thumb|A magmatic dike (vertical) cross-cutting horizontal layers of sedimentary rock, in [[Makhtesh Ramon, Israel]]
In geology, a dike or dyke is a sheet of rock that is formed in a fracture of a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into a crack then solidifies as a sheet intrusion, either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack.
Magmatic dike
thumb|A dike of [[lamprophyre near the Shiprock volcanic plug, New Mexico, that has resisted the erosion that removed some of the softer rock into which the dike was originally intruded]]
A magmatic dike is a sheet of igneous rock that cuts across older rock beds. It is formed when magma fills a fracture in the older beds and then cools and solidifies. The dike rock is usually more resistant to weathering than the surrounding rock, so that erosion exposes the dike as a natural wall or ridge. They also record ancient episodes of extension of the Earth's crust, since large numbers of dikes (dike swarms) are formed when the crust is pulled apart by tectonic forces. The dikes show the direction of extension, since they form at right angles to the direction of maximum extension. and the opposite walls are roughly parallel, so that a dike is more or less constant in thickness. The thickness of different dikes can range from a few millimeters to hundreds of meters, but is most typically from about a meter to a few tens of meters. The lateral extent can be tens of kilometers, and dikes with a thickness of a few tens of meters or more commonly extend for over 100 km. Most dikes are steeply dipping; in other words, they are oriented nearly vertically. Subsequent tectonic deformation may rotate the sequence of strata through which the dike propagates so that the dike becomes horizontal.
thumb|Simplified map of the en echelon dike set at Jagged Rock, Arizona, US. The total dike length is about 1.45 km. and the dikes of [[oceanic spreading centers.
Dikes range in composition from basaltic to rhyolitic, but most are basaltic. The texture is typically slightly coarser than basalt erupted at the surface, forming a rock type called diabase. The grain size varies systematically across the dike, with the coarsest grains normally at the center of the dike. Dikes formed at shallow depth commonly have a glassy or fine-grained chilled margin 1 to 5 cm thick, formed where the magma was rapidly cooled by contact with the cold surrounding rock. Shallow dikes also typically show columnar jointing perpendicular to the margins. Here the dike rock fractures into columns as it cools and contracts. These are usually 5- to 6-sided, but 3- to 4-sided columns are also common. These are fairly uniform in size within a single dike, but range from a few centimeters to over 0.3 meters across in different dikes, tending to be thicker in wider dikes. Larger columns are likely a consequence of slower cooling.
Dike rock is usually dense, with almost no vesicles (frozen bubbles), but vesicles may be seen in the shallowest part of a dike. When vesicles are present, they tend to form bands parallel to walls and are elongated in direction of flow. Likewise, phenocrysts (larger crystals) on the margins of the dike show an alignment in the direction of flow.
In contrast to dikes, which cut across the bedding of layered rock, a sill is a sheet intrusion that forms within and parallel to the bedding.
Formation
Mafic magma (fluid magma low in silica) usually reaches the surface through fissures, forming dikes.
Dike swarms (also called dike complexes) are exposed in the eroded rift zones of Hawaiian volcanoes. As with most other magmatic dikes, these were fissures through which lava reached the surface. The swarms are typically 2.5 to 5 km in width, with individual dikes about a meter in width. The dike swarms extend radially out from volcano summits and parallel to the long axis of the volcanic shield. Sills and stocks are occasionally present in the complexes. They are abruptly truncated at the margins of summit calderas. Typically, there are about 50 to 100 dikes per kilometer at the center of the rift zone, though the density can be as high as 500 per kilometer and the dikes then make up half the volume of the rock. The density drops to 5 to 50 per kilometer away from the center of the rift zone before abruptly dropping to very few dikes. It is likely that the number of dikes must increase with depth, reaching a typical value of 300 to 350 per kilometer at the level of the ocean floor. In some respects, these dike swarms resemble those of western Scotland associated with the flood eruptions that preceded the opening of the Atlantic Ocean.
Dikes often form as radial swarms from a central volcano or intrusion. Good examples of ring dikes and cone sheets are found in the Ardnamurchan peninsula of Scotland.
Clastic dike
thumb|upright|[[Clastic dike (left of notebook) in the Chinle Formation in Canyonlands National Park, Utah]]
A clastic dike (also known as a sedimentary dike) is a vertical body of sedimentary rock that interrupts other rock layers. It can form in two ways:
- When 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. Sediment driven by fluid pressure breaks through overlying layers and forms a dike.
- When a soil is under permafrost conditions the pore water is totally frozen. Cracks formed in such rocks may fill up with sediments that fall in from above.