At what geological condition do ductile properties of rocks become predominant?

Ductile properties of rocks become predominant below the brittle-ductile transition. This is a key concept in geology, relating to how rocks deform under varying conditions of temperature and pressure.

Above this transition zone, typically occurring at depths of around 10 to 15 kilometers depending on the geothermal gradient and rock type, rocks behave in a brittle manner, meaning they fracture and break when stress is applied. However, as depth increases, temperature and pressure also rise, allowing rocks to flow and deform plastically. This plasticity is what defines ductile deformation.

Thus, below the brittle-ductile transition zone, rocks can undergo significant changes in shape without failing, behaving more like a viscous material. This ductile behavior is essential in understanding tectonic processes, such as the bending of rocks in mountain building and the movement of tectonic plates.

Other options refer to specific locations or conditions that do not directly relate to the fundamental understanding of rock deformation mechanics at varying depths. For example, being at sea level or surface level does not account for temperature and pressure conditions that significantly influence a rock’s state of deformation. The crust-mantle boundary, while being a significant geological boundary, is not the defining parameter for when ductility becomes predominant