In geology competence refers to the degree of resistance of rocks to either erosion or deformation in terms of relative mechanical strength. In mining 'competent rocks' are those in which an unsupported opening can be made. Competent rocks are more commonly exposed at outcrop as they tend to form upland areas and high cliffs or headlands, where present on a coastline. Incompetent rocks tend to form lowlands and are often poorly exposed at the surface. During deformation competent beds tend to deform elastically by either buckling or faulting/fracturing. Incompetent beds tend to deform more plastically, although it is the 'competence contrast' between different rocks that is most important in determining the types of structure that are formed. The relative competence of rocks may change with temperature, such as in metamorphosed limestones, which are relatively competent at low metamorphic grade but become highly incompetent at high metamorphic grade.
Resistance to erosion
Resistance to deformation
Unmetamorphosed or weakly metamorphosed sedimentary and volcanic rocks
Most sedimentary and volcanic sequences show layering between different lithologies. When affected by deformation the response of the sequence depends on the competence of the various layers.
Crystal plasticity starts to become more important as the temperature increases and, from low-grade metamorphic conditions upwards, the relative deformability of individual minerals begins to control the competence of particular rock units.