An instantaneous fossil
When a bolt of lightning strikes silica-rich sand, the electrical discharge can generate temperatures exceeding 1,800 degrees Celsius. This intense heat, about five times hotter than the surface of the sun, vaporizes and melts the sand along the lightning's path, fusing the mineral grains into hollow glass tubes in about a second. The resulting structures are called fulgurites, from "fulgur," the Latin word for lightning.
These formations are essentially glass, a type of mineraloid called lechatelierite. Their shape mirrors the path the lightning took as it dissipated into the ground, often resulting in branching, root-like structures. The interior surface of a fulgurite tube is smooth and glassy from the rapid cooling, while the exterior is rough, coated with partially melted sand grains. Fulgurites are fragile and difficult to excavate whole. They can penetrate deep underground, sometimes as far as 15 meters (49 feet) below the surface. While they are found worldwide, they are most common in clean, dry desert or beach sands, like those in Egypt's Western Desert.
A bubble of ancient climate
The rapid formation of a fulgurite traps sand and other materials. As the molten silica cools, it captures bubbles of the surrounding air and soil gases. These bubbles are microscopic samples that preserve a sample of the atmosphere and soil conditions from the moment of the lightning strike. Scientists can analyze these trapped gases to reconstruct past environments, a field known as paleoclimatology.
One study of a fulgurite from the Libyan Desert, near the Egyptian border, used thermoluminescence dating to determine it was formed approximately 15,000 years ago. Analysis of the gases inside its bubbles revealed compositions of carbon dioxide, carbon monoxide, and nitric oxide. The specific carbon isotopes were characteristic of vegetation found in hot, arid climates, such as grasses and shrubs. Today, the area is bare sand. The findings suggest that 15,000 years ago, the environment was more like the semi-arid Sahel region, which is now located hundreds of kilometers to the south. The presence of numerous fulgurites in a region that currently sees very little lightning activity provides physical evidence of a different climate in the past.