Not all rock is as dense as the granite and marble we use to build with. Some rocks are much more porous and permeable. This permeability allows air and water to enter into the rock body, where physics and chemistry take over to explode the rock when it heats up.
Types of Rock
There are three basic types of rock: igneous, sedimentary or metamorphic. Igneous rock is the result of volcanic activity throwing out hot lava, which then cools to a rock solid. Sedimentary rock is the product of many layers of ocean or lake sediment settling on the ocean or lake floor. Metamorphic is rock that was once either igneous or sedimentary and has been heated and compressed over time into something that is much more dense than the original rock.
The most common hard rocks are granite, marble, and slate. Marble and slate are both metamorphic rocks, which have been formed over time by the compression of limestone and shale, respectively. Granite is an igneous rock that was never exposed to air or water as it cooled down, allowing it to solidify in a very dense form.
Softer rocks, like sandstone, limestone and pumice, are not as dense as granite or marble; there is more space in between the molecules that make up the stone. This makes them both lighter, softer and more permeable to water. Both sandstone and limestone are sedimentary rocks that were formed without a great deal of compression, thus allowing for their high levels of permeability. Pumice stone is also porous, but is an igneous rock that forms when hot volcanic lava mixes with colder air or water, forming bubbles that weaken the rock structure, making it very porous.
How Rocks Explode
Air- and water-permeable rocks are much more likely to explode than dense non-permeable rocks. This is because air or water is absorbed by the rock when it is cool, and then the air or water molecules trapped inside the rock expand faster than the solid rock when it heats up next to the fire . If there is a high enough volume of water in a hot, porous rock, the rock will explode when the force of the expanding steam gas inside is greater than the rock can contain.