Fulgurites (from the Latin fulgur meaning thunderbolt) are natural hollow carrot-shaped glass tubes formed in quartzose sand or soil by lightning strikes. In the right kind of sand the extreme heat generated will form silica glass shapes that trace the path of the lightning. These structures are also sometimes referred to as petrified lightning. The glass formed is called lechatelierite which may also be formed by meteorite impact and volcanic explosions. As it is amorphous it is classified as a mineraloid.
The tubes can be up to a couple of centimeters in diameter, and meters long. Their color varies depending on the composition of the sand they formed in, ranging from black or tan to green or a translucent white. The interior is normally very smooth or lined with fine bubbles; the exterior is generally coated with rough sand particles. They are rootlike in appearance and often show branching or small holes. Fulgurites occasionally form as glazing on solid rocks (sometimes referred to as an exogenic fulgurite).
Fulgurites are a very rare phenomenon. A very large one was found in South Amboy, New Jersey. This was roughly nine feet long with a diameter of three inches (7.6 cm) near the surface of the ground, and tapered to roughly three sixteenths of an inch (5 mm) in diameter at the deepest point recovered. As is often the case due to the fragile nature of fulgurites, scientists were unable to extract it in one piece and the largest recovered fragment was a mere six inches (15.2 cm) long.
Fulgurites are notably found high on Mount Thielsen ("the lightning rod of the Cascade Range") where they form a brownish-green glaze on rocks (especially on the final five or ten feet of the summit pinnacle) and on the shores of the Great Lakes.
Possibly the finest fulgurite sample on display can be seen in Philadelphia, USA, at the Academy of Natural Sciences. It was discovered in 1940.
Fulgurites can also be produced when a high voltage electrical distribution network breaks and the lines fall onto a concutive surface beneath which sand is present, example of such held by the Canadian National Geologic Survey and was donated by R. Scott Holliday. Occurred when a 14,400 volt high voltage electrical line was broken by a falling branch and the wires fell onto a dirt road, resulting in Fulgurite being formed UNDER the roadway, discovered during annual road maintenance three to fives years after the electrical line was severed. Mostly green glass, but there are other colors present in the samples. - R. Scott Holliday
The largest fulgurite known is at the Peabody Museum of Natural History at Yale University, which has on display a 13-foot (4 m) long fulgurite from the shores of Lake Congamond in northern Connecticut. The fulgurite has been on display at the Museum since the 1950s, and is viewable in the new Hall of Minerals, Earth and Space as of May, 2006.
A specimen of fulgurite over 3 metres long is in the Natural History Museum in London. It is preserved in sections of over 50cm.
- Fulgurites in New Scientist
- Glossary of Meteorology definitions (including Fulgurite).
- Petrified Lightning by Peter E. Viemeister (pdf)
- Mindat with location data
- W. M. Myers and Albert B. Peck, A Fulgurite from South Amboy, New Jersey, American Mineralogist, Volume 10, pages 152-155, 1925
- Vladimir A. Rakov, Lightning Makes Glass, 29th Annual Conference of the Glass Art Society, Tampa, Florida, 1999
fulgurite in German: Fulgurit
fulgurite in Spanish: Fulgurita
fulgurite in French: Fulgurite
fulgurite in Lithuanian: Fulguritas
fulgurite in Dutch: Fulguriet
fulgurite in Japanese: 閃電岩
fulgurite in Polish: Fulguryt
fulgurite in Portuguese: Fulgurite
fulgurite in Russian: Фульгурит
fulgurite in Finnish: Fulguriitti
fulgurite in Swedish: Fulgurit
fulgurite in Chinese: 閃電熔岩