Weathering refers to the process by which rocks are broken apart, or chemically altered, to become sediment. This process can be further subdivided into two categories: physical/mechanical weathering and chemical weathering. Physical/mechanical weathering, as the name implies, is when the rock is physically broken into smaller pieces. This physical breaking apart results from the exposure to the atmosphere and environment: temperature changes, moisture, biological activity, etc.
One common type of physical weathering is ice or frost wedging. Frost wedging is a natural result of the fact that water expands when it freezes. If water gets into a fracture in a rock and freezes, it can expand and put pressure on the rock from within the fracture. Over time, successive cycles of freezing and thawing can cause this fracture to expand. Eventually, a piece of the rock may be broken off along this fracture.
Similarly, root wedging is the process by which a plants root system grows into existing fractures and expands these fractures as the roots grow.
Thermal expansion describes the effects of heating and cooling on a rock. Over the course of a day, week, year, etc. a rock is heated and cooled repeatedly if exposed to the elements. Consider a rock outcrop in a moderate climate. During the day, this rock is exposed to sunlight, gradually heating the rock and causing it to expand. As the temperature drops overnight, the rock begins to cool and contract. Moreover, the rock may not be uniformly heated or cooled depending on its orientation and a variety of other factors. The repeated heating and cooling places stress on the rock which can cause it to fracture and break. This may happen because existing fractures in the rock are exacerbated, or as a result of minerals responding differently to the heating and cooling with a resulting fracture along the natural mineral boundaries in the rock.
The fourth main type of physical weathering results from unloading. Unloading occurs when overlying material, such as soil or another rock stratum, is removed (most commonly through erosion) and confining pressure on the underlying rock is decreased. In response, the rock generally fractures into sheets which lie perpendicular to the direction in which pressure is released. Since the most common occurrence is the removal of a horizontal layer of material above the rock, the results of unloading are often seen as sheets of rock which lie parallel to the surface topography.