When rocks decay and break down into smaller pieces, sometimes into unusual shapes, the process is called weathering. It is ordinarily a very gradual process, involving both physical pressures and chemical changes, that takes place over many years or even centuries. However, the sudden temperature change produced by a forest fire can sometimes have a rapid effect on weathering.
The two basic types of weathering are physical and chemical. Physical weathering refers to weathering caused by frost damage, abrasion from contact with other rocks, tree roots growing through cracks in the rock or any other kind of physical pressure. Chemical weathering refers to weathering caused by changes in the chemical structure of the rock. Some types of physical weathering are caused by temperature changes, although under most situations, the effects of this kind of weathering are very gradual.
Water and Ice
When a rock becomes wet -- for example, as a result of a rainstorm or a stream flowing over the rock -- the water is able to flow into every tiny crack or hole in the rock's structure. If the temperature drops suddenly before the water evaporates, it will expand as it turns to ice. Ice takes up about 10 per cent more space than liquid water, and the sudden expansion of the frozen water in a limited space puts enough pressure on the rock to drive the edges of a small crack apart from each other and make it larger. When the rock gets wet again, even more water can get into the larger crack, and when that water freezes it will make the crack larger still.
Changes in temperature can affect weathering in a rock indirectly when water freezes, but they can also affect it directly due to changes in the rock itself. When a rock is exposed to sunlight, the temperature of the rock will increase and the rock itself will expand a little. After the sun sets, the rock cools down and contracts back down to its former size. Over a long period of time, this can actually cause the molecules that make up the rock to decay and cause cracks in the rock's structure. Of course, water can then get into those cracks and accelerate the process.
A sudden change in temperature can contribute to weathering in a rock through either ice damage or expansion and contraction, but the changes caused by a forest fire are both more sudden and more extreme. When a rock is exposed to the high temperatures generated by a fire, it can expand enough to crack the rock. The effect of such a sudden temperature change is much greater than the gradual weathering caused by the daily expansion and contraction of the rock as it heats and cools, which often produces no noticeable changes.