Physical weathering refers to the process of rock fracturing due to mechanical processes. It is one of two types of weathering, or breakdown of rock, the other being chemical weathering. Physical weathering is often caused by temperature changes over time, although the growth of organisms or simple changes in the load on rocks can also contribute to weathering.
There are two types of weathering directly related to temperature changes. One is temperature changes in the rock itself. The shifts in temperature between day and night, and between seasons, causes rock to expand and contract. Both the heating and cooling often take place unevenly throughout the rock structure, so this expansion and contraction causes stress in the rock, and may create or enlarge a fracture. The second type is frost wedging. Water expands as it freezes, so if water gets into a crack or crevice in the rock and turns to ice, it can expand the fracture.
Root wedging is similar to frost wedging, except that instead of expanding ice, plant roots in the normal process of growth infiltrate and expand cracks in the rock. Unloading, which occurs when overlaying soils or other materials are eroded away, also causes weathering. In this case, the rock often fractures in layers parallel to its surface.