Fulgurite: What Happens When Lightning Strikes Sand

Jul 21, 2015 2 comments

A single bolt of lightning can deliver 5 gigajoule of energy enough to power an average U.S. household for more than a month. When such a powerful lightning bolt strikes a sandy area like a beach or a dune, the sand particles can melt and fuse together in less than a second. Sand melts at about 1800 degrees Celsius, but the temperature in a bolt of lighting can reach 30,000 degrees, or more than five times the temperature on the surface of the sun. If conditions are right, the fused sand forms long hollow tubes called fulgurite. The term comes from the Latin word fulgur, which means "lightning". Although lightning strikes earth at least a million times each day, only rarely does fulgurites form.

Fulgurites are usually found beneath the surface of the sand, generally decreasing in diameter and sometimes branching outs as they descend. Their shape reflects the path lightning bolt took as it dispersed into the ground. Because of this, fulgurites are sometimes called “fossilized lightning”.


A 14 inch fulgurite found near Queen Creek, Arizona. Photo credit

Fulgurites look like roots, due to its branching, and have a rough surface, covered with partially melted sand grains. But the inner surfaces are usually smooth and glassy due to rapid cooling and solidification of the sand. The size and length of a fulgurite depends on the strength of the lightning strike and the thickness of the sand bed. Many sand fulgurites average an inch or two in diameter and can be up to 30 inches long, but fulgurites as long as 16 feet have been found. Some fulgurites can penetrate deep into the soil, sometimes occurring as far as 49 feet below the surface that was struck.

Fulgurites can also form when lightning strikes rock, occurring as coatings or crust of glass and sometimes as veins on the rock surface lining preexisting fractures within the host rock.

Fulgurites have been described as early as 1711 and are found all all over the world, from mountain peaks to the Sahara desert, but are considered to be rare. They aren’t precious but are appreciated by many for their scientific value. By studying the distribution of fulgurites over a specific area, for instance, one can infer the occurrence of thunderstorm activity in the area during a certain period, which in turn can help understand past climates. 250-million-years-old fulgurites found in the Sahara has shown, or rather confirmed, that the desert was once a fertile region where rain thunderstorms were common.



6.5" hollow sand coated fulgurite from Sahara Desert in Boujdour Province. Photo credit


Close up of the hollow opening on the previous specimen. Photo credit


Sand fulgurites from Algeria. On display at the San Diego County Fair, California, USA. Photo credit


Fulgurite found in the Mauritanian desert. Photo credit


Fulgurite found in the Libyan Desert, Southwestern Egypt. Photo credit


This image is often circulated in social media sites as an example of fulgurite. It’s actually a sculpture (driftwood with sand toppings) created by “Sandcastle Matt”. You can see his work on his Flickr stream.


Another falsely captioned image. Fulgurites form beneath the surface embedded in sand, and only appear above it if the sand has eroded away around them. Also, as the Museum of Hoaxes rightly observes, “A fulgurite of that size would be extremely valuable, and most likely would already have been carted away to a museum. It wouldn't be sitting on a beach surrounded by holiday-goers, any one of whom could potentially break it.”

Sources: Wikipedia / Minresco / Utah Geological Survey / Fossilized Lightning by Jeff J. Person

Also see:

libyan-desert-glass trinitite


  1. When I was a boy learning to hunt in east Texas, I spent more of my time digging up root fulgurites than I did hunting. A root fulgurite is the result of lightning striking a tree. Much of the soil in the area was sandy-clay. As the lightning bolt traveled down the tree into the ground, the sandy-clay around the roots would be melted about an inch or so around the root. When the tree died, the roots would rot away over many years and leave the tubular "rocks" about four to eight inches long. Periodic geological and ecological events (floods, for example), would turn these artifacts up to the surface. They were quite intriguing to a young boy, (moreso than shooting deer, actually) but it was years before I discovered what they really were and how they had come to be.


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