What defines the exact point at which a rock transitions from brittle to ductile?

The point at which a rock transitions from brittle behavior to ductile behavior is closely related to its composition. Different minerals and rock types have distinct mechanical properties, which influence how they respond to stress. For example, some rocks may fracture under relatively low stress, while others can deform plastically when subjected to higher stress levels. Composition affects factors such as mineral bonding and crystal structure, ultimately determining the rock's capacity for deformation.

While temperature, pressure, and depth also play significant roles in the behavior of rocks, they do not specifically define the transition point from brittle to ductile behavior. Temperature and pressure generally contribute to the conditions under which these transitions occur; typically, higher temperatures and pressures favor ductile deformation. However, it is the intrinsic properties of the rock's composition that fundamentally dictate how it will behave under stress, marking the crucial transition point between the two states.