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A dike or dyke in geology is a sheet of rock that formed in a crack in a pre-existing rock body. However, when the crack is between the layers in a layered rock, it is called a sill, not a dike. It is a type of tabular intrusion, or sheet intrusion, that either cuts across layers in a planar wall rock structures, or into a layer or unlayered mass of rock.
Dikes can therefore be either intrusive or sedimentary in origin. For example, when molten rock intrudes into a crack then refreezes, it is an igneous dike. When limestone forms by precipitation in a pre-existing crack, it is a sedimentary dike.
Magmatic dikes 
An intrusive dike is an igneous body with a very high aspect-ratio, which means that its thickness is usually much smaller than the other two dimensions. Thickness can vary from sub-centimetre scale to many metres, and the lateral dimensions can extend over many kilometres. A dike is an intrusion into an opening cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies that a dike is always younger than the rocks that contain it. Dikes are usually high-angle to near-vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata through which the dike propagates so that the dike becomes horizontal. Near-horizontal, or conformable intrusions, along bedding planes between strata are called intrusive sills.
Sometimes dikes appear in swarms, consisting of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event. The world's largest dike swarm is the Mackenzie dike swarm in the Northwest Territories, Canada.
Dikes can vary in texture and their composition can range from diabase or basaltic to granitic or rhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vast volumes of mantle-derived magmas through fractured lithosphere throughout Earth history. Pegmatite dikes comprise extremely coarse 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.
Sedimentary dikes 
Sedimentary dikes or clastic dikes are vertical bodies of sedimentary rock that cut off other rock layers. They can form in two ways:
- When a shallow unconsolidated sediment is composed of alternating coarse grained and impermeable clay layers the fluid pressure inside the coarser layers may reach a critical value due to lithostatic overburden. Driven by the fluid pressure the sediment breaks through overlying layers and forms a dike.
- When a soil is under permafrost conditions the pore water is totally frozen. When cracks are formed in such rocks, they may fill up with sediments that fall in from above. The result is a vertical body of sediment that cuts through horizontal layers: a dike.
See also 
|Wikimedia Commons has media related to: Dike (geology)|
- Ring dike
- Fissure vent
- Runamo, formerly interpreted as a runic inscription.
- Dike swarm
- Essentials of Geology, 3rd Ed, Stephen Marshak
- Supressing Varying Directional Trends Retrieved on 2007-07-28