Fulgurite
Typical broken fulgurite sections.
Fulgurites (from the Latin fulgur, 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 mineraloid lechatelierite. 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 quartzose sand, 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.
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.
Description
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 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).
Peak temperatures within a lightning channel exceed 30,000 K, with sufficient pressure to produce planar deformation features in SiO2, a kind of polymorphism. This is also known colloquially as shocked quartz.
Classification
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
Significance
Paleoenvironmental indicator
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 record
Many
high-pressure, high-temperature materials have been observed in
fulgurites. Many of these minerals and compounds are also known to be
formed by synthetic or meteoritic sources such as nuclear weapon tests, hypervelocity impacts, and cosmic dust. Shocked quartz was first described in fulgurites in 1980. Other materials, including highly reduced silicon-metal alloys (silicides), the fullerene allotropes C60 (buckminsterfullerene) and C70, as well as high-pressure polymorphs of SiO2, have since been identified in fulgurites.
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.
In material culture
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.
