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(geology), the occurrence of relatively young strata of rock on the erosion surface of older strata. There are two main types of unconformities: parallel and angular. In parallel unconformities (disconformities), strata of different ages lie parallel to one another and are separated only by an erosion surface—a stratigraphic break. In angular unconformities (disconformities), strata of different ages are not only separated by an erosion surface but dip at different angles; as a result, the ends of the old strata abut the base of the younger overlying stratum. Angular unconformities are formed when previously deposited strata have been tilted or folded and then the upper parts of the strata destroyed (by marine erosion, for example) and layers of a new stratum deposited on the erosion surface.
The study of unconformities is of great importance in elucidating the history of the movements and deformations of the earth’s crust. Analysis of unconformities also helps date magmatic activity, ore formation, and other geological processes.