A fourth mechanism of mechanical weathering is CRYSTAL GROWTH.  Most often the crystals involved are simple, ionically bonded precipitates that form as water in cracks evaporates.  Such compounds are called SALTS.  Ice crystals and other minerals that form in near-surface environments can also do the work.

Any crystal is able to exert a force in the direction it is growing -- a force called the FORCE OF CRYSTALLIZATION.  This force is often directed against the walls of any cavity or crevice in which the crystal grows because most crystals growing in such places nucleate on one wall and grow their elongate dimension directly perpendicular to it.  Thus, the force of crystallization is directed against the opposite wall (or against the ends of other crystals growing from the opposite wall).  The diagram illustrates the effect.

Animals and plants, and most particularly tree roots, also contribute to mechanical weathering by helping to open cracks.  The outward growing force of a tree root has the same effect, though at a lower pressure, that the force of crystallization of a crystal.