On Occasion, Lightning will strike an outcropping of rock (or other hard material like concrete). When this happens, the lightning will melt a channel through the rock, creating a "rock fulgurite". Rock fulgurites are a part of the surrounding material, and trace the exact path the lightning took as it passed through the material. The heat of the lightning is sufficient to change the material as the bolt passes through it, leaving a kind of "fossil" behind.
Let's start with an interesting fact: A typical cloud-to-ground lightning bolt is hot. Very hot. The air surrounding a lightning stroke is superheated plasma, which can be anywhere between 30,000 and 50,000 degrees Fahrenheit. By way of comparison, the outer layer of the sun is around 11,000 degrees Fahrenheit.
Worldwide, there are about six thousand lightning flashes per minute. Not all these flashes are cloud-to-ground strokes, but even so, that's a lot of lightning strikes every minute.
Some of these lightning bolts hit sand. Sand is primarily silicon dioxide, SiO2. When sand is heated above about 900 degrees Fahrenheit or so, it melts and fuses into an amorphous solid that you're probably familiar with; in this form, it's called "glass."
When lightning meets sand, all the energy in the lightning bolt spreads out, melting it into glass as it goes. The result is a formation called a "fulgurite," a brittle glass tube that traces the pattern of the lightning in the sand.
CSI MiamI
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