The correct option is D.
Explanation
In geomorphology and structural geology, the deformation of Earth materials is governed by the principles of mechanics. The interaction between internal resistance and external forces determines the stability and transformation of landforms.
- Stress (Option D): This is defined as the force applied per unit area. In the context of Earth materials, stress can be compressive, tensional, or shearing. It is the primary driver that induces deformation in rocks and regolith.
- Load (Option A): This refers to the total weight or mass of the overlying material (such as ice, water, or rock) exerting pressure on a surface. While related to stress, it represents the force itself rather than the force distributed over a specific area.
- Strain (Option B): This is the resultant deformation or change in shape and volume of the material caused by the application of stress. It is a dimensionless quantity representing the ratio of change to the original state.
- Shear (Option C): This is a specific type of stress (shear stress) or deformation (shear strain) where forces act parallel to a surface, causing layers to slide past one another. It is a subset of the broader concept of stress.
Key Takeaway: Stress is the force per unit area applied to a material, whereas strain is the physical deformation resulting from that stress.