A dike in geology refers to a tabular intrusiveigneous body. The thickness is usually much smaller than the other two dimensions. Thickness can vary from sub-centimeter scale to many meters in thickness and the lateral dimensions can extend over many kilometers. A dike is an intrusion into a cross-cutting fissure, meaning a dike normally cuts across or through other pre-existing layers or bodies of rock. Dikes are usually high angle to near vertical in orientation, but subsequent tectonic deformation may rotate the including sequence. Near horizontal or conformable intrusions along bedding planes between strata are called intrusive sills.
Shiprock, New Mexico a volcanic neck in the distance, with radiating dike on its south side. Photo credit: USGS Digital Data Series
Dikes often form as either radial or concentric swarms around plutonic intrusives or around volcanic necks or feeder vents in volcanic cones.
Dikes can vary in texture and composition from diabase or basaltic to granitic or rhyolitic. Pegmatite dikes are extremely coarsely crystalline granitic rocks often associated with late stage granite intrusions or metamorphic segregations. Aplite dikes are fine grained or sugary textured intrusives of granitic composition.
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