Rocks can resist compression very well, with granite having a compressive strength around 140 MPa. The topographic deviatoric stresses created by mountain ranges easily exceed the tensile strength of crustal rocks. The strength of brittle rocks increases linearly with pressure, and thus very often with depth too. Brittle fracturing, or cataclasis , occurs in this microscopic or thin-section view when tensional stresses become great enough to break across mineral grains, or sever the bonds between them and cause rows of them to roll. Shear fracture will take place on complementary planes oriented about 60 degrees from the direction of .
Elastic deformation is temporary and reversible until fracture is achieved.
Each of these mechanisms has an exponential constitutive relation in which strain increases exponentially with applied stress. Ductile constitutive relations are more sensitive to temperature and composition than to other factors. Plastic Flow produces limited, permanent strains at high stresses when the yield strength of mineral grains is exceeded and they deform by gliding along internal dislocation planes or grain boundaries. Most minerals melt at temperatures between 400 and 1800 C, so power-law creep takes place in the lower crust or deeper. For this mechanism the constitutive relation gives the strain rate with time as proportional to an exponential power of the applied stress, with the exponent being 3 or greater.