Deformation/strain and stress

The necessary vocabulary, fundamentals and definitions for ‘deformation’, ‘strain’ and ‘stress’ are provided. Types of deformation, incremental or progressive, pure shear and simple shear, deformation regimes, flow lines and vorticity number, shortening, extension and strain measurements are explained. The concept of stress acting on a surface, through its normal and shear components is presented, along with their graphical representation using the Mohr diagram. In the elastic domain that characterizes very small strains, the relationship between stress and strain is discussed through the elastic constants among which the shear modulus and Poisson’s coefficient are notable. Finally, the stress–strain relationships for the ductile (plastic) and viscous behaviours, characteristic of large deformations, are discussed. These form the basis of understanding the rheology of the Earth, and hence Tectonics.