It's hard to believe that something as ephemeral as lightning could be frozen in time for thousands of years. But that's just what happens with fulgurites--glassy, hollow tubes that form when lightning melts sand. For the first time, researchers have successfully dated these unusual geological formations, and the findings are providing a unique insight into the long-ago climate and ecology of the Sahara desert.
Fulgurites record the past by providing direct evidence of thunderstorms and rain. But when Rafael Navarro-González, a chemist at the National Autonomous University of Mexico in Mexico City, came across an unusual specimen from the Sahara desert in southwestern Egypt, he realized that some fulgurites may have a much more interesting story to tell. Unlike most fulgurites, this one was rounder and solid, but what really caught his eye were tiny, embedded glass bubbles. He wondered if they might contain gas.
Using a laser, Navarro-González carefully opened the bubbles, releasing trapped carbon dioxide, carbon monoxide, and nitric oxide. This gas was created when heat from the lightning had oxidized organic matter in the soil. By measuring ratios of carbon isotopes in the gas, Navarro-González and his colleagues determined that the organic matter came from grasses and shrubs typical of hot, semi-arid environments. Today, the area of the Sahara where the fulgurite was found is too hot and dry for even these hearty plants to thrive.
To determine when this vegetation grew, team member Shannon Mahan, a geochronologist at the United States Geological Survey in Denver, Colorado, dated the fulgurite using a technique called thermoluminescence. The approach relies on the fact that when natural ionizing radiation in the ground passes through crystalline objects, tiny defects can trap electrons, which then accumulate over time. When the object is heated in the lab, these electrons escape, releasing light. The amount of light reveals how long ago the object was last heated, which in this case was when the lightning struck. The Sahara fulgurite, the researchers report in this month's issue of Geology, formed roughly 15,000 years ago. These findings corroborate other evidence that the Sahara was less arid and more hospitable to plant life during this period.
"It's a really creative approach," says Steve Forman, a geochronologist at the University of Illinois at Chicago. They "squeeze[d] the most information out of a real geological oddity." Barbara Sponholz, a physical geographer at the University of Würzburg in Germany, is excited that fulgurites are living up to their promise and hopes more scientists start studying them. Forman, for one, says, "I'll keep my eyes open for fulgurites in the future."