What does undercutting refer to in the context of mass movement?

Undercutting, in the context of mass movement, specifically refers to the process where water erosion occurs at the base of a slope. This type of erosion can weaken the stability of the slope by removing material from underneath, creating an overhang or a steep grade that may eventually collapse due to lack of support.

When water flows against a slope, it can wash away soil and rock at its base, which undermines the structural integrity of the materials above. This erosion is a significant factor in natural processes such as landslides, as it can lead to a sudden failure of the slope when the support is no longer adequate to hold up the mass above.

The other options involve different processes or actions that do not directly pertain to the erosive process at the base of slopes. Adding vegetation can help stabilize soil and reduce erosion but does not define undercutting. Human modifications might alter the landscape or affect erosion but do not represent the natural occurrence of undercutting. Animal activity could contribute to localized soil disturbance but is not synonymous with the broader geological process of undercutting caused by water erosion.