Mechanical weathering is the process of breaking down rocks into smaller pieces without changing their chemical composition. There are several causes of mechanical weathering, but not all processes that affect rocks are related to mechanical weathering.
The options listed include:
Temperature changes: This is a cause of mechanical weathering. When rocks are exposed to fluctuating temperatures, they expand when it's hot and contract when it's cold. Over time, these repeated expansions and contractions can cause the rocks to crack and break apart.
Frost action: Also known as freeze-thaw weathering, this is another form of mechanical weathering. Water can seep into cracks in the rocks, and as temperatures drop, the water freezes and expands. This expansion exerts pressure on the rock, gradually breaking it apart.
Alternate wetting and drying: This causes mechanical weathering by changing the volume of clay minerals found in some rocks. When these rocks get wet, the clay swells, and when dry, it contracts. This constant expansion and contraction lead to mechanical breakdown.
Rain action: This is not primarily a cause of mechanical weathering. While rain can contribute to weathering processes, such as chemical weathering where it reacts with minerals in rocks to form new compounds, its primary role in mechanical weathering is minimal compared to other factors.
Therefore, among the options provided, rain action is the one that is not a direct cause of mechanical weathering.