|Two Type I (arenaceous) fulgurites: a common tube fulgurite and a more irregular specimen.|
|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 Latin fulgur, meaning "lightning") are classified generically as a variety of the mineraloid lechatelierite, 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 natural, commonly hollow assemblages of glassy, protocrystalline, and heterogeneously-microcrystalline tubes, crusts, slags, vesicular masses, and clusters of dielectric and refractory materials that often form during the discharge phase of lightning strikes propagating into silica-rich quartzose sand, 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; in carbon-rich fulgurites, which clearly have achieved liquid phase, temperatures unquestionably have exceeded the first triple point of carbon (4600 K, at 10800 kPa), with sufficient pressure to produce planar deformation features, or "shock lamellae" in SiO2 polymorphs. It is assumed that the process of fulguritization 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. Pseudo-fulgurites can also be produced when the cables of a high voltage electrical distribution network are severed, discharging alternating current into a conductive surface beneath, producing a linear trace structurally-distinct from natural fulgurite distribution due to the alternating polarity and duration of discharge. Arcing from power lines due to direct lightning strikes and related surges can produce hybrid pseudo-fulgurites. It is a common and conceptually-limiting belief that is widely propagated that fulgurites are exclusively tubaceous, columnar structures forming in sand or sandy soil.
Fulgurites occasionally form as glazed tracks on rock, or as internal diffusion limited aggregation networks completely metamorphosing the target rock. Ejected droplets and irregular sub-rounded structures are associated with such rock (type IV) fulgurites, as well as type II (mixed soil) and clay fulgurites; these unmixed and processed ejected materials, often quenching into bizarre forms - typically aerodynamically-contoured and displaying some degree of structural complexity, are classified as exogenic fulgurites (Type V), and can resemble many confirmed and putative crater glass forms, impactites, and tektites.
Their color varies widely, depending on specific composition and degree of hydric saturation of sediments, rock, and/or biomass in which they formed, ranging from black or tan, to green, blue, metallic blue-grey, or a translucent white. 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, or smooth, or structurally-complex. Fulgurites display some degree of self-similarity and structural scale invariance as a macroscopic or microscopic diffusion limited aggregation network of rootlike branches and fractally-distributed voids. Fulgurites formed in sand or loose soil are mechanically fragile, making the field collection of large specimens difficult.
The primary SiO2 phase in ordinary Type I (sand) fulgurites, lechatelierite, also may be formed by meteorite impacts and during volcanic eruptions (often including a lightning-induced formation mechanism), but the products of such events do not share the characteristic self-similar branching, bifurcated contours, and chaotically-deterministic external morphologies unique to fulgurites. Because their groundmass is generally amorphous in structure, fulgurite is classified as a mineraloid.
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 University Peabody 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). Peculiarly, its spatial orientation conforms to true geographic North.
Through chemical analysis and thermodynamic modeling of the morphologically-irregular and chemically-heterogeneous, the reduced (deoxidized) Winans Lake fulgurite[s], recovered approximately 15 km north of Ann Arbor, Michigan in 1984, were determined to have reached temperatures in excess of 2000 K, due to the presence of many chemically-reduced mineral and elemental phases previously believed to occur only in meteorites, Earth's mantle, and/or as a result of modern industrial processes, such as (Fe,Ni)3P (schreibersite), Fe2Si (hapkeite), Fe3Si7, FeTiSi2, FeSi (ferrosilicon), and elemental silicon; specimens also contained elemental carbon (graphite), native gold, and native silver.
An ad hoc typology for fulgurite classification, incorporating five generalized subtypes, has been developed (Pasek et al., 2012) that encompasses the type of sediment in which the fulgurite formed, with an additional category for exogenic fulgurites (e.g. spheroidal, botryoidal, filamentous, or aerodynamic), which are associated with Type II and Type IV fulgurites.
This typology is 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
- Type V - exogenic fulgurites, which are hollow or filled spherules, clusters of rounded structures, filaments, aerodynamically-distorted forms, or a combination of the aforementioned habits. Exogenic fulgurites are formed from liquefied materials resulting from a powerful lightning strike that were thrown into the atmosphere above the lightnin's point of impact and solidified in the air.
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.
Fractal geometric scale-invariance in subtle proportional features do recur, however, in fulgurites, often apparent upon rotation of a specimen on any of three axes, or recursively-embedded within a larger structure. This property is also reflected in their redox phase distributions as revealed by linear analytical transects.
Role in biological and ecological systems
Carbon-based fulgurites, produced directly on biomass, such as wood, or as scavenged biogenic (humic), sediment, or precipitation (atmosphere-derived) carbon, are now known to exist; these are termed phytofulgurites. Carbon fixation can thus be achieved directly through a physical pathway within thunderstorms and in sediments. Reduction of metals and other elements in fulgurites may not require the oxidation of carbon, however, and boron or carbon may be involved with metals in a reciprocally-complex redox catalytic system. In a phytofulgurite found in Russia - the type specimen - abundant non-racemic amino acids, with preference for left-handed chirality, and other organic compounds (complex hydrocarbons) have been analytically-identified, although there is still uncertainty as to which fraction had survived the extreme electro-pyrolytic conditions, or conversely, had been authigenically-synthesized during its electro-pyrometamorphic formation. The long-term survivability of chiral organic compounds native to organisms has been demonstrated reliably in carbonaceous chondrites such as the Murchison, a CM chondrite. Reduced phosphorus as phosphides and phosphites have 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) - terrestrially extremely rare, but common on meteorites, comets, interplanetary dust, and some planetary bodies - and TiP, which is unique to fulgurites. Reduced phosphorus species were necessary for the development of DNA and RNA in proto-biotic systems that eventually emerged as primitive life, and is essential for the production of ATP, among other key functions in cellular physiology - availability of which within ecosystems is a critical factor that effectively limits population growth. There is also experimental evidence that lightning is responsible for greater than 50% of all terrestrial nitrogen fixation.
