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Fulgurites: When Lightning Meets Sand

By Edited Dec 3, 2013 1 2
Fulgurite a lightning bolt captured on sand
Credit: Ji-Elle through Wikimedia Commons

Fulgurites are the result of a lightning bolt striking on soil or rock and forming a crystal – all under one second. Their name comes from the Latin word fulgur, which means lightning.

This is another precious little secret that the universe has for us. It is an edaphological singularity worth preserving, because it is greatly enigmatic and beautiful. Moreover, the rays themselves reproduce the same fractal structure than Fulgurite roots, as a result of the impact. In other words, when the lightning hits the ground, it creates an object with a similar shape. Amazing, right? The fulgurites tend to be clearer (better structured) and more common in sandy landscapes (eg, dunes), but they also appear in clay soils. These are amazing formations that civilizations have considered to have all types of properties, including healing and divine. Let’s discover a bit more about them.

Fulgurites: the science behind the fact

Fulgurites are one of the three major glass formations that exist in Nature: volcanic, meteorite impact and the impact of lightning. They all result in glass formation due to the quick melting of the soil (at least at 1800ºC) and fast cooling, which leave no time for crystallization of the mineral phases. Crystals in sand and other soil types have a very organized and compact structure. However, when melted and quickly cooled down, they transform into amorphous crystals that are not as compact and therefore have different properties. In the case of glass for instance, it becomes transparent. Therefore when the lightning impacts the ground, it is transformed into fulgurites, which is first molten sand/soil, and as it comes into contact with cold air it solidifies, giving rise to these stalactite-like formations. They often contain gas bubbles due to the quickness of the process. Fulgurites usually take the shape of a hollow cylinder. The inside of the tubular structure tends to have a shiny material, while the outside is rough, due to the presence of mineral fragments that have not completely merged with the molten part or were attached during the process of formation. The structure of the stone thus tends to be dendritic, like the branches and roots of plants, reflecting the path that the bolt has followed to penetrate into the ground. The fulgurites can appear in different colors, influenced by the composition of the molten material. Bubbles of different sizes can usually be seen on the inside. They are more frequent in sandy materials because the lightning penetrates better into them. When the rays fall on a rocky surface, their branching takes a different form, essentially conditioned by the preferential fracture directions of these lithic substrates.

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Fulgurites: incredible facts

Here is a list of incredible facts about fulgurites:

  • It is believed that there are up to 1.4 billion lightning flashes every year, therefore lightning happens 40-50 times every second somewhere in the world
  • Fulgurites can be several meters long, and the deepness of the soil below the ground affected can be up to 15m.
  • They are particularly abundant in the desert of the Sahara, where it is believed that there were once many electrical storms, and the fact that there is just sand helped.
  • Also in very high mountains, where the lightning strikes preferentially (because it’s closer, acting like lightning rods), many fulgurites and marks of lightning strikes can be observed.
  • Lightning bolts typically have a diameter of between 2 and 5 cm, a speed of 4.4x105 meters per second on average (study by the University of Florida[1]), a temperature that can exceed 50,000 Kelvins (nearly 50,000ºC) and a peak current of 10,000 to 30,000 amps
  • Lightning happens more frequently in the tropical areas of the earth, because the atmospheric convection (the flow between cold and hot masses of air) is the highest and the moisture levels vary
  • Lightning hotspots in the world include Catacumbo in Venezuela, Singapore, Teresina in Brazil and Lightning Alley in Central Florida
World lightning distribution
Credit: Citynoise through Wikimedia Commons
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Fulgurites as seen in the movies

Fulgurites are pretty unknown for most of us, however some might have come to know them through these productions:

  • Sweet Home Alabama: two kids are kissing in the beach. As if to seal their love, a lightning strikes and turns sand into a beautiful branching glass structure. Sounds familiar? In this romantic comedy, Reese Witherspoon plays Melanie Carmichael, a fashion designer from New York who is to be married to the best man in town. However, she first has to convince the southerner she married when she was in high school to grant her divorce. So of course she goes back to Alabama to chase him for his signature. And that’s when… lightning happens. However the fulgurite seen in the film is a Hollywood version of real fulgurites (it is all transparent and glassy, when real ones tend to have unmolten sand granules sticked on the surface, and are not as regular and pretty)
  • The Reaping: The story focuses on a former Christian missionary (Hillary Swank) who tragically lost her family (and altogether her spiritual faith). Since then she becomes a renowned expert in unveiling religious myths. But when she travels to a small religious town in Louisiana to investigate what appears to be the apparitions of biblical plagues and science cannot explain the events that occur, she is forced to bring her faith back to fight the dark forces threatening the community. Fulgurites appear at the beginning of the movie
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May 21, 2013 9:18am
Very interesting article
May 21, 2013 9:29am
Thanks! :)
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  1. Thomson, E. M.; Uman, M. A.; Beasley, W. H. "Speed and current for lightning stepped leaders near ground as determined from electric field records." Journal of Geophysical Research . (1985): 8136.

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