Fulgurite

Cloud-to-ground lightning transfers electrical energy into the ground along a narrow path. The resulting intense heating can fuse or melt grains and rock minerals, which cool into natural glass. In sand, the product often follows a hollow, branching or irregular tube-like path through the sediment; the inner surface may be glassy while the outside retains partly fused grains. In rock, glassy material may occur as veins, channels or coatings. Shape depends on the path of the discharge and on the material it encounters, so not all fulgurites are identical tubes.

Fulgurites are commonly described by the substrate in which they form. Sand and soil examples may incorporate silica-rich grains, clay and other local minerals, while rock fulgurites can preserve melted material along fractures or on exposed surfaces. Composition and texture therefore vary with the host material and the conditions of heating and cooling. Similar glassy formations may also result from high-voltage electrical accidents, a distinct origin from a natural lightning strike; their context matters when interpreting a specimen.

Because fulgurites are created by lightning, their occurrence provides direct physical evidence of a discharge interacting with the ground. Some specimens retain trapped gases or other material that can help researchers investigate the environment in which they formed. A study reported in Science described analyses of gases sealed in a Sahara fulgurite and thermoluminescence dating that placed its formation at roughly 15,000 years ago, consistent with evidence that the region then supported more vegetation than today. Such interpretations depend on particular samples and methods; a fulgurite does not by itself provide a complete climate record.

Lightning is difficult to study at the moment and place it naturally strikes, so laboratory experiments provide a controlled complement to field specimens. In a 2023 open-access study, researchers used a triggered high-voltage discharge on volcanic ash and produced fulgurite-like material that resembled natural examples in state and texture. Reproducible experiments allow scientists to vary materials and discharge conditions, helping test how fulgurites form. They do not reproduce every feature of natural strikes, which involve diverse substrates and conditions.