Forged in an instant
A bolt of lightning carries immense energy, heating the air in its path to temperatures of 30,000°C—five times hotter than the surface of the sun. When this electrical discharge strikes silica-rich sand, the ground itself is superheated. The strike lasts microseconds, but in that moment, the sand along the lightning's path is vaporized, while the surrounding silica melts and fuses. This rapid heating, followed by immediate cooling, forms a hollow tube of natural glass.
These formations are called fulgurites, from the Latin word "fulgur," for lightning. The glassy material they are made of is a mineraloid known as lechatelierite, an amorphous form of silicon dioxide. Unlike quartz, which has a crystalline structure, lechatelierite is a glass because its atoms did not have time to organize into a lattice as the molten sand cooled. The pressure from the lightning strike contributes to the formation of the hollow central tube, recording the exact path of the electricity as it dissipated into the ground. The exterior of a fulgurite is rough, coated in partially melted sand grains, while the interior is smooth and glassy.
A record in glass
Fulgurites are geological records. They are exceptionally stable and can survive for thousands of years. Scientists have used a technique called thermoluminescence to date fulgurites found in the Egyptian desert to approximately 15,000 years ago. This suggests the region was once subject to more frequent thunderstorms than it is today.
Trapped inside the glassy tubes are small bubbles that preserve samples of the atmosphere and soil gases from the moment of formation. Analysis of these bubbles in fulgurites from the Libyan Desert showed compositions different from today's atmosphere. The trapped gases indicated the presence of organic material from grasses and shrubs, showing the now-barren area was a more vegetated, semi-arid environment during the late Pleistocene.
Fulgurites vary greatly in size. While many are just a few inches long, some can penetrate deep into the ground. In 1996, researchers in Florida excavated a fulgurite with two branches measuring 4.9 and 5.2 meters (16 and 17 feet) respectively. Charles Darwin noted reports of fulgurites in England reaching a length of 9.1 meters (30 feet). Because they are extremely fragile, recovering a large, intact specimen is very difficult.