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It seems that every tutorial I read and show I watch shows people using rocks for everything from a cooking surface to boiling water. However, I have had rocks crack in half on me, and I've always been warned that they can explode from rapidly expanding moisture embedded within them.

How much of a risk is this and how is it best avoided? A nice flat rock is a great cooking surface in the woods, so if there's a safe way to use them, I would like to.

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don't put flint rocks in a fire. very sharp shards and they do split nearly always. A good way to get some rudimentary tools tho. –  dadacucu Nov 17 at 17:18

2 Answers 2

up vote 24 down vote accepted

There's two main things that generally cause this, the first being the moisture content in the rock and the second being the type of rock. If the rock is wet and you heat it rapidly, any water will turn to steam and put pressure on the rock, forcing shards of it to break off rapidly. Secondly the type of rock matters, layered rocks such as sandstone are much more likely to split and perhaps explode because of the weaker bonds between their layers. As pointed out below, watch out for very smooth rocks - a sign that they may have been on a river bed at some point in their lives and therefore have water trapped deep in them.

If you choose hard, dry, un-layered rocks then you should be fine. When in doubt, you can build your fire on top of the rock the first night and heat it up safely covered to drive out whatever moisture may be trapped.

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Sandstone is a particularly bad rock too. –  Kevin Feb 1 '12 at 16:28
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Watch out for river rock - rocks with very smooth, rounded surfaces. They are likely to have ancient water trapped inside. –  Jay Bazuzi Feb 1 '12 at 18:01
    
@JayBazuzi Good point, I've udpated my answer! –  berry120 Feb 1 '12 at 19:49
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Can you provide any sources for this information? –  Mr.Wizard May 5 '12 at 2:55

Here is a good article on the types of rocks that explode: http://www.ehow.com/list_7360348_rocks-explode-around-fire-pits.html

Generally if you rub two of the same rocks together and they crumble easily, then they are not safe to use.

Hard rocks:

The following rocks are not very likely to explode, but should be approached with with common sense. When wet, don't use them.

Granite:

Granite

Marble:

Marble

Slate:

Slate

Soft rocks:

-These will explode if they contain moisture.

Sandstone:

Sandstone

Limestone:

Limestone

Pumice:

Pumice

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