Editing Intrusive rock

{{Short description|Magmatic rock formed below the surface}}

[[File:Intrusive big.png|thumb|upright=1.5|[[QAPF diagram]] for the classification of plutonic rocks]]
[[Image:A Yool DevilsTower 04Sep03 exif.jpg|thumb|[[Devils Tower]], United States, an igneous ''intrusion'' exposed when the surrounding softer rock eroded away]]

'''Intrusive rock''' is formed when [[magma]] penetrates existing rock, crystallizes, and solidifies underground to form ''[[Igneous intrusion|intrusions]]'', such as [[batholith]]s, [[dike (geology)|dikes]], [[Sill (geology)|sills]], [[laccolith]]s, and [[volcanic neck]]s.<ref name="kids-fun-science.com">Intrusive Rocks: [http://www.kids-fun-science.com/intrusive-rocks.html Intrusive rocks], accessdate: March 27, 2017.</ref><ref name="geocases2.co.uk">Igneous intrusive rocks: [http://www.geocases2.co.uk/printable/rocks.htm Igneous intrusive rocks] {{Webarchive|url=https://web.archive.org/web/20180512145313/http://www.geocases2.co.uk/printable/rocks.htm |date=2018-05-12 }}, accessdate: March 27, 2017.</ref><ref name="britannica.com">Britannica.com: [https://www.britannica.com/science/intrusive-rock intrusive rock | geology | Britannica.com], accessdate: March 27, 2017.</ref>

Intrusion is one of the two ways [[igneous rock]] can form. The other is [[extrusive rock|extrusion]], such as a [[Types of volcanic eruptions|volcanic eruption]] or similar event. An intrusion is any body of intrusive igneous rock, formed from magma that cools and solidifies within the crust of the [[planet]]. In contrast, an ''extrusion'' consists of extrusive rock, formed above the surface of the crust.

Some geologists use the term '''plutonic rock''' synonymously with intrusive rock, but other geologists subdivide intrusive rock, by crystal size, into coarse-grained plutonic rock (typically formed deeper in the [[Earth's crust]] in batholiths or [[stock (geology)|stocks]]) and medium-grained [[Subvolcanic rock|subvolcanic or hypabyssal rock]] (typically formed higher in the crust in dikes and sills).<ref name="philpotts-ague-2009-53">{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |page=52 |edition=2nd}}</ref>

==Classification==
Because the solid [[country rock (geology)|country rock]] into which magma intrudes is an excellent insulator, cooling of the magma is extremely slow, and intrusive igneous rock is coarse-grained ([[phaneritic]]). However, the rate of cooling is greatest for intrusions at relatively shallow depth, and the rock in such intrusions is often much less coarse-grained than intrusive rock formed at greater depth. Coarse-grained intrusive igneous rocks that form at depth within the Earth are called ''abyssal'' or ''plutonic'' while those that form near the surface are called [[Subvolcanic rock|subvolcanic]] or ''hypabyssal''.<ref name="philpotts-ague-2009-53"/>

Plutonic rocks are classified separately from extrusive igneous rocks, generally on the basis of their [[mineral]] content. The relative amounts of [[quartz]], [[alkali feldspar]], [[plagioclase]], and [[feldspathoid]] are particularly important in classifying intrusive igneous rocks, and most plutonic rocks are classified by where they fall in the [[QAPF diagram]]. [[Diorite|Dioritic]] and [[gabbro]]ic rocks are further distinguished by whether the plagioclase they contain is [[sodium]]-rich, and sodium-poor gabbros are classified by their relative contents of various [[iron]]- or [[magnesium]]-rich minerals ([[mafic]] minerals) such as [[olivine]], [[hornblende]], [[clinopyroxene]], and orthopyroxene, which are the most common mafic minerals in intrusive rock. Rare [[ultramafic rocks]], which contain more than 90% mafic minerals, and [[carbonatite]] rocks, containing over 50% carbonate minerals, have their own special classifications.<ref name="iugs-1991">{{Cite journal|last1=Le Bas|first1=M. J.|last2=Streckeisen|first2=A. L.|title=The IUGS systematics of igneous rocks|journal=Journal of the Geological Society|volume=148|issue=5|pages=825–833|doi=10.1144/gsjgs.148.5.0825|bibcode=1991JGSoc.148..825L|year=1991|citeseerx=10.1.1.692.4446|s2cid=28548230}}</ref><ref name="BGS-1999">{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}</ref>

Hypabyssal rocks resemble volcanic rocks more than they resemble plutonic rocks, being nearly as fine-grained, and are usually assigned volcanic rock names. However, [[dike (geology)|dikes]] of [[basalt]]ic composition often show grain sizes intermediate between plutonic and volcanic rock, and are classified as [[diabase]]s or dolerites.  Rare ultramafic hypabyssal rocks called ''[[lamprophyre]]s'' have their own classification scheme.{{sfn|Philpotts|Ague|2009|p=139}}

== Characteristics ==
[[Image:NotchPeakDike.JPG|thumb|An intrusion (pink [[Notch Peak]] [[monzonite]]) inter-fingers (partly as a [[dike (geology)|dike]]) with highly metamorphosed black-and-white-striped host rock ([[Cambrian]] [[carbonate rock]]s) near Notch Peak, [[House Range]], [[Utah]], United States]]

Intrusive rocks are characterized by large [[crystal]] sizes, and as the individual crystals are visible, the rock is called ''[[phaneritic]]''.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |pages=12–13 |edition=2nd}}</ref> There are few indications of flow in intrusive rocks, since their texture and structure mostly develops in the final stages of crystallization, when flow has ended.{{sfn|Philpotts|Ague|2009|p=48}} Contained gases cannot escape through the overlying strata, and these gases sometimes form [[vesicle (geology)|cavities]], often lined with large, well-shaped crystals. These are particularly common in granites and their presence is described as ''miarolitic texture''.{{sfn|Blatt|Tracy|1996|p=44}} Because their crystals are of roughly equal size, intrusive rocks are said to be ''[[equigranular]]''.<ref name="auckland.ac.nz">rocks and minerals: [https://flexiblelearning.auckland.ac.nz/rocks_minerals/glossary.html#e Geology - rocks and minerals], accessdate: March 28, 2017.</ref>

Plutonic rocks are less likely than volcanic rocks to show a pronounced [[porphyritic]] texture, in which a first generation of large well-shaped crystals are embedded in 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 [[xenomorph (geology)|xenomorphic]].

There are also many other characteristics that serve to distinguish plutonic from volcanic rock. For example, the alkali feldspar in plutonic rocks is typically [[orthoclase]