Two Type I (arenaceous) fulgurites: a section of a common tube fulgurite and one exhibiting a branch
Stereo image
Left frame
Right frame
Parallel view ()
Cross-eye view ()
Two small Type I Saharan Desert fulgurites.[1] In a planar view the specimen on the right has a blade-like morphology, but its tubular nature is dramatically shown in a stereo view.
Typical broken fulgurite sections
Fulgurites (from Latinfulgur 'lightning' and -ite), commonly called "fossilized lightning", are natural tubes, clumps, or masses of sintered, vitrified, or fused soil, sand, rock, organic debris and other sediments that sometimes form when lightning discharges into ground. When composed of silica, fulgurites are classified as a variety of the mineraloidlechatelierite.
When ordinary negative polarity cloud-ground lightning discharges into a grounding substrate, greater than 100 million volts (100 MV) of potential difference may be bridged.[2] Such current may propagate into silica-rich quartzose sand, mixed soil, clay, or other sediments, rapidly vaporizing and melting resistant materials within such a common dissipation regime.[3] This results in the formation of generally hollow and/or vesicular, branching assemblages of glassy tubes, crusts, and clumped masses.[4] Fulgurites have no fixed composition because their chemical composition is determined by the physical and chemical properties of whatever material is being struck by lightning.
