When rocks are stressed, the resulting strain can be elastic, ductile, or brittle. This change is generally called deformation. Elastic deformation is a strain that is reversible after the stress is released. For example, when you stretch a rubber band, it elastically returns to its original shape after you release it. Ductile deformation occurs when enough stress is applied to a material that the changes in its shape are permanent, and the material is no longer able to revert to its original shape. For example, if you bend a metal bar too far, it can be permanently bent out of shape. The point at which elastic deformation is surpassed and strain becomes permanent is called the yield point. In the figure, the yield point is where the line transitions from elastic deformation to ductile deformation (the end of the dashed line). Brittle deformation is another critical point of no return when rock integrity fails and the rock fractures under increasing stress.
The type of deformation a rock undergoes depends on pore pressure, strain rate, rock strength, temperature, stress intensity, time, and confining pressure. Pore pressure is exerted on the rock by fluids in the open spaces or pores embedded within rock or sediment. Strain rate measures how quickly material is deformed. For example, applying stress slowly makes it is easier to bend a piece of wood without breaking it. Rock strength measures how easily a rock deforms under stress. Shale has low strength and granite has high strength. Removing heat, or decreasing the temperature, makes materials more rigid and susceptible to brittle deformation. On the other hand, heating materials make them more ductile and less brittle. Heated glass can be bent and stretched.
Table showing the relationship between factors operating on rock and the resulting strains:
|Increase Strain Rate
|Increase Rock Strength