Great Sand Dunes rises against the Sangre de Cristo Mountains in Colorado's San Luis Valley, the tallest dunes in North America and a dunefield wide open to the sky. Lightning can and does strike it during storms. When a bolt hits the sand, the National Park Service explains, it vaporises the channel the current runs through and melts the surrounding sand into dark glass. The result is a fulgurite, a blackish tube of glass that holds the shape of the strike.
The name comes from fulgur, Latin for lightning. The park notes that Cicero used the phrase condere fulmina, to dig up thunderbolts, a sign that Romans knew these tubes from the sandy ground of Italy. The heat involved is extraordinary. The park and the National Weather Service both put lightning at about 50,000 degrees Fahrenheit, hotter than the surface of the sun, and Mark Milligan of the Utah Geological Survey told Rock & Gem magazine that sand needs around 1,800°C to melt instantly into fulgurite glass, which can trap bubbles of air.
The wind that built the dunes is also what uncovers the tubes. Creeks carried sand into the valley, prevailing winds pushed it against a bend in the mountains, and opposing storm winds squeezed it into towering ridges that still shift back and forth. Andrew Valdez, a geologist at the park, told the magazine that one strike can create a lot of fulgurites and that they turn up exposed after big wind events. Once exposed, he said, they break down quickly, within a week, and need to be treated like very thin glass.
The park asks anyone who finds one to study and enjoy it where it lies and leave it in the sand for the next visitor, since natural resources in national parks are protected by law for future generations. A fulgurite underfoot is also a reminder of how it got there, and when a storm builds over the valley the park's advice is short: come down off the dunes.
Fulgurite (lightning glass)
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