Role in planetary processes
Many observations have been made in fulgurites of high-pressure, high-temperature materials more commonly assumed to be the exclusive products of asteroid impacts, comet airbursts, or cosmic dust influx. Such materials - as a suite - formerly considered to be unique to hypervelocity impacts, have been identified in fulgurites, including the fullerene allotropes C60 (buckminsterfullerene) and C70, as well as high-pressure polymorphs of SiO2, in fulgurites.
Erosion of mountains may have a primary mechanism for initial fracture to otherwise mechanically-invulnerable rock faces in lightning discharges. Conversely, many sedimentary formations may be preserved by lightning discharges by resultant progressive fulguritic networks in loose sediments, or baking, induration, and polymerization in argillaceous sediments - preserving otherwise erosion-susceptible landforms. Paleomagnetic data are also skewed and corrupted by the phenomenon of lightning-induced remanent magnetism; this effect must be filtered from instrumental vector readings to correct for this error.
Fulgurites are appreciated by many for their scientific value as permanent tangible evidence of transient lightning strikes. For instance, the fact that fulgurites are abundant in the Sahara Desert demonstrates that lightning once was a frequent occurrence in that region. Ancient fulgurites constitute the primary data source for paleolightning research and its applications in paleoclimatology. There is philological and archaeological evidence that many ancient cultures venerated fulgurites, and recognized their relationship to cloud-ground lightning. A fulgurite was found within the contents of the ash altar at Lykaion, conforming to the Type II typology (mixed clayey/silty soil). It was assumed that the object was contemporaneously transported and deposited post factum, and not produced authigenically; this interpretation does not necessarily withstand scrutiny, as it was falsely surmised by the scientist[s] involved with the geological interpretation of the site that fulgurites require sand to form, and since no sand was found within the altar, it must therefore indicate that the fulgurite represents a symbolic sacrificial offering, although it is also possible that this fulgurite formed in situ, reinforcing belief in the power of its divine "host." Fulgurites also are popular among hobbyists and collectors of natural specimens.
Interdisciplinary implications and prospects
Although geochronological experimental and analytical studies are limited by a discipline-wide underestimation of total lightning discharge frequency (with models of periodically-fluctuating global and local rates of cloud-ground lightning naive at best, and ignored at worst), empirical evidence has been established to support the hypothesis that lightning discharges and other energetic events in thunderstorms may promote variations in cosmogenic isotope production, with obvious implications for dendrochronological radiocarbon dating of tree ring sequence incongruities and their calibration. The total contribution of thunderstorm-mediated radiocarbon production, thus, to absolute global accumulation from cosmic and endogenous sources of 14C, relative to stable carbon isotopes, may exceed background insignificance. L. M. Libby and H. R. Lukens, in 1973, empirically and analytically-demonstrated that direct lightning discharge into terrestrial carbon-rich substrates may account for 1% of total 14C production, which by extension could also explain, ceteris paribus, at least some fraction of secular radiocarbon calibration error. Ion condensation in target material by route of chemical infusion by a "dusty" or "dirty" plasma, or direct synthesis by hot/fast neutrons of 14C from 14N, 13C, or 17O, with minor productive reactions involving 15N, 16O|3He, and 20Ne||21Ne, is suspected to play a chief role in this process.
Currently, no exhaustive typology unequivocally accounts for chemical and structural attributes common in hybrid surface-formed fulgurites occurring in organic-rich substrates; indeed, no universally-applied classification scheme accounting for the very recently established diversity of fulgurites has yet been standardized.
Type I (sand) fulgurite, Okeechobee, Florida.
Type I (sand) fulgurites, highly weathered (paleofulgurites) Algeria
Type I (sand) Fulgurite, Mauritanian desert
Type I (sand) Fulgurites, Mauritanian desert
Type II (soil) tubaceous fulgurite, Iwamizawa, Japan
Type II (granitic soil, with abundant plagioclase clasts) tubaceous fulgurites, Coachella Valley, California
Type II (soil, clastic) tubaceous fulgurite, Lancaster, California
A Type III (calcareous/caliche) fulgurite, Atlas Mts., Morocco
Type IV (rock) fulgurite in situ - with arched geometric scar from groundcover loss due to the recent lightning discharge that created the fulgurite, Washtenaw County, Michigan
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|Wikimedia Commons has media related to Fulgurites.|
- The Bibliography of Fulgurites
- Petrified Lightning by Peter E. Viemeister (pdf)
- Interview (The Event: Petrified Lightning from Central Florida) with artist Allan McCollum along with an historical archive of sixty-six
- 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