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ssss1 (a) Edge dislocation with an extra half plane of atoms; this is a line defect because the base of the half plane can be represented by a dislocation line (⊥). (b) Screw dislocation, where a plane of atoms has been rotated relative to the adjacent plane.

Source: Klein and Hurlbut (1985). © John Wiley & Sons.

Dislocations are extremely important in the plastic deformation of crystalline materials that leads to changes in rock shape and volume without macroscopic fracturing. Dislocations permit rocks to flow plastically at very slow rates over long periods of time. Detailed discussions are available in many books on mineralogy (e.g., Wenk and Bulakh 2016) and structural geology (e.g., van der Pluijm and Marshak 2004; Davis et al. 2011). ssss1 shows how an edge dislocation can migrate through a crystal by breaking a single bond at a time. The result of dislocation migration is a crystal shape change accomplished without rupture. Dislocations are important plastic deformation processes allowing rocks to change shape without visible rupture (ssss1).