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An intrusion is an igneous rock body that forms from crystallized magma under Earth's surface. Magma slowly pushes up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly cools into a solid, the different parts of the magma crystallize into minerals. Many mountain ranges, such as the Sierra Nevada in California, are formed mostly by intrusive rock, large granite (or related rock) formations. Intrusions are one of the two ways igneous rock can form; the other is extrusive, that is, a volcanic eruption or similar event. Technically speaking, an intrusion is any formation of intrusive igneous rock; rock formed from magma that cools and solidifies within the crust of the planet. In contrast, an ''extrusion'' consists of extrusive rock; rock formed above the surface of the crust. Intrusions vary widely, from mountain-range-sized batholiths to thin veinlike fracture fillings of aplite or pegmatite. When exposed by erosion, these cores called batholiths may occupy huge areas of Earth's surface. Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons. Coarse-grained intrusive igneous rocks that form at depth within the earth are called abyssal while those that form near the surface are called hypabyssal. 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, otherwise it is discordant. A well-known example of an intrusion is Devils Tower. ==Structural types== Intrusions can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes: :Batholith: a large irregular discordant intrusion :Dike: a relatively narrow tabular discordant body, often nearly vertical :Laccolith: concordant body with roughly flat base and convex top, usually with a feeder pipe below :Lopolith: concordant body with roughly flat top and a shallow convex base, may have a feeder dike or pipe below :Phacolith: a concordant lens-shaped pluton that typically occupies the crest of an anticline or trough of a syncline :Volcanic pipe or volcanic neck: tubular roughly vertical body that may have been a feeder vent for a volcano :Sill: a relatively thin tabular concordant body intruded along bedding planes :Stock: a smaller irregular discordant intrusive :Chonolith: an irregularly-shaped intrusion with a demonstrable base 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Intrusive rock」の詳細全文を読む スポンサード リンク
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