I heard somewhere , a long time ago , that some types of rocks will explode if they get too hot from a fire . Anyone else hear of this happening ?

It's not about the rocks themselves, it's the moisture they may contain. A lot of rocks are porous. If they have been saturated with water, and you heat them in a hurry, the water can flash into steam. Massive expansion, with great force. Boom! with potential shrapnel. So, rocks from a stream bed are a poor choice for heating -- at least directly in a fire.

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.

One year we had to build a fresh fire-pit at elk camp (some wahoo took all the stones from our old, well established one). All there was available at the time was fractured basalt that had been covered in early snow. That stuff popped and hissed at us for days. We had to keep our distance or else get shrapneled out, and even at 20 feet we were still getting stung by little bits and pieces. Lesson learned.

So , you shouldn't use rocks from a river , stream or pond . But what about rocks that have been covered by snow , ice or frost - what then ?

Generally, I doubt a little bit of snow/ice/frost would penetrate deep enought to have much effect. Not enough volume of water there. But add that to a wet year, a fair bit of rain, etc., and you could have some highly interesting action. It's all cumulative, and you won't have any knowledge of most of the "accumulation" when you show up in camp.

You could, of course, warm the rocks slowly. Build a rough cairn around a small-stick fire and keep it going. You're unlikely to create enough heat to make steam, and the rocks will still heat up slowly. Over time, you will get what you want.

On the other hand, if in doubt, and wood is plentiful, fire 'em all hot and stand well back. Once the fire is down to ash and coals, you're unlikely to have any big booms in the dead of night. So you can sleep tight. (Don't look in the mirror next day, though; you'll be wearing enough soot and ash to make small children go screaming to mommy. :-)

Stay away from sand stone since it is very porous. harder igneous rocks are much safer as far as those being picked up from high ground.

Generally speaking, if you see layers in the rock (sedimentary rocks formed by millenia of deposits of sand, clay, etc., that have compacted under high pressure), there is a high degree of probability that there are pockets of moisture bound inside some of those layers. Heat the rock enough, and the water turns to steam. It has nowhere to go, so it fractures the rock - sometimes explosively.

Other rocks have a more solid appearance - think granite. Those are generally safe, even if they get wet, because they don't have water bound inside. Any moisture that soaked in can escape as steam.

The first year we moved to our property, I didn't know the difference. I dug a firepit in the beach, and lined it with nice, flat rocks that had lots of pretty colored stripes (layers). I built a nice, hot fire with the dry driftwood that was readily to hand. An hour later, we were all taking cover behind a dune in best "Sands of Iwo Jima" fashion.

Handyman and Dougwalkabout,

See my post right above yours. The stones we used around our fire ring were fractious basalt that had been sitting under 3 to 6" inches of snow for only a couple weeks; snow that had been subject to considerably thaw which would've kept the rocks nice and moist.

As a general rule, if you must use rocks that are under the snow, try and select solid igneous types that are beneath well drained soil. Even better, find ones that are beneath heavy litter (leaves, debtritous), which can act as a good moisture barrier.

Point taken. And thanks for the cautionary tale. Wet, melting snow would certainly add enough moisture to cause problems. I was thinking about the dry snow we get locally when I sent that post.