In areas that have extreme temperature fluctuations rocks may be broken down by the continual expansion and contraction of the rock. If there is water present then this may be also cause the rocks to break up.
In the investigation you will designing you should model rapid heating and cooling of a rock sample. The control for this investigation could be a rock that is not heated but plunged into water an equal number of times. The independent variable would be the treatment that you give to the rock such as heating in a bunsen flame and then plunging into water to cause rapid cooling. The dependent variable would be the amount of crumbling that occurs. All other variables such as the type and size of the piece of rock should be kept equal.