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Definition
Weathering is the dismantling of rock into smaller constituents through atmospheric and biological processes.
Mechanical Weathering
Rock broken down physically but not altered chemically has been mechanically weathered. The most significant form of mechanical weathering is the splitting of rock by freeze/thaw cycles: Water seeped into any opening in rock expands when frozen. Even large boulders may be cleft in this manner. Such rocks are commonly seen in high mountains above timberline.
Chemical Weathering
Oxidation, hydrolysis and carbonation are examples of chemical weathering, where rock denudes due to chemical reactions.
Biological Weathering
Biotic processes also break down rocks: They may be invaded by plant roots, weakened by lichens that leach away minerals, and compromised by the burrowing of small mammals.
Exfoliation
Exfoliation, which likely involves both mechanical and chemical weathering, is the wearing away of curving rock layers. The process may occur on a grand scale, as in exfoliation domes; Yosemite's Half Dome is a prime example.
Denudation Continues
In mountains, the fractured rock that results from weathering then undergoes further denudation: mass wasting, which is the transport of rock materials downslope by gravity.