A dike is a formation of igneous rock that can form exposed vertical or linear ridges. Dikes are formed underground and are an intrusive plutonic rock formation. Intrusive formations form when upwelling magma cools and solidifies beneath the surface. As the magma rises it intrudes into the overlying country rock (older rock that is also termed "host" rock).
Dikes are vertical formations and thus form at a steep or often near right angle to the surface. Dikes are planar intrusions that, in contrast to horizontal sills, have a discordant form of contact with the host rock into which they intrude. A discordant contact is one that transverses or cuts across the established bedding planes of the country or host rock (e.g., at right angles to surrounding sedimentary bedding planes).
Dike texture varies from aphanitic (no visible mineral crystals) to phaneritic (visible mineral crystals). The texture is determined by the time needed for the upwelling magma to cool and solidify. Because dikes are vertical, gradients of textures can be established within the same dike (e.g., a change form aphanitic to phaneritic texture within the same dike. The longer the magma cooling time, the greater the extent and size of mineral crystal formation in the igneous rock comprising the dike. If, for example a diker a region of a dikeools rapidly, the texture becomes uniform, smooth and without mineral crystals that are discernable upon visual inspection. If the magma in the dike cools over a long period of time, visible crystals form and the texture is described as phaneritic. Variation in the texture of dikes can also result from multiple intrusions of magma.
When exposed, dikes may form visible cliffs. In addition, dikes can form a network of underground passages for magma and when resistance to upward magma flow is encountered, a dike formation may give way to a horizontal sill intrusion.
Dikes can vary greatly in thickness across a range from a few inches to hundreds of yards. At the extreme, the great Dike of Zimbabwe extends more than 350 miles and has an average width of about six miles.
The formation of dikes often follows or reflects the fracturing of surrounding country rock. Accordingly, dikes often form in clumps or "swarms" and can occur in radial distributions about a deeper upwelling.
By definition, dikes form and cool underground. However, if the same form of magma upwelling reaches the surface, it results in a usually low viscosity volcanic fissure formation. Such formations are common in the Hawaiian islands (formed from an upwelling of a magmatic "hot spot") and Iceland (formed as part of the magma upwelling associated with the Mid-Atlantic Ridge and in other areas of volcanic activity.
The search for such cliff formations is an important part of extraterrestrial studies conducted by the Mars Global Surveyor and probes sent to explore the moons of Jupiter. Identification of dike formations provide easily visible evidence of past volcanic activity, mantle plumes, and other forms of plate tectonic activity.
See also Igneous rocks; Pluton and plutonic bodies; Stratigraphy; Volcanic eruptions; Volcano
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