Final answer:
When stress is applied at shallow levels, rocks initially behave elastically and return to their original shape. However, as the stress increases, rocks undergo plastic deformation, bending or fracturing instead of returning to their original shape.
Step-by-step explanation:
When stress is applied at shallow levels, rocks initially behave in an elastic manner, returning to their original shape after deformation. However, as the stress increases, rocks reach their elastic limit and undergo plastic deformation, leading to bending or fracturing. The extent of deformation depends on various factors such as the type of stress, the rock type, depth, and pressure and temperature conditions.