If you have seen the movie Sweet Home Alabama, you may remember the dialog, "That's what happens when lightning strikes sand." The words are accompanied by a beautiful glass sculpture found on the beach.
Although seemingly perfect for a Hollywood movie scene, the transparent and clear glass sculpture shown in the movie is not at all close to the actual geological formation resulting from a lightning strike.
We have learned about the variations in the speeds of light and sound through the example of lightning; we hear thunder after we see the flash in the sky. Similarly, let us try to understand the science behind the interaction of lightning and sand, the formation of fulgurites, and their significance.
What Happens When Lightning Strikes Sand
Lightning is one of the two phenomena (the other being meteorites) that result into the formation of natural glass. There is high amount of silica present in beach sand at some shores. This is silicon dioxide, or quartz (colorless). Lightning, being a very high voltage electric discharge, is characterized by very high temperatures.
So, it is the release of such high atmospheric electricity through the clouds that comes in direct contact with the ground, and this process occurs in the span of approximately one second or less.
What results from this instant interaction is also known as petrified lightning, hinting at the stone-like stiffened structures formed of tiny sand granules. Lightning hits the sand, passing through the ground like a spread-out root structure of a tree.
Patterns of this tree-root spreading have been found on rock and ground surfaces. Lightning merges the sand particles into root-like structures or dendritic shapes.
What are Fulgurites?
When lightning strikes silica, quartzose sand, and soil through the ground, it creates long and hollow tubes known as fulgurites (though glass tubes, they are not transparent or clear-glass structures).
The name fulgurite stems from the Latin word 'fulgur', meaning thunderbolt. They also denote the path followed by a lightning bolt underground. Of about 100 lightning occurrences per second, one-third affect the ground, and thus, may form fulgurites.
These unusual geological formations are lechatelierite (silica glass or amorphous SiO2) mineraloids, which means that they are mineral-like, but not crystalline. They are of light, smoky-gray color, with smooth and sandy outer surfaces.
The interior looks more or less opaque. Tiny bubbles of gas are trapped inside them. There are two types of fulgurites - sand and rock fulgurites.
The ones found on beaches and in deserts are sand fulgurites; these are also the more commonly spotted than rock fulgurites. Their diameter is generally 1 to 2 inches, and their length can be up to 30 inches. The length and overall size of a fulgurite is decided by the strength of the lightning discharge and the thickness of the sand bed.
The glass coatings or crusts formed over rocks due to lightning strikes are rock fulgurites. These appear like veins or channels on the rock surfaces. They are found over mountain tops, near the summits, and thus across the world's mountain ranges.
Significance of Lightning Strikes Causing Fulgurites
Fulgurite specimens found in the Sahara desert have helped provide an insight into the climate and ecology of the region in the past. This specimen contained embedded bubbles, which were opened using laser. They released the gases of carbon dioxide, carbon monoxide, and nitric oxide.
Lightning had caused oxidation of organic matter in the soil, leaving behind these gases. Ratios of carbon isotopes in the gases determined that the organic matter originated from grasses and shrubs of the hot, semi-arid environment. The current climate of southwestern Egypt is far from this type of climate where fulgurite specimens had formed.
Using the technique of thermoluminescence (property of luminescence from accumulated energy used in dating old objects), this Sahara fulgurite was estimated to have formed around 15,000 years ago.
Human interference or activities also result in causing the same effect as formation of fulgurites. High voltage electrical cables breaking off and falling down on the sand, aided by conductive surfaces underneath, is an apt example of this.