Which of the following processes is primarily responsible for breaking down rocks by mechanical means?

The process primarily responsible for breaking down rocks by mechanical means is physical weathering. Physical weathering involves the physical alteration of rocks without changing their chemical composition. This could include processes like temperature changes causing expansions and contractions, freeze-thaw cycles causing ice to wedge into cracks, or the mechanical action of wind or water impacting the rock surface.

In contrast, chemical weathering involves chemical reactions that lead to changes in the mineral composition of rocks, such as oxidation or hydrolysis. Biological weathering, on the other hand, occurs when living organisms, such as plants or burrowing animals, contribute to the breakdown of rocks through their biological activities. Erosion refers to the movement of sediments and soil due to natural forces like water, wind, or ice, rather than the breakdown of the rocks themselves. Therefore, the correct answer focuses on the direct mechanical breakdown, which is characteristic of physical weathering.