Non-elastic deformation is conveniently classified into brittle and plastic (ductile) deformation based on the scale of discontinuous deformation. Although the main focus of this book is plastic (ductile) deformation, some knowledge of brittle deformation is critical to the understanding of rheological stratification of the lithosphere. Also an understanding of brittle to plastic transition is essential for the understanding of the origin of deep and intermediate earthquakes. This chapter provides a brief summary of brittle deformation and the major differences between brittle and plastic deformation. The nature of brittle–plastic transition is discussed based on the different dependence of strength in two regimes on temperature and pressure and other parameters.

Key words micro-cracks, faulting, Coulomb–Navier's law, Byerlee's law, pore fluid pressure, brittle fracture, plastic flow, brittle–ductile transition, semi-brittle deformation, brittle–plastic transition.