fulgurite

A fulgurite is a natural glass formation created when a lightning bolt strikes silica-rich material, fusing the surrounding substrate into a glassy mass along the strike path. The term comes from fulgur, the Latin word for lightning.

Formation occurs when the arc's extremely high temperature rapidly melts the material it travels through. In sandy soils, beaches, deserts, or quartz-rich bedrock, the molten silica refracts and flows before rapidly cooling to form a glassy tube, lump, or network. The interior channel may remain hollow, reflecting the path of the electrical discharge, or be partially filled with glass and mineral inclusions. The outside is typically a glassy crust that can be darker due to impurities.

Fulgurites come in several forms. Sand fulgurites are the most common and are found where loose silica-rich material is present, such as beaches or dune fields. Rock fulgurites form when lightning strikes sandstone, limestone, or other crystalline rocks and produce glassy, sometimes branched structures within the rock. The resulting specimens often show branching or tubular geometries that trace the original arc.

Distribution is global, occurring wherever suitable substrates are available and lightning activity occurs. They are most frequently found in arid regions, deserts, and coastal environments but have been reported in diverse settings. Size varies widely, from a few centimeters to decimeters in length, with diameters typically a few centimeters; exceptionally long specimens exceeding a meter have been documented.

Significance lies in their ability to record a high-energy atmospheric event and the properties of the substrate at the strike point. Fulgurites are studied by geologists and archaeologists to infer past lightning activity, substrate composition, and conditions during formation. They are distinct from meteorites and other glassy geological materials.