Intrusion
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"Intrusive" redirects here. For other uses, see Intrusive (disambiguation).
In geology, an intrusion is a body of igneous rock that has crystallized from a molten magma below the surface of the Earth. Bodies of magma that solidify underground before they reach the surface of the earth are called plutons, named for Pluto, the Roman god of the underworld. Correspondingly, rocks of this kind are also referred to as igneous plutonic rocks or igneous intrusive rocks. This is to be contrasted with extrusive rocks. The rock surrounding a pluton is called country rock.
[edit] Varieties
Intrusive rocks include all varieties of igneous rocks from coarse-grained, phaneritic granites of large batholiths to very fine grained, aphanitic, rhyolites in volcanic necks or feeder pipes. In composition, intrusive rocks also include the entire sequence of igneous rock types from the dense and dark ultramafic peridotites to the very light-colored and low-density alkali granites and syenites. A well-known example of an igneous intrusion is Devil's Tower in Wyoming, USA.
[edit] Structural types
Intrusive rocks also exist in a wide range of forms from mountain range sized batholiths to thin vein-like fracture fillings of aplite. Intrusive structures are often classified according to whether or not they are parallel to the bedding planes or foliation of the country rock: if the intrusion is parallel, the body is concordant, while if it cuts across the country rock, it is discordant. Structural types include:
- batholith: large irregular intrusions.
- stock: smaller irregular discordant intrusions.
- dike: a relatively narrow tabular discordant body, often with near-vertical attitude.
- sill: a relatively thin tabular concordant body intruded along bedding planes, often near-horizontal when emplaced, but may be intruded into tilted beds or the entire package may be tilted by later deformation.
- pipe or volcanic neck: circular or tube shaped nearly vertical body which may have been a feeder vent for a volcano.
- laccolith: concordant body with essentially flat base and dome shaped upper surface, usually has a feeder pipe below.
- lopolith: concordant body with a relatively flat to sagging top and a shallow covex base (spoon-shaped), may have a feeder dike or pipe below.
- phacolith: a concordant lense shaped pluton that typically occupies the crest of an anticline or the trough of a syncline.
