Types of Weathering Mechanical or Physical - the breakdown of rock material into smaller and smaller pieces with no change in the chemical composition of the weathered material. Chemical - the breakdown of rocks by chemical agents. Obviously the chief chemical agent is water which carries dissociated carbonic acid. Mechanical Weathering Expansion and Contraction - the thermal heating and cooling of rocks causing expansion and contraction. Frost Action - Water freezes at night and expands because the solid occupies greater volume.
Action wedges the rocks apart.
Exfoliation - process in which curved plates of rock are stripped from a larger rock mass. Example Half Dome. Other types - Cracking of rocks by plant roots and burrowing animals. Quartz remains quartz. Grains are rounded.