It seems like a good idea to heat that rock up in the fire--and it may work just fine, but rocks have the potential of being explosive!
Rocks can explode in a campfire because of rapid expansion due to trapped water inside the rock, or through uneven heating. Although virtually all rocks have some amount of water inside them, porous and more permeable rocks have more water and are thus more dangerous inside a fire.
I had heard about this phenomenon, and I have personally see rocks pop inside a fire but not with a ton of force. I decided to do some research on this subject and find out how dangerous rocks in a fire really are. So, let’s talk about why you would want to use rocks in a fire, and which rocks are the safest to use.
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Some Terminology to Help the Discussion
Porosity is the measure of empty space within a rock. The higher the porosity, the more empty space is within the rock.
Permeability is the ability of the water within the pores of a rock to travel from one pore to the other.
Clay, for example, has lots of pores, but very low permeability--hence why clay is used to make pottery that can hold water, because the water does not soak through. Clay is described as impermeable, or not permeable.
Theories Why Rocks Explode in a Fire
The truth is that nobody knows the exact reasons why every rock explodes in a campfire. But there are some good guesses.
The most common theory people have about exploding rocks is the heating of trapped water within pores of the rock. The water, trapped within inaccessible pores within the rock, heats up and thus builds up tremendous energy, causing the rocks to crack, and even to explode violently.
Another theory is that because rocks are often made up of different densities of the same rock or of another type of rock entirely (for example, a piece of shale might have bits of quartz in it), thus, because the different types of rock expand differently when exposed to heat, the rock expands asymmetrically, which can cause a violent shattering of the rock when exposed to high heat.
Rapid heating of a cold rock could mean a more dangerous rock due to either theory.
Almost All Rock Has Some Water
Water is almost everyone on the planet, even within the pores of rock. Although some rocks are more porous than others, even the hardest rocks can have water contained inside. For example, granite is considered a denser, drier rock, but even granite contains water.
Limestone and sandstone are more porous than granite, so their capacity to hold water is greater.
The porosity of rock is a factor of whether a rock is dangerous in a fire, but perhaps even more important is the rock’s permeability. If the rock is more permeable (meaning it’s more like a sponge), then the trapped water can escape easier, meaning that explosions, if they are to occur, will likely be less violent.
Many people, anecdotally, since this subject hasn’t been chosen for a research study, have experienced exploding rocks with shale, a less porous and less permeable rock than limestone or sandstone. Therefore, because shale is less permeable and less porous, the trapped water has a harder time escaping, and thus if the water inside the shale is heated up, can cause a dangerous explosion.
The more water that is trapped in a rock, the more dangerous the rock is in a campfire.
The Most Dangerous Rocks in a Campfire
Rocks that Contain Water
As mentioned, since all rocks have the potential to contain water, caution should be used with any type of rock you are heating up to very high temperatures.
Very porous rocks should be avoided, such as limestone, pumice, shale, and sandstone. Even these rocks have varying densities (even throughout a single rock), which means that some water could very easily get trapped inside, and crack or explode when heated.
Any rock that has been lying in a river bed, or in any body of water is suspect, no matter the type of rock. In fact, the shape of the rock may indicate how porous the rock is. Rocks that are rounded (as you find in rivers) are much more likely to carry water, because rounder grains of a rock indicates bigger pores (source).
Rocks That Are More Likely to be of Multiple Types
As explained above, one of the theories of why rocks explode is that rocks made up of multiple types of rock expand unevenly in a fire, thus causing them to crack or explode.
Conglomerate rocks are simply rocks made up of multiple types of rocks
Metamorphic rocks, are rocks that have changed from one type to another.
Examples of metamorphic rocks you’ll commonly find in the wilderness are slate and marble.
Metamorphic rocks could be a potential hazard if it’s true that a rock made up of different types of rocks heating and expanding unevenly will cause the rock to split.
Rocks Safest to Use in a Campfire
From our discussion, it seems that the best rocks to use are dense, heavy, non-porous, angular, dry, continuous (all the same rock material) and non-smooth.
The rock type is less important since even heavily compacted sandstone can fit inside this category. In general though, granite fits many of these categories and is one of the safer rock types to use in a fire.
Since granite isn’t everywhere, go for the denser rocks such as marble, slate, quartzite.
Remember that even if a rock is safer to use than another, to do all the reasonable safety precautions.
