Fulgurites (from Latin Fulgur, lightning) are produced by lightning striking exposed surfaces.
These fulgurites have been found at the feet of sand dunes of the Great Sand Sea, in the SW of Egypt.
Their tubular shapes are typical of lightning stricking sands. On rocks, they may have different shapes.
It has been estimated that each flash releases an average energy of 1 billion Joules. Air temperatures may reach momentarily 10,000°K, even sometimes 30,000°K. The peak currents measured range from 10 kA to 100 kA but they last only a few microseconds.
This high energy will melt or vaporize the sands or rocks with formation of amorphous glass, named by A. Lacroix who studied this phenomenon since 1915: Lechatelierite (after the french mining engineer, Le Chatelier).
Tubular fulgurites are know that exceed 8 meters but their recovery as a whole is impossible; they are very fragile and are usually found in pieces of 10 to 15 cm, with diameters ranging from 5 to 20 mm.