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).