A lightning strike can do something surprising to sand: turn part of it into glass. When a bolt enters sandy ground, intense heat can melt the grains along its path. The molten material cools rapidly, leaving behind a glassy formation called a fulgurite. It is a lasting trace of an event that happened in an instant.
Fulgurites rarely look like the clear glass in a window. Many are hollow, irregular tubes, with a smooth or glassy interior and a rough outside coated in sand. Some branch underground, following the route the electrical discharge took. Others form crusts where lightning strikes rock rather than loose sand.
Why does sand work? Much sand contains quartz, a mineral made of silica. Lightning can heat the material enough to melt it; as it cools, some of that melted silica becomes natural glass. The Utah Geological Survey’s guide to fulgurites explains why these formations may be tubes or crusts rather than neat, solid pieces.
You need not imagine a pristine crystal buried under every strike. A fulgurite may be dark, gritty, and easy to mistake for an ordinary clump of earth. At Great Sand Dunes National Park, lightning-formed glass tubes are blackish. The park asks visitors who find one to enjoy it where it lies and leave it for others.
The safest way to appreciate this phenomenon is after the storm, never by searching for a fresh strike. If you hear thunder, head indoors: the National Weather Service’s lightning safety advice says there is no safe place outside when thunderstorms are nearby. A piece of lightning-made glass is fascinating, but no discovery is worth staying out in its path.