A fulgurite is a natural glass structure created when lightning strikes sand, soil, or rock and melts it almost instantly. The word comes from the Latin “fulgur,” meaning lightning, and fulgurites are sometimes called “petrified lightning.” They typically form as hollow, irregular tubes with a glassy interior and a rough, sand-coated exterior, and they can range from less than an inch to several feet long.
How Lightning Creates Glass
Fulgurite formation requires more than just the initial lightning bolt. A cloud-to-ground lightning strike happens in stages: first, a massive current surge (on the order of thousands of amps) lasting less than a millisecond, followed by a longer, lower-energy “continuing current” of a few hundred amps that can last up to half a second. Lab experiments replicating lightning strikes have shown that the initial high-energy burst alone only partially melts sand grains. It’s the sustained continuing current that transfers enough heat to fully melt the material and form a fulgurite body.
When lightning hits, it superheats the ground material along its path. Sand is mostly quartz (silicon dioxide), which melts and then rapidly cools into a glass called lechatelierite, a naturally occurring fused silica. This is the same type of glass found in tektites, the small glassy objects created by meteorite impacts. The composition of the surrounding soil or rock determines how much energy the lightning needs to start the melting process, which is why fulgurites form more easily in some materials than others.
What Fulgurites Look Like
From the outside, a fulgurite looks rough and unassuming. Unmelted sand grains cling to the outer wall, giving it a crusty, irregular texture. The color is typically darker than the surrounding soil, often brown or grayish. Cut one open, though, and the interior tells a different story: the central channel is entirely glassy and smooth, lined with tiny gas bubbles (called vesicles) of various shapes and sizes. The wall thickness varies from about 1 to 3 centimeters, with partially melted grains scattered through the glass and fully unmelted grains concentrated toward the outer edge.
Most fulgurites are tube-shaped, following the path the lightning took through the ground. Larger specimens branch like tree roots. One natural specimen studied by researchers measured about 112 cm long and 105 cm wide, with several small branches connected to two main branches. Many fulgurites found in the wild are much smaller, sometimes just an inch or two.
Five Types of Fulgurites
Not all fulgurites look the same. Researchers classify them into four main types and one minor type based on what material the lightning struck:
- Type I (sand fulgurites): The most familiar kind, with thin, glassy walls. These form in sandy environments like beaches and deserts.
- Type II (clay fulgurites): Formed in clay-rich soils, with thicker walls that contain more melted material.
- Type III (caliche fulgurites): Formed in calcium carbonate-rich desert crusts, with thick walls but relatively little glass.
- Type IV (rock fulgurites): Created when lightning hits exposed rock, producing a glassy coating or crust on the rock surface rather than a hollow tube.
- Droplet fulgurites: A minor type that looks quite different from the others. These are small, bead-like formations related in composition to clay or rock fulgurites.
Where Fulgurites Are Found
Fulgurites have been found worldwide but are relatively rare. Sand fulgurites, the most common type, turn up in beach and desert regions where the sand is clean, dry, and free of fine-grained silt or clay. Rock fulgurites are primarily found on or near mountain summits, where exposed rock faces frequent lightning strikes.
Rock fulgurites have been documented on Mont Blanc in the French Alps, across the Pyrenees, and throughout western U.S. mountain ranges including the Sierra Nevada, Cascades, Rockies, and Utah’s Wasatch Range. In the Wasatch, they appear mainly on summits made of quartzite, at elevations above 10,000 feet. Geologists have found both sand and rock fulgurites on peaks like Mount Timpanogos (11,749 feet) and Broads Fork West Twin (11,328 feet). Desert areas in the American Southwest also produce sand fulgurites regularly.
What Fulgurites Tell Scientists
Fulgurites are more than geological curiosities. The tiny gas bubbles trapped inside them capture a snapshot of the atmosphere and soil chemistry at the exact moment lightning struck, and that information can survive inside the glass for millions of years.
In one striking example, researchers analyzed gas bubbles in a fulgurite from the Libyan Desert dating to the Late Pleistocene. By measuring the carbon dioxide, carbon monoxide, and nitrogen oxide trapped in the glass, they determined that the soil at the time of the strike contained about 0.1% organic carbon. The chemical signature of that carbon matched vegetation adapted to hot, semi-arid conditions, similar to what grows today in the Sahel region of Africa, which sits roughly 500 miles farther south. This meant the Sahel’s semi-arid grasslands once extended much farther north than they do today, reaching at least to 24°N latitude. The age of the fulgurite could be determined through luminescence dating of the glass itself, giving scientists both a “what” and a “when.”
This approach, combining gas analysis with age dating, opens a window into ancient landscapes. Fulgurites preserve direct chemical evidence of local soil and vegetation at a specific point in time, making them a unique tool for reconstructing how environments like the Sahara have shifted over thousands of years.
Composition and Chemistry
The primary component of most fulgurites is silicon dioxide, the same compound that makes up quartz sand. When lightning melts quartz, it loses its crystalline structure and becomes amorphous (non-crystalline) fused silica, or lechatelierite. This material is technically classified as a mineraloid rather than a true mineral because it lacks the ordered atomic structure that defines minerals.
Most fulgurites contain at least two distinct components: a lechatelierite glass core and a surrounding groundmass of more varied composition that depends on whatever else was in the soil. In rock fulgurites, the glass also contains significant aluminum oxide alongside the silica. The glass itself has a porosity of about 5 to 7 percent, meaning a small fraction of its volume is made up of those trapped gas bubbles that prove so useful to researchers.