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The components of deformation of a rock body are rotation, translation,
distortion, and dilation. During deformation one or more of these four
components may be zero. If, for example, during deformation the rock body
undergoes no distortion or no volume change, then deformation consists
of either a rigid-body translation, a rigid-body rotation, or includes
components of both translation and rotation. In contrast, if volume change,
translation, and rotation are all zero, then deformation consists of a
non-rigid body distortion or strain. Though commonly confused with each
other, strain is only synonymous with deformation if there has been distortion
without any volume change, translation, or rotation. In short, strain
represents only one of four possible components involved in the overall
deformation of a rock body where it has been transformed from its original
position, size, and shape to some new location and configuration (van
der Pluijm and Marshak, 1997). Strain describes the changes of points
in a body relative to each other, or, in other words the distortions a
body undergoes. The reference frame for strain is thus internal.
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