Reasons to Use Rocks in a Campfire
Whatever the reason, it’s important to stay safe. Fire is inherently dangerous, and can be totally safe if treated with respect. Never handle a rock near a fire unless you’ve made sure it is cool enough to be handled (put your hand right above it… if you can feel any heat, it’s probably too hot to handle with bare hands)
Rocks come in handy for a campfire. One common use of rocks in a campfire is to create a protective ring around the flame. This ring serves two purposes:
- Creates a barrier between the fire and the rest of the forest, keeping the rest of the woods safe.
- Creates a wind barrier which can help get the fire going, especially in damp and windy conditions
Now We’re Cooking with … Rocks!
There are several other reasons why people heat up rocks, though. One being cooking.
Ash in your food isn’t exactly a beloved delicacy. But ash in your food happens frequently whenever you put your cooking pan in the middle of coals. Additionally, the heat of the fire in direct contact with your pot also will scorch your food, not to mention leaving those sooty marks all over your cookware.
Scorched ashy food could be considered the fun of the camping experience--it all depends on your sense of adventure. However, one way people try and get around this is to put a rock that can act as a platform for their cookware, thus keeping their pan away from direct contact with the coals.
Super Cheap Shoe Dryers
Another way people use rocks is to dry their shoes. To learn more about how to dry your clothing over a campfire and use rocks to dry your shoes, check out our article on this subject.
So… I really don’t recommend trying this. My dad did this when he was a kid: he heated up some rocks and put them in his sleeping bag.
Although it did work in warming up his sleeping bag, he managed to burn a hole straight through the bag fabric--somewhat counterproductive!
Anyway, if you are extremely careful and the rocks are not too hot to handle with your bare hands, than this can work to keep your feet warm for a short time.
What Damage Can Exploding Rocks Do?
I’ve never personally been hurt by an exploding rock. I definitely have seen rocks crumble, crack, and even pop a bit when heat was added, but I did some research to find out what can happen.
In bladeforums, people are relating personal stories of being hit by exploding rocks--one person witnessed a friend get a hole burned in his sleeping bag because of a rock chip that flew out of the fire, while another saw a rock chip hit a friend in the face, while another person has personally experienced rock chips hitting his shins.
On bushcraftusa, one person used a large rock near a stream edge that shot a rock chip through his aluminum pot, while another person had a friend get a rock chip shot at them near their eye.
YouTube creator Manifestation of Imagination captured this on video with river rocks at the bed of a fire.
While the majority of the cracks and pops of rocks in a fire are harmless, some do have enough force in the explosion to cause injury.
Best Ways to Avoid Exploding Rocks
When you’re out in the wilderness, or even at a campground, using rocks for your campfire definitely has some advantages. But they do carry a risk when heated up. Ultimately, the best thing to do is to avoid rocks and fire altogether, but, there are things to do to make it less risky. This is a list of some of the things you can do to prevent any rock explosions.
Don’t Use a Fire Ring
A ring of stones around a campfire location is commonly called fire ring. It kind of sounds like a relic in dungeons in dragons, though. Fire rings have some advantages, but they aren’t essential--especially if there isn’t a fire ring already present at your campsite. You can soak the perimeter around a fire with water and clear any dead vegetation to avoid any fire spreading, and if weather conditions are good, there isn’t really any benefit to making a fire ring.
You should always douse your fire when not using it, but especially if you aren’t using a fire ring, make special care to always douse your fire completely when not attended.
Spread Out Your Fire Ring
You don’t always have to have your flame touching your fire ring. You can easily spread out the rocks and put your campfire in the middle to avoid as much contact with the fire and the rocks. The rocks, at this distance will be heated, but not as quickly and dramatically as they would if you light the fire in contact with the fire ring.
For best results, make your fire ring 3-4 feet across to give your campfire as much room as you need.
Do Not Put Rocks Directly in the Coals of a Fire
Throwing a rock into the coals means you will be rapidly expanding the rock in a short period of time. This is inviting trouble. You can still heat up a rock by putting it close to the fire, and gradually getting it closer to coals. Rapid expansion can fracture a rock.
Even a gradually heated rock is still dangerous, so continue to be cautious.
Choose the Best Rocks
Check out the rock selection section, above.
Choose dense, non-porous stones.
Don’t Use River Rocks
River rocks have had water splashed on them for potentially hundreds of years, it’s likely they’ve absorbed some of that water.
Avoid Using Rock as your Fire Bed
Having a flat, non-wet surface as the bed of your fire makes starting it much easier, but using rocks as your fire bed means that those rocks will receive the hottest coals. You can use wrist-thick logs, close together as your fire bed instead if you want a surface to build a fire on.