i). Mechanical weathering leads to a physical disintegration of the original rock mass into smaller particles. This can be caused by any one of several natural agents. For example, freezing of water within a crack produces an expanded wedge of ice which forces the walls of the crack apart.

ii). If the process is repeated by alternate thawing and freezing, fragments from the outer surface of the rock eventually break off to form loose scree. The same mechanical effect may be produced locally in the rock by chemical reactions between certain minerals and water that has penetrated along cracks.

iii). The hydration of these minerals produces a local increase in volume, and local pressure causes disintegration of the rock. Similarly, entry of water into the minute void spaces in rocks may allow salts to crystallize there and press against the walls of the void, thus weakening the rock.

iv). Other processes involving the action of water in its various forms include erosion by ice, wave action at coasts, river erosion, and slopes being made unstable by the presence of water in the ground.

v). Expansion and contraction of the outer skin of a rock mass as it was heated by the sun and cooled at night were formerly thought to be an important agent of weathering, but careful modern studies show that this process (called exfoliation) requires water to be present for it to work.