How Lightning Strikes Sand and Creates a Fulgurite Glass Tube

When a lightning bolt strikes sand, the electrical discharge can heat it intensely enough to melt silica-rich grains in a fraction of a second. Almost instantaneous cooling then turns some of the molten material into glass, creating a structure beneath the surface known as a fulgurite. It is often hollow, uneven and branched, resembling a root frozen within the ground.

What is the glass tube created after a lightning strike?

A fulgurite is a natural formation created when the vast energy of a lightning bolt is transferred into sand, soil or rock. In sandy deposits, its most recognisable form is a glass tube with an outer coating of partly melted grains, leaving it rough on the outside and vitrified within.

Not every lightning strike reaching the ground produces a formation that can be easily located and preserved. The mineral composition, silica content, moisture level, discharge strength and features of the terrain all affect the outcome. Under favourable conditions, however, the process can leave behind a recognisable structure long after the storm has passed.

How can sand turn into glass so quickly?

The channel of an electrical discharge reaches extreme temperatures, transferring part of that heat rapidly into the ground. Studies of fulgurites indicate temperatures close to 1,800 °C in areas struck by lightning, which is enough to melt silica-rich components before the surrounding environment can absorb all the energy.

The sequence occurs extremely quickly, with heating and cooling allowing little opportunity for minerals to return to an organised crystalline structure:

• the lightning bolt contacts the sandy surface;
• the electric current spreads through the ground in several directions;
• grains close to its path are heated and partly melted;
• the liquid material gathers around the discharge channel;
• the temperature drops quickly once the current has passed;
• the silica solidifies into a glass-like mass.

Why are fulgurites hollow and full of branches?

Their root-like shape arises because electricity does not necessarily follow a straight route after entering the ground. The discharge travels along paths of least resistance and may form several offshoots. Sand fulgurites are therefore described as hollow, branching underground tubes, with a geometry that reflects part of the way energy spread through the soil.

The central cavity is associated with the superheated channel created during the discharge, as well as the rapid expansion of gases and vaporisation of materials. Around this area, the melted sand hardens and retains a glassy wall. These structures may feature:

• empty or partly empty internal channels;
• branches resembling small roots;
• a glassy interior made from melted silica;
• an exterior covered in attached sand grains;
• bubbles and small cavities trapped in the material;
• dimensions that vary greatly between individual discharges.

Why do many of these tubes remain hidden beneath the sand?

A lightning bolt can penetrate below the surface and disperse its current underground, meaning that much of the glass tube forms while buried. In deserts, dunes and other sandy landscapes, wind and shifting sediment can conceal the structure even further after a storm. Some fulgurites are only exposed when sand is naturally removed or during excavations.

Does a fulgurite really preserve the route taken by lightning?

A fulgurite's form acts as a physical record of the discharge in the ground, although it is not a perfect reproduction of the entire lightning flash seen in the sky. The tube follows the area through which sufficient energy travelled to melt the material, and studies have used its diameter and morphology to estimate how lightning energy was distributed per unit length.

These structures can retain information beyond the geometry of the discharge. Ancient fulgurites have preserved gases in bubbles within their glass and have been used to investigate environmental conditions from thousands of years ago. What begins as an electrical discharge lasting an exceptionally short time can therefore become a fragile underground tube that remains in the soil far longer than the storm that created it.