Two Type I (arenaceous) fulgurites: a common tube fulgurite and a more irregular specimen.
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 (from the Latinfulgur, meaning "lightning") are classified generically as a variety of the mineraloidlechatelierite, although their absolute chemical composition is dependent on the physical and chemical properties of target material affected by the discharge of cloud-ground lightning. They are commonly hollow and/or branching assemblages of glassy, protocrystalline, and heterogeneously-microcrystalline tubes, crusts, slags, vesicular masses, and clusters of refractory materials that often form during the discharge phase of lightning strikes propagating into silica-rich quartzosesand, mixed soil, clay, caliche and other carbonate-rich sediments, humic sediments, conductive biomass (such as peat, water-saturated wood, or dung), or anthropogenic materials having similar compositions (e.g. concrete, brick, asphalt, tile, etc.). Colloquially, they have been referred to as petrified lightning. Fulgurites are homologous to Lichtenberg figures, which are the branching patterns produced on surfaces of insulators during dielectric breakdown by high-voltage discharges, such as lightning.
Fulgurites are formed when lightning with a temperature of at least 1,800 °C (3,270 °F) melts silica or other common conductive and semiconductive minerals and substrates, fusing, vitrifying, oxidizing and reducing mineral grains and organic compounds; the fulgurite mass is the rapidly-quenched end-product. The temperature peak within a lightning channel, however, is known to exceed 30,000 K, with sufficient pressure to produce planar deformation features, or "shock lamellae" in SiO2polymorphs. It is assumed that the process of forming a fulgurite occurs over a timespan of the order of a single second, following the termination of the return stroke sequence, and leaves direct evidence of the dissipation path and its dispersion over the surface or into the earth. Artificial fulgurites can also be produced when the controlled arcing of electricity into a fusable medium. This has been documented in cases of downed high voltage power lines; current was discharged into the ground, producing blue fulgurite-like lechatelierites, colored by copper from the power line.
The color of fulgurites varies widely, depending on composition and chemical impurities. It can range from black or tan, to green, blue, metallic blue-grey, or a translucent white. More colorful variants are usually synthetic and reflect incorporation of synthetic materials. The interior of Type I (sand) fulgurites normally is very smooth or lined with fine bubbles, while other types are often both vesicular and dense or porous and scoria-like; their exteriors generally can be coated with rough sedimentary particles and can be porous, smooth, or structurally-complex. Fulgurites display some degree of self-similarity and structural scale invariance as a macroscopic or microscopic network of root-like branches. Fulgurites formed in sand or loose soil are mechanically 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, but may form directly on appropriate 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
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
Droplet fulgurites, which are exogenous (e.g. spheroidal, botryoidal, filamentous, or aerodynamic), related by composition to Type II and Type IV fulgurites
A sixth category is proposed to include fulgurites that formed on carbon-rich biological materials, and a supplementary taxonomy that can account for formation pathways, chemical varieties, and clastic combinations is emerging as analysis continues and differentiation of polarity-specific properties becomes feasible.
Fulgurites are appreciated by many for their scientific value as permanent tangible evidence of transient lightning strikes. Fulgurites also are popular among hobbyists and collectors of natural specimens.