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Geology is the Way


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 o 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.

Mesozoic lamprophyre dyke crosscutting Ordovician sedimentary rocks near Leading Tickles, in Northern Newfoundland, Canada. This dyke is part of a swarm comprising hundreds of dykes in the region related to rifting prior to the opening of the Atlantic Ocean. Photo by Alexander Peace via Imaggeo.
Oligocene lamprophyre dyke, exposed by erosion. This outcrop highlights the wide lateral extent of the dyke along two directions, contrasting to its limited thickness. Ship Rock volcanic neck of Navajo Volcanic Field, New Mexico, USA. Photo by James St. John.
Dykes of dark-colored dolerite (about 1100 million years old) crosscutting light-coloured migmatitic paragneiss (1800 million years old) at Yttre Ursholmen (Kosterhavet National Park, Koster Islands, Sweden). Photo by Thomas Eliasson (Geological Survey of Sweden).
Basaltic dyke intruded in volcanic rocks and exposed by erosion. Fayal Island, Azores, Portugal. Photo by Heidi Soosalu.
Let’s make things complicated! Dolerite dyke (black) crosscutting a pegmatite dyke (white and coarse grained) crosscutting intrusive rocks with evidence of magma mingling (black enclaves in granitic host). Precambrian intrusive complex located in Kosterhavet National Park (Yttre Ursholmen Island, Koster Islands, Sweden). Photo by Thomas Eliasson (Geological Survey of Sweden).

Neves granite with dykeNeves panorama interpreted
A mafic dyke (lamprophyre) crosscutting granodioritic rocks. Neves Glacier, SudTirol, Italy.

Lamprophyre dykes crosscutting granodioritic rocks. Neves Glacier, SudTirol, Italy.

Anderson, E. M. (1939). XVII.—The Dynamics of Sheet Intrusion. Proceedings of the Royal Society of Edinburgh58, 242-251.
Kavanagh, J. L., Boutelier, D., & Cruden, A. R. (2015). The mechanics of sill inception, propagation and growth: Experimental evidence for rapid reduction in magmatic overpressure. Earth and Planetary Science Letters421, 117-128.
Delaney, P. T., & Pollard, D. D. (1982). Solidification of basaltic magma during flow in a dike. American Journal of Science282(6), 856-885.
Kjøll, H. J., Galland, O., Labrousse, L., & Andersen, T. B. (2019). Emplacement mechanisms of a dyke swarm across the brittle-ductile transition and the geodynamic implications for magma-rich margins. Earth and Planetary Science Letters518, 223-235.
Petford, N., Kerr, R. C., & Lister, J. R. (1993). Dike transport of granitoid magmas. Geologia21(9), 845-848.
Pollard, David D. “Derivation and evaluation of a mechanical model for sheet intrusions.” Tectonophysics 19.3 (1973): 233-269.
Pollard, D. D., Muller, O. H., & Dockstader, D. R. (1975). The form and growth of fingered sheet intrusions. Geological Society of America Bulletin86(3), 351-363.


Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies


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