Anatomy of a lightning fossil
When a lightning bolt strikes silica-rich ground, it is a geologic event with immense power. The electrical discharge can heat the air to 30,000°C, and the ground itself to at least 1,800°C. This instantaneous, extreme heat vaporizes the sand, soil, or rock along the lightning's path. As the current disperses underground, it fuses the material into hollow glass tubes in a process that takes about one second. The resulting structures are called fulgurites, from the Latin word "fulgur," for lightning.
The glass that makes up a fulgurite is a mineraloid known as lechatelierite, a naturally occurring silica glass. Unlike quartz, which has a crystalline structure, lechatelierite is amorphous, its atoms locked in a disordered state by the rapid cooling. Sand fulgurites, the most common type, typically have a rough exterior coated in partially melted sand grains and a smooth, glassy interior. They are often a few centimeters in diameter and can burrow deep into the subsoil, sometimes reaching depths of 15 meters (49 feet). While most are fragile, some impressive specimens have been excavated. One of the longest ever found, unearthed in northern Florida, was over 4.9 meters (16 feet) long. Charles Darwin noted fulgurites in the UK that reached 9.1 meters (30 ft).
A record of ancient storms
Fulgurites are a geological curiosity and are a direct physical record of past atmospheric conditions. The study of ancient lightning strikes is called paleolightning, and fulgurites are its primary evidence. Their presence indicates past thunderstorm activity, scientists use them to reconstruct climate patterns in areas where storms are now rare. For example, the abundance of fulgurites in the Sahara Desert shows that the region was once a fertile area with frequent storms. One fulgurite found there was dated to 250 million years ago.
Scientists can determine the age of fulgurites using techniques like thermoluminescence. This method measures the faint glow emitted by minerals when heated, which corresponds to the amount of background radiation they have absorbed over time. A fulgurite from southwest Egypt was dated to 15,000 years ago using this method. Tiny gas bubbles trapped within the fulgurite's glass can preserve samples of the ancient atmosphere and soil gases. Analysis of the gases from the Egyptian fulgurite revealed molecules from grasses and shrubs, proving the area was far more vegetated than the bare sand that exists there today.