fulgurite that had been collected in 1999, Navarro-González and his colleagues found that it had formed 15,000 years ago. The team measured the luminescent glow that the fulgurite’s minerals gave off when heated. Over time, exposure to cosmic rays and to the decay of radioactive elements in the soil produce defects in the material. The more defects, the brighter the heated material glows.

Chemical analyses of the gases trapped in bubbles inside the glass revealed that there have been major changes in the ancient landscape. Today, it’s bare sand, but 15,000 years ago, it was hospitable to shrubs and grasses.

The tests, which are the first to look at the chemical composition of a fulgurite’s gas bubbles, revealed a small amount of argon, the atmosphere’s most abundant inert gas today. In an average modern sample of Earth’s atmosphere, argon outweighs carbon dioxide about 25:1. In the fulgurite gases, however, carbon dioxide was more than 100 times as common as argon, says Navarro-González. That extra carbon dioxide was generated when the lightning bolt vaporized organic material in the once-fertile soil, the researchers propose in the February Geology.

The ratio of carbon-12 to carbon-13 isotopes that the team measured in the trapped gases is typical of that generated by the photosynthesis of grasses and shrubs adapted to hot, arid climates. Today, such vegetation grows in southwestern Niger, about 600 kilometers south of the site where the team’s fulgurite was recovered. The ratios of elements in fulgurite’s gases were typical of those in the modern soils of that region.

All these clues suggest that 15,000 years ago, near the end of the most recent ice age, the climate in southwestern Egypt was similar to that found today in Niger.

Because fulgurites are mainly glass, they’re chemically stable and aren’t very susceptible to erosion, says Barbara Sponholz, a physical geographer at the University of Würzburg in Germany. That makes fulgurites and the gases that they contain long-lasting indicators of climate, she notes.

Analyzing the Egyptian fulgurites is “an interesting way of showing that the climate in this region has changed,” agrees Kenneth E. Pickering, an atmospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

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