geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...

competence refers to the degree of resistance of rocks to deformation or flow. In mining 'competent rocks' are those in which an unsupported opening can be made.

Characteristics

Competent rocks are more commonly exposed at

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...

s as they tend to form upland areas and high cliffs, or

headlands A headland, also known as a head, is a coastal landform, a point of land usually high and often with a sheer drop, that extends into a body of water. It is a type of promontory. A headland of considerable size often is called a cape.Whittow, J ...

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 In structural engineering, buckling is the sudden change in shape ( deformation) of a structural component under load, such as the bowing of a column under compression or the wrinkling of a plate under shear. If a structure is subjected to a ...

or faulting and 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

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

s, which are relatively competent at low metamorphic grade, but become highly incompetent at high metamorphic grade. Most sedimentary and volcanic sequences show layering between different

lithologies The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lith ...

. When affected by deformation the response of the sequence depends on the competence of the various layers. Crystal plasticity becomes more important as the temperature increases and the relative deformability of individual minerals controls the competence of particular rock units.

References

{{Structural geology Structural geology