What is the primary process responsible for the breakdown of rocks and soil by mechanical means?

The primary process responsible for the breakdown of rocks and soil by mechanical means is physical weathering. This process involves the physical disintegration of rocks without any change to their chemical composition.

Physical weathering can occur through various mechanisms, such as freeze-thaw cycles where water seeps into cracks in the rock, freezes, expands, and eventually causes pieces to break off. Other examples include abrasion from wind or water carrying sediment that grinds against rock surfaces, and pressure release, where overlying materials are removed and the underlying rock expands and fractures.

While chemical weathering involves changes to the mineral composition of rocks due to chemical reactions, and biological weathering refers to the breakdown caused by organisms, including plants and animals, these processes are distinct from the purely mechanical breakdown that characterizes physical weathering. Thermal expansion also contributes to rock breakdown, but it is one of the many mechanisms of physical weathering rather than a standalone process. Therefore, physical weathering is the correct answer as it encapsulates the broad range of mechanical forces acting on rocks and soil.