Fulgurites, the rocks created by lightning strikes on the sand

Throughout the centuries, many cultures interpreted the discovery of polished stones, generally pointed, as objects of celestial origin, produced by lightning strikes on Earth. Healing properties were attributed to them and they passed into the mythology of each place as such with different names.

In Japan they were called thunder stonesin Sweden Thor’s tightsin India lightning arrowsthe romans called them ceraunias and the greeks astropelekia (axes from heaven). Starting in the 16th century, several authors began to show their doubts about it, and today we know that all those stones were of human origin: Paleolithic axes, handaxes, and Neolithic tools.

But the point is that we also know that there is a type of metamorphic rock that is created by lightning strikes. They are rare and very difficult to find, extract and preserve, due to their fragility. Even so, some specimens are kept in museums and scientific collections around the world.

Fulgurite found in the desert in Mauritania | photo Ji-Elle on Wikimedia Commons

It is called Fulgurite and it is composed of vitrified silica in the shape of an elongated tube, which is formed when lightning strikes a sandy soil and propagates through the quartz sand, melting and vitrifying the grains. This is possible due to the high temperatures that are reached, up to 4,000 degrees Celsius, penetrating the beam to a depth of 15 meters and forming the fulgurite tubes, which are usually between 2 and 50 millimeters in diameter.

The color of the fulgurites depends on the composition of the soil and its chemical impurities. Most are usually gray in various shades, although elements such as iron can give it a greenish coloration, and other materials give it a bronze or even translucent white appearance.

Fulgurite tubes / photo John Alan Elson on Wikimedia Commons

The first documented discovery of a fulgurite was made by David Hermann in Germany in 1706. Since then they have been found virtually everywhere in the world, even in places like the Sahara where there is little lightning activity today.

For this reason they can be used as paleoenvironmental indicators, in the African case confirming that in prehistoric times the conditions were very different in that region.

The largest known mass of fulgurite was recovered from the Winans Lake, Michigan locality, extending for about 30 meters. However, the largest piece of this mass measures 4.88 meters in length. Darwin, in his work The Voyage of the Beagle, states that the fulgurite tubes from Drigg, in Cumberland, United Kingdom, were 9.1 meters long. The oldest specimen found has been dated to about 250 million years.

The most common type of fulgurite is beach sand, which is formed when lightning strikes the sand directly creating natural hollow glass tubes.

For this, it is necessary that the temperature exceeds 1,800 degrees Celsius for at least one second, the period of time in which the grains merge, leaving evidence of the path of the lightning and its dispersion on the surface or on the ground.

Since it is electricity that causes their formation, there are also fulgurites created by high-voltage power lines. This occurs when the cables break and fall onto sandy soil, forming streaks of colorful tubes on the surface.


The Agatelady / Lightning Makes Glass (Vladimir A.Rakov) / Allpe Environment / Wikipedia.

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