A dike (also spelled dyke) is a sheet tabular intrusion that crosscuts preexisting country rocks. In the vast majority of cases, a dike consists of igneous rocks. However, sedimentary processes may also produce sediment-filled cracks called clastic or sedimentary dikes. A dike always show discordant relationships with the foliation, bedding, or other structures in the host rocks. On the contrary, a sill is a tabular intrusive body emplaced parallel to the structures of the country rocks. Both dikes and sills are tabular inclusions, i.e. they extend laterally in two dimensions and have very limited thickness, and both are younger than the rocks they crosscut. Dikes may pass laterally to sills or other types of intrusive bodies and divide into several segments. A dike that brings magma into another intrusive body is a feeder dike. Dikes may occur as sets of dikes with similar age, composition, and orientation in the same region, known as dike swarms.
Lamprophyre dykes crosscutting granodioritic rocks. Neves Glacier, SudTirol, Italy.
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