From AMS Glossary
A glassy, rootlike tube formed when a lightning stroke terminates in dry sandy soil.
The intense heating of the current passing down into the soil along an irregular path fuses the sands. Concurrently, vaporization of soil moisture and possibly even vaporization of the sandy materials causes the molten material to be expanded into a tube with a diameter that may be over 2.5 cm (1 in), but with a very thin wall. Fulgurites have been recovered in lengths of over 1.5 m (5 ft).