Fulgurites: how lightning fossilizes its underground silhouette by forging pure glass tubes

A common lightning bolt carries millions of volts and unleashes temperatures that can exceed 30,000 degrees Celsius, more than five times the surface heat of the Sun. When this plasma projectile strikes ground rich in quartz sand, physics and geology undergo an almost instantaneous transformation: the birth of a fulgurite.

At that excessive temperature, the melting point of quartz (around 1,700 °C) is greatly exceeded in a matter of microseconds. The lightning bolt vaporizes internal soil moisture and melts silica grains along its electrical path underground. The sudden expansion of gases pushes the liquefied material outward, leaving a hollow conduit that cools at lightning speed. The result is a cylindrical, branching tube made of lechatelierite, a natural amorphous silica glass identical to a petrified electrical root.

These geological formations exhibit a fascinating morphological duality:

• The interior: It is completely smooth, shiny, and glassy in texture, filled with tiny gas bubbles trapped during the express cooling.
• The exterior: It remains rough and coarse, covered by partially melted sand grains that adhered to the tube's outer walls.

Although most measure just a few centimeters in diameter and a few spans in length, continuous specimens several meters deep have been unearthed. Beyond their sculptural beauty, fulgurites are authentic climate fossils: they preserve trapped atmospheric bubbles and organic remains from the exact moment of impact, allowing scientists to reconstruct prehistoric climate and vegetation.