What is mechanical exfoliation?
Mechanical exfoliation is the weathering of a rock structure by freeze/thaw cycles or temperature differentials between the rock surface and interior. The exfoliation is the breaking off of the outer layer from the intact interior, akin to the removal of an onion's layers being peeled from the onion. Some rock exfoliation occurs from chemical weathering, thus the distinction from mechanical exfoliation.
The weathering process involved in the formation of tors is exfoliation (onion skin weathering). Exfoliation can be mechanical or chemical. Mechanically, exfoliation may be a result of the different expansion rates between exterior and interior layers. Chemically, the exterior of the rock can be chemically altered to minerals of higher volume, causing it to slough off from the rock to which it was formerly attached.
Exfoliation or 'onion skin' weathering occurs mainly in granitic rocks from temperature differences in the surface and subsurface due to heat, cold, and sunlight. The temperature differences can cause minute cracking which is further attacked by mechanical and chemical weathering, and the outer skin in shed, giving the rock body a rounded look.
When rocks are broken down without any change to their chemical compositions it is mechanical weathering. Causes of mechanical weathering are Freezing and melting of water, Abrasion, when the rock is weather by an abrasive agent such as wind and sand, and exfoliation occurs when a rock is brought to the surface.
Mechanical (or physical) weathering is where physical forces (abrasion, exfoliation, frost action, root wedging) break the rock into smaller pieces but the composition of the rock is fundamentally unchanged. in other terms: Mechanical weathering is the process of erosion where larger rocks are broken down into smaller rocks without changing any of its chemical nature. The rocks are basically crashed by physical breakdown, rather than chemical. :)