Internal pressure-just like when water freezes from the outside -> inward. rock form from the outside inward. when some things solidify (water) they expand [glass jar will break if filled with water and put in the freezer]. so just like when ice cubes are dropped into water and the outer layer starts to melt [releasing the inside pressure] and crack and pop. so do rocks, the outer layer is holding in pressure and when weakened the rocks can crack, pop, or actually explode outward(flying rock very dangerous to miners).