EVERYONE knows what happens to a tree when it gets struck by lightning, but what about when lightning strikes the ground? Given the right conditions, such as sandy soil or a beach, the lightning bolt鈥檚 path is scorched vertically into the earth, preserved as a network of delicate, glassy tubes called a fulgurite.
The name was coined in the 19th century 鈥� fulgur is Latin for 鈥渓ightning鈥�. Fulgurites form when lightning melts and fuses mineral grains in sand. Knobbly and branched like tree roots, they can penetrate several metres. Estimates of the energy required to create a fulgurite vary, but the temperature reached is thought to be about 4000 掳C.
Fulgurites are quite rare, and the best place to look for them is in shifting desert sand, not only because the necessary raw materials are abundant there, but also because desert breezes blow their cover away. Good specimens fetch high prices among collectors and merchants 鈥� not least because their fragility makes them difficult to extract from the ground in decent-sized chunks.
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Fulgurites also have a value that goes beyond scarcity and aesthetics: they can persist for thousands of years and are able to provide scientists with information about past climates. In a recent study, researchers led by Rafael Navarro-Gonzalez at the National Autonomous University of Mexico analysed bubbles of gas in a 15,000-year-old fulgurite from the Libyan desert. Levels of carbon dioxide, carbon monoxide and nitrogen oxide within the bubbles suggest that the soil conditions at the moment the lightning struck were similar to those in present-day Libya鈥檚 more southerly Sahel region. This indicates that the Libyan desert may once have been a semi-arid savannah covered with annual grasses and shrubs (Geology, vol 35, p 171).
Geologists recently discovered another type of fulgurite, high on a mountain in Greenland. A team from the Geological Survey of Denmark and Greenland came across a branched trail running for many metres down the rocky hillside. Lichen had been burned away along the trail and the rock surface had been transformed into a smooth glass, coloured bright blue, red and yellow (Geological Magazine, vol 143, p 737). This particular mountain top consists of banded rock formations rich in iron 鈥� definitely a place to avoid during a thunderstorm.
鈥淭he surface had been turned into a smooth glass, coloured bright blue, red and yellow鈥�
