Fulgurites: With the look and shape of lightning bolts

Deborah Painter (USA)

If the characteristics referred to in the title were their only quality, fulgurites would be fascinating. However, they have other unusual qualities that make them even more amazing. For example, some hold ancient air within that can offer a window into palaeo-environments. Fulgurites are natural tubes or, in the case of rock fulgurites, crusts of glass formed by the fusion of silica (quartz) from a lightning strike. They are categorised in four main types: clay, sand, caliche and rock fulgurites. In the case of the sand or clay fulgurites, the shape mimics the path of the lightning bolt as it enters the ground. All lightning strikes hitting the ground are capable of forming fulgurites, but not all lightning strikes will do so. A temperature of 1,800^oC is required to melt sand and form a fulgurite, but this is not usually an impediment, since most lightning strikes have a temperature of 2,500^oC.

In addition to the four main types mentioned above, there are the droplet fulgurites, which obviously resemble droplets, but, in composition, are similar to the clay and caliche fulgurites. Sand fulgurites tend to have rather fragile glass walls. Rock fulgurites are found not as discrete structures, but as veins or branching channels on a rock surface, or as a lining of fractures, which existed before the lightning strike.

Since mountain summits are the parts of mountains most likely to sustain a lightning strike, most rock fulgurites are found at or near summits. Clay fulgurites feature thicker glass walls with melt features, and caliche fulgurites have low glass content and thick walls. In the Wasatch Range of the American state of Utah, rock fulgurites appear to be confined to mountaintops composed chiefly of quartzite, but summits consisting of other rock types could feature them as well. Some of the fulgurites found in mountainous areas can owe their existence in part to lightning being attracted by steel buildings near the summits.

Sand fulgurite tubes have a glassy interior due to rapid cooling and solidification of the sand after the lightning strike. The length of a sand fulgurite depends on the thickness of the sand formation and the total voltage of the strike. Many sand fulgurites average 5cm in diameter and can be up to 10cm long. Sand fulgurites are the easiest to find.

But how do you find them?

Just go to a sand dune or beach containing dry sand and little clay or silt, and start digging. It is best when seeking fulgurites in the USA (and probably other areas) that one wears shoes and socks to avoid being bitten on the feet by sand mites. Some fulgurites resemble roots or branching tube-like structures that have a rough surface, covered with partially melted sand grains. Some are smoother in appearance, have smaller sand grains and resemble soda straws or ‘chips’ as in ‘fish and chips’.

Many years ago, I taught earth science and, since my students were adolescents, I felt it best to maintain a stoic demeanour and never laugh or express any emotion. At the beginning of each month, I would pass an unknown object around the classrooms in a box and the assignment was for the students to write in their notebooks what they thought the object or objects were. After they exercised their speculation muscles, we would then discuss what the object really was. The day I had the students pass around a small box containing two tube-shaped sand fulgurites, the fulgurites came back intact, but with a chip (or French fry) the same shape and size as one of the fulgurites. In spite of myself, I burst out laughing uncontrollably.

The sand fulgurites in the box are from the same collection as the ones pictured here, found at Jockey’s Ridge State Park in Nags Head, North Carolina in the USA, by James Pritchard. These fulgurites are now in the private collection of Pamela Painter. Jockey’s Ridge is a large sand dune slightly over one kilometre in width, surrounded by maritime forest and developed areas.

Interestingly, many fulgurites are hundreds or even thousands of years old. Large and intact hollow fulgurites can act as a time capsule for atmospheric conditions at the time of formation. A 2007 paper authored by Navarro-Gonzalez et al. (2007) discussed a rare opportunity these investigators put to use when they were able to examine gases trapped in a Libyan fulgurite. Thermoluminescence dating revealed an age of approximately 15,000 years BP for the fulgurite, placing it in the Pleistocene epoch. The results of the gas analysis strongly suggest that the semiarid Sahel (at 17°N) extended at least to 24°N and the fulgurite formed when the ground contained atmospheric compositions similar to those found in the Holocene Sahel, which supports plants adapted to a semiarid environment. 

About the author

Deborah Painter is an environmental scientist with a large consulting firm with its headquarters in London. Her specialties are in wetland delineation and permitting, National Environmental Policy Act documents and hazardous materials site assessments.

Further reading

Navarro-Gonzalez, Rafael, Shannon A. Mahan, Asok K. Singhvi, Rafael Navarro-Aceves, Jean-Louis Rajot, Christopher P. McKay, Patrice Coll and Francois Raulin. 2007. Paleoecology Reconstruction from trapped gases in a fulgurite from the late Pleistocene of the Libyan Desert. Geology, v. 35, p. 171-174.

Pasek, Matthew, Kristen M. Black and Virginia Pasek. 2012. Fulgurite Morphology: A Classification Scheme and Clues to Formation. Contributions to Mineralogy and Petrology 164, issue 3, p. 477-492.

Utah Geological Survey: http://geology.utah.gov/map-pub/survey-notes/glad-you-asked/what-are-fulgurites-and-where-can-they-be-found/.