The Guide to Rocks and Minerals

In focus: Fulgurite

Chemical Composition: Fused Quartz (SiO₂)
Colour: Black or Tan , Green, translucent White.

Fulgurite is a name given to Fused Quartz, Si02. The fusing takes place as a result of lightning striking the Earth and melting the sand where it hits. The name comes from the latin Latin word 'fulgur' meaning 'thunderbolt'. The best known Fulgurites are found in Quartz sands, where the Fulgurites take the form of tubes. The tubes can be up to a couple of centimeters in diameter, and meters long. They are normally very smooth inside or lined with fine bubbles. On the outside they are coated with rough sand particles. They have a very characteristic root-like shape which follows the path of the lightning.

When the lightning strikes the Earth and moves through the sand, the sand is instantaneously superheated. The Quartz particles melt into glass which then cools down, showing the path of the lightning as a tube of melted sand. The process involves extreme temperatures. The melting point of the quartz is about 1620^oC. This is why Fulgurites are also referred to as Petrified Lightning. Fulgurites are not very common. However, a number of them have been found. A Fulgurite found in South Amboy, New Jersey was roughly nine feet long with a diameter of three inches near the surface of the ground. The largest Fulgurite known is at the Peabody Museum of Natural History at Yale University. It was found in the shores of Lake Congamond in northern Connecticut. and was almost 4 meters long.

Fulgurites are yet another example of a transformed Earth material due to exposure to extreme heat. We have previously discussed Tektites (a general term used for pieces of natural glass which originate from meteorite impact ) and desert glass created by a huge fireball created by exploding rocks in the Earth's atmosphere. The process involved - melting and rapid cooling of the rocks - is very similar in all three cases. However, in each case the source of heat is different.

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