Fulgurites (from the Latinfulgur, meaning "lightning") are natural tubes, clumps, or masses of sintered, vitrified, and/or fused soil, sand, rock, organic debris and other sediments that sometimes form when lightning discharges into ground. Fulgurites are classified as a variety of the mineraloidlechatelierite. When lightning strikes a grounding substrate, upwards of 100 million volts (100 MV) are rapidly discharged into the ground. This charge propagates into and rapidly vaporizes and melts silica-rich quartzosesand, mixed soil, clay, or other sediments. This results in the formation of hollow, branching assemblages of glassy, tubes, crusts, and vesicular masses. Because of the high temperature differential between the core of a fulgurite and the surrounding soil, many fulgurites show evidence of progressive crystallization: in addition to glasses, many are partially protocrystalline or microcrystalline. 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.
Parallel view ()
Cross-eye view ()
Two Type I (arenaceous) fulgurites: a section of a common tube fulgurite and one exhibiting a branch.
Parallel view ()
Cross-eye view ()
Two small Type I Saharan Desert fulgurites. 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.
Fulgurites are formed when lightning strikes the ground, fusing and vitrifying mineral grains. The primary SiO2 phase in common tube fulgurites is lechatelierite, an amorphous silica glass. Because their groundmass is generally amorphous in structure, fulgurites are classified as mineraloids.
Material properties (color, surface texture) of fulgurites vary widely, depending on bulk composition, interface dynamics, and trace elements. Most natural fulgurites fall on a spectrum from colorless (transparent), to white, to black. Iron oxide is a common impurity that can result in deep brownish-green coloration. Lechatelierite similar to fulgurites can also be produced via controlled (or uncontrolled) arcing of artificial electricity into a medium. Downed high voltage power lines have produced brightly-colored lechatelierites, due to copper or other materials from the power lines themselves. Brightly-colored lechatelierites resembling fulgurites are usually synthetic and reflect the incorporation of synthetic materials. However, lightning can strike man-made objects, resulting in colored fulgurites.
The interior of Type I (sand) fulgurites normally is smooth or lined with fine bubbles, while their exteriors are coated with rough sedimentary particles or small rocks. Other types or fulgurites are usually vesicular, and may lack an open central tube; their exteriors can be porous or smooth. Branching fulgurites display fractal-like self-similarity and structural scale invariance as a macroscopic or microscopic network of root-like branches, and can display this texture without central channels or obvious divergence from morphology of context or target (e.g. sheet-like melt, rock fulgurites). Fulgurites are usually fragile, making the field collection of large specimens difficult.
Fulgurites can exceed tens of centimeters in diameter and can penetrate deep into the subsoil, sometimes occurring as far as 15 m (49 ft) below the surface that was struck.  Or they may form directly on sedimentary surfaces. One of the longest fulgurites to have been found in modern times was a little over 4.9 m (16 ft) in length, and was found in northern Florida. The Yale UniversityPeabody Museum of Natural History displays one of the longest known preserved fulgurites, approximately 4 m (13 ft) in length.Charles Darwin in The Voyage of the Beagle recorded that tubes such as these found in Drigg, Cumberland, UK reached a length of 9.1 m (30 ft). The Winans Lake fulgurite[s] (Winans Lake, Livingston County, Michigan), extended discontinuously throughout a 30 m range, and arguably includes the largest reported fulgurite mass ever recovered and described: its largest section extending approximately 16 ft (4.88 m) in length by 1 ft in diameter (30 cm).
Fulgurites have been classified by Pasek et al. (2012) into five types related to the type of sediment in which the fulgurite formed, as follows:
Type I - sand fulgurites with tubaceous structure; their central axial void may be collapsed
Type II - soil fulgurites; these are glass-rich, and form in a wide range of sediment compositions, including clay-rich soils, silt-rich soils, gravel-rich soils, and loessoid; these may be tubaceous, branching, vesicular, irregular/slaggy, or may display a combination of these structures, and can produce exogenic fulgurites (droplet fulgurites)
Type III - caliche or calcic sediment fulgurites, having thick, often surficially glazed granular walls with calcium-rich vitreous groundmass with little or no lechatelierite glass; their shapes are variable, with multiple narrow central channels common, and can span the entire range of morphological and structural variation for fulguritic objects
Type IV - rock fulgurites, which are either crusts on minimally altered rocks, networks of tunneling within rocks, vesicular outgassed rocks (often glazed by a silicide-rich and/or metal oxide crust), or completely vitrified and dense rock material and masses of these forms with little sedimentary groundmass
Type V - [droplet] fulgurites (exogenic fulgurites), which show evidence of ejection (e.g. spheroidal, filamentous, or aerodynamic), related by composition to Type II and Type IV fulgurites
The presence of fulgurites in an area can be used to estimate the frequency of lightning over a period of time, which can help to understand past regional climates. Paleolightning is the study of various indicators of past lightning strikes, primarily in the form of fulgurites and lightning-induced remanent magnetization (LIRM) signatures.
Place in planetary processes and the geologic recordEdit
Reduced phosphorus as phosphides and phosphites has been identified through quantitative analyses of a representative sample of 10 fulgurites recovered from most continents, in the form of schreibersite (Fe3P, (Fe,Ni)3P), and titanium(III) phosphide (TiP). Many of these reduced compounds are otherwise rare on Earth due to the presence of oxygen in Earth's atmosphere, which creates oxidizing surface conditions.
An object initially believed to be a fulgurite was found within the contents of the ash altar at the temple of Lykaian Zeus at Mount Lykaion in Greece. However, after nearly two decades, the "fulgurite" has not yet been analyzed or confirmed to be a fulgurite, and it has not been described in any peer-reviewed publications. The two published reports of the excavations at Mt. Lykaion notably omit any references to, or description of, a "fulgurite."
Fulgurites are popular among hobbyists and collectors of natural specimens.
^Carter, Elizabeth A.; Hargreaves, Michael D.; Kee, Terence P.; Pasek, Matthew A.; Edwards, Howell G. M. (2010-06-07). "A Raman spectroscopic study of a fulgurite | Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 368 (1922): 3087–3097. doi:10.1098/rsta.2010.0022. PMID20529946.