Yes, it does happen. It is caused by heating rocks that have been exposed to water for long periods of time, e.g. river bed rocks. When heated, the water that has absorbed into the rock evaporates and builds pressure inside the rock, eventually causing it to fracture, fragment, and fly all over the place.

Edit: Oh, well. dougwalkabout beat me to it. wink


Edited by JCWohlschlag (10/03/07 11:09 PM)
Edit Reason: Too slow.
_________________________
“Hiking is just walking where it’s okay to pee. Sometimes old people hike by mistake.” — Demitri Martin