Scientists Say: Fulgurite

These glassy deposits are formed when lightning hits sand or rock


These are fulgurites, tiny glassy deposits formed when lightning hits the right piece of ground. 

John Alan Elson/Wikimedia Commons

Fulgurite (noun, “FUHL-grr-eyte”)

A glassy deposit that is often in the shape of a tube. Fulgurites are formed when lightning strikes sand, soil or rock. If that surface has a lot of silica in it, the heat and electricity from lightning may fuse it and other minerals together into glass. Silica is a grid made of silicon atoms with oxygen atoms attached, and is used in making glass.

In a sentence

When volcanoes erupt, lightning can heat the volcano’s ash in the air, forming tiny specks of glass. These are similar to fulgurites that form after lightning hits the ground.

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Power Words

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fulgurites   Glassy deposits that form in rocks or in sediment when a lightning bolt strikes the ground.

glass    A hard, brittle substance made by melting sand (silica) together with lime (a highly alkaline material obtained by heating limestone) and other ingredients, then cooling the mix quickly. Glass usually is transparent and fairly inert (chemically nonreactive).

silica  A silicon atom attached to two oxygen atoms. It is commonly found in quartz and crystal, and is used to make glass.

silicon  A nonmetal, semiconducting element used in making electronic circuits. Pure silicon exists in a shiny, dark-gray crystalline form and as a shapeless powder.


Edited 3/24/15, 9:39AM ET: The article was updated to clarify the structure of silica.

Bethany Brookshire was a longtime staff writer at Science News Explores and is the author of the book Pests: How Humans Create Animal Villains. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.

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