Plasticity is the capacity to resist plastic deformation (dislocation movement of a solid material undergoing non-reversible changes of shape in response to applied forces), while toughness measures the ability of a material to resist crack propagation.
The rocks, subjected to external forces, are deformed continuously and permanently, without however being subjected to rupture phenomena and without returning to their original shape once the external stress has ceased. The field of plastic deformation is between the plasticity limit and the breaking one.
Plasticity, and therefore yielding, are representative, at the macroscopic level, of phenomena that are explained at the microscopic level, on the scale of the molecular organization of the material. In metallic materials, plasticity can be explained on the basis of irreversible changes of their crystal lattice, that is in terms of dislocation theory (this conclusion was reached simultaneously in 1934 by scholars Egon Orowan, Michael Polanyi and Geoffrey Ingram Taylor).
In other materials, such as polymers, the plastic behavior cannot be explained by the aforementioned dislocation theory, because they lack a regularity of molecular structure (the crystal structure). In such a case, plasticity is explained as an effect of stress that induces a regularity, oriented according to the stress, in the chaos of the polymer’s molecular chains.
Rocks, subjected to external forces, deform continuously and permanently, but without undergoing fracture phenomena and without returning to their original shape once the external stress ceases. The range of plastic deformation is between the plasticity limit and the rupture limit. The first one, also called limit of elasticity, divides the field of elastic deformation from the one of plastic deformation and corresponds to the minimum value that a force must have to deform in a gradual and permanent way a rock; this value depends, in addition to the intrinsic characteristics of the rock, on the modalities with which the force is applied (in particular the time of application and the direction according to which the stress is exerted in anisotropic rocks) and on the conditions of pressure and temperature. The fracture limit marks the boundary between plastic and clastic deformation, beyond which, due to the establishment of discontinuity surfaces, there is usually movement of the rock parts.
The plasticity of a sedimentary rock is very high in the stage prior to lithification because of the inconsistency between the granules and the presence of water in the interstices: the deformations that occur in a sediment during diagenesis have an almost fluid aspect. Rocks apparently rigid can behave plastically if the time of application of deforming forces is long enough: the tectonic forces that act in the course of geological times can in fact plastically deform most rocks, even those normally more rigid.
Plasticity increases if there are discontinuity surfaces in the rock: thickly stratified rocks, or schistose, are more easily bent than massive rocks or in banks. As temperature increases, even the most rigid rock can deform plastically; thus, in high-grade metamorphic environments all rocks can deform in the plastic field, especially if a fluid phase is present. A rock that plastically absorbs the stresses to which it is subjected is called incompetent, whereas if it reacts rigidly it is called competent.