Final answer:
A rock that bends under stress and does not return to its original shape exhibits ductile behavior, but this occurs beyond its elastic limit, leading to plastic deformation.
Step-by-step explanation:
A rock that behaves in a manner where it will bend under stress and does not return to its original shape is described as exhibiting ductile behavior. When rocks are subjected to stress and strain beyond their elastic limit, they undergo plastic deformation. This means that the rock has reached a point where it no longer behaves elastically but instead deforms permanently.
Plasticity in materials signifies this irreversible deformation, typically seen in materials such as metals which show a gradual decrease in stress with increasing strain until they reach the breaking point. In contrast, elastic behavior is observed when a material returns to its original shape after the removal of the load that caused its deformation, a property of materials like polymers and certain metals at lower levels of stress.