I've heard that putting wet rocks on a campfire is a really bad idea. If they are porous enough, the trapped water turns to steam and presto! Exploding rock!

I'd like to know how true this is, and see how catastrophically the Mythbusters can demonstrate this effect. It has the added benefit of acting as a PSA of sorts (you can dress Jamie up as Smokey the Bear :þ

I've only seen it happen once, it wasn't a really big explosion, and it was actually a chunk of concrete in my case rather than a naturally occurring rock.

That's a pretty well known occurrence.

In fact, it's occasionally used to find some kinds of fossils. The idea is that if there is an impression fossil in a stone (lets say a nice flat image of a leaf) and you drop the stone in a bucket of water for a few days, it will absorb some water. Putting the stone in a fire will cause the water to expand into steam, particularly in the void where the impression of the leaf and break it apart. The surface where the impression of the fossil is almost always going to be the weakest structural division, so you usually get two cleanly divided halves of a fossil.

That said, yes, it can be extremely dangerous and should not be attempted. (Making it perfect for the show).

wet, exploding rocks

For someone to ask that question, implies you have not been in the construction industry. That is the worst hazard of a asphalt plant. The rocks are tumbled at a high temperature until they are dry. Adding any moisture to the cementatious oil will result in a rather nasty explosion.

DingDao

Wouldn't the rock being sedimentary or igneous make a difference?

Definitely sedimentary. Number one igneous rocks are not permeable. Neither are metamorphic. The exploding rock is most likely limestone. Sandstone is too porous, if heated the water could escape as steam. Shale is another possibility.

I have another theory. maybe it is the fact that the cooling water makes the rock colder, and the aalready hot fire might cause a reaction of sorts. I'm not sure, but I remember that heating and ooling makes things more brittle. I'm not sure of this but I'm wondering...

I grew up beside a river with a huge firepit. River rocks, rocks that have been soaking for years near the rivers bank, will explode into thin blades and shoot as far as 100 feet. I woke up one morning with plate-like blades of rocks sitting along the windowsills and on the cars. They cant be very powerful if they stopped at the windows and fell straight down, but they did travel 100 feet. My theory is that over the years of water absorption and hot summer days the rocks soaking and drying create slice-like pattern and a little heat and steam will send these flying like frisbees