In this section we briefly define terms such as elasticity, viscosity and plasticity so that our use of these terms later in the discussion is clear.
All rocks show an elastic response to an imposed stress or strain rate.
This means that there is a reversible, instantaneous deformation once the stress or strain rate is applied. We explore these effects further in Section 5. It is common in geological arguments to specifically neglect elasticity under the pretext that the folding process is slow and is therefore intrinsically controlled by viscous behaviour, the elastic deformation being small compared to the viscous part of the total deformation. This is true so long as the deformation remains homogeneous throughout with no development of instabilities such as folds or shear bands. A useful concept here is that of the Deborah Number, D e .