Competence (geology)

For other uses, see Competence (disambiguation).

Boudinage of relatively competent metabasic sheet within quartzofelspathic gneiss

In geology competence refers to the degree of resistance of rocks to either erosion or deformation in terms of relative mechanical strength.^[1] In mining 'competent rocks' are those in which an unsupported opening can be made.^[2] 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

Certain rock types, such as granite and limestone are more resistant to erosion due to their relatively high mechanical strength.

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.

Metamorphic sequences

Crystal plasticity starts to becomes important as temperature increases and from low-grade metamorphic conditions upwards, the relative deformability of individual minerals begins to control the relative competence of particular rock units.

References

1. ^ Fossen, H. (2010). Structural Geology. Cambridge University Press. p. 430. ISBN 9780521516648. http://books.google.co.uk/books?id=01PI5jhjiiQC&pg=PA263&dq=competent+rock+-musician+structure&hl=en&ei=Bq2VTZ2DMZWT4ga4x-3oDA&sa=X&oi=book_result&ct=result&resnum=7&ved=0CEkQ6AEwBjgK#v=onepage&q=competent&f=false. Retrieved 1 April 2011. 
2. ^ Cristescu, N. (1989). Rock rheology. Mechanics of elastic and inelastic solids. 7. Springer. p. 1. ISBN 9789024736607. http://books.google.co.uk/books?id=ocYLhBzuRA4C&pg=PA1&dq=competent+rock+definition&hl=en&ei=LqWVTbjeJc6DhQfG1bHkCA&sa=X&oi=book_result&ct=book-preview-link&resnum=9&sqi=2&ved=0CF8QuwUwCA#v=onepage&q=competent%20rock%20definition&f=false. Retrieved 1 April 2011.