Flow and Deformation

A hunter who investigates tracks in muddy ground near a waterhole may be able to reconstruct which animals arrived last, but older tracks will be partly erased or modified. A geologist faces similar problems to reconstruct the changes in shape that a volume of rock underwent in the course of geological time, since the end products, the rocks that are visible in outcrop, are the only direct data source. In many cases it is nevertheless possible to reconstruct at least part of the tectonic history of a rock from this final fabric. This chapter treats the change in shape of rocks and the methods that can be used to investigate and describe this change in shape. This is the field of kinematics, the study of the motion of particles in a material without regard to forces causing the motion. This approach is useful in geology, where usually very little information can be obtained concerning forces responsible for deformation. In order to keep the discussion simple, the treatment is centred on flow and deformation in two dimensions.