||This article is largely based on an article in the out-of-copyright Encyclopædia Britannica Eleventh Edition, which was produced in 1911. It should be brought up to date to reflect subsequent history or scholarship (including the references, if any). When you have completed the review, replace this notice with a simple note on this article's talk page. (January 2011)|
Intrusive rock forms within Earth's crust from the crystallization of magma. 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 magma slowly cools into a solid, the different parts of the magma crystallize into rocks. Many mountain ranges, such as the Sierra Nevada in California, are formed mostly from large granite (or related rock) intrusions; see Sierra Nevada Batholith.
Intrusions are one of the two ways igneous rock can form; the other is extrusive rock, 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.
Coarse-grained intrusive igneous rocks that form at depth within the earth are called abyssal while those that form near the surface are called subvolcanic or 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 called parallel the body is concordant, otherwise it is called discordant.
How they vary
When exposed by erosion, such batholiths may occupy large areas.
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
- Boss: a small stock
- Chonolith: an irregularly-shaped intrusion with a demonstrable base
Intrusive rocks are characterized by large crystal sizes, and as the individual crystals are visible, the rock is called phaneritic.  This is as the magma cools underground, and while cooling may be fast or slow,  it is slower than cooling on the surface, allowing larger crystals to grow,. If it runs parallel to rock layers, it is called a sills. If an intrusion makes rocks above rise to form a dome, it is called a laccolith. 
Deep-seated intrusions are recognized by the way they have burst through the overlying strata. Veins spread out into branches or branchlike parts result from filled cracks, and the high temperature involved in this process is evident as they alter adjacent country rock. As heat dissipates slowly, and since the rock is under pressure, crystals form and no vitreous rapidly chilled matter is present.
As the intrusions have had time to rest before crystallizing, they do not show lines, in that they flowed while solidifying. Their contained gases were not able to escape through the thick strata, and thus form cavities, which can often be observed. Such gases have also resulted in many important modifications in the crystallization of the rock. Because their crystals are of approximately equal size these rocks are said to be equigranular,. 
There is typically no distinction between a first generation of large well-shaped crystals and a fine-grained ground-mass. The minerals of each have formed in a definite order, and each has had a period of crystallization that may be very distinct or may have coincided with or overlapped the period of formation of some of the other ingredients. Earlier crystals originated at a time when most of the rock was still liquid and are more or less perfect. Later crystals are less regular in shape because they were compelled to occupy the spaces left between the already-formed crystals. The former case is said to be idiomorphic (or automorphic); the latter is xenomorphic. There are also many other characteristics that serve to distinguish the members of these two groups. For example, orthoclase is typically feldspar from granite, while its modifications occur in lavas of similar composition. The same distinction holds for nepheline varieties. Leucite is common in lavas but very rare in plutonic rocks. Muscovite is confined to intrusions. These differences show the influence of the physical conditions under which consolidation takes place.
Intrusive rocks formed at greater depths are called plutonic or abyssal. Some intrusive rocks solidified in fissures as dikes and intrusive sills at a shallow depth beneath the surface and are called subvolcanic or hypabyssal. As might be expected, they show structures intermediate between those of extrusive and plutonic rocks. They are very commonly porphyritic, vitreous, and sometimes even vesicular. In fact, many of them are petrologically indistinguishable from lavas of similar composition.
- Ellicott City Granodiorite
- Extrusive rock
- Guilford Quartz Monzonite
- Methods of pluton emplacement
- Norbeck Intrusive Suite
- Volcanic rock
- Woodstock Quartz Monzonite
- Intrusive Rocks: Intrusive rocks crystallize and harden beneath the Earth's surface. Some of the rock formations that form are plutons, batholiths, dikes, sills, laccoliths, and volcanic necks., accessdate: March 27, 2017
- Igneous intrusive rocks: Igneous intrusive rocks, accessdate: March 27, 2017
- Britannica.com: intrusive rock | geology | Britannica.com, accessdate: March 27, 2017
- Wilkinson, Bruce H.; McElroy, Brandon J.; Kesler, Stephen E.; Peters, Shanan E.; Rothman, Edward D. (2008). "Global geologic maps are tectonic speedometers—Rates of rock cycling from area-age frequencies". Geological Society of America Bulletin. 121: 760–779. doi:10.1130/B26457.1.
- Oxford Academic: Crustal Contamination of Picritic Magmas During Transport Through Dikes: the Expo Intrusive Suite, Cape Smith Fold Belt, New Quebec | Journal of Petrology | Oxford Academic, accessdate: March 27, 2017
- 9/28/94: 9/28/94, accessdate: March 27, 2017
- Intrusive and Extrusive Rocks: Intrusive and Extrusive Rocks, accessdate: March 28, 2017
- Sciencing: Characteristics of Intrusive Rocks | Sciencing, accessdate: March 28, 2017
- rocks and minerals: Geology - rocks and minerals, accessdate: March 28, 2017
- One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Petrology". Encyclopædia Britannica. 21 (11th ed.). Cambridge University Press. pp. 323–333.