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ssss1 Two‐dimensional depiction of how an edge dislocation created by slip due to shear can migrate through a crystal by breaking one bond at a time, so that no fractures develop as the crystal changes shape during deformation (steps 1–6).

Source: Adapted from Hobbs et al. (1976). © John Wiley & Sons.

ssss1 Three types of planar defect (shown in two dimensions): (a) intergranular grain boundary between two different crystals; (b) intragranular mechanical twin boundary resulting from mechanical slip; (c) intragranular subgrain boundary within a crystal, separated by a wall of dislocations. Imagine each extending in a second dimension perpendicular to the page and note how (b) and (c) accommodate changes in crystal shape.

Planar defectstwo‐dimensional defects

Point defects, line defects, and planar defects are critically important in the study of deformed rocks, particularly in the elastic and plastic deformation processes discussed in ssss1 (ssss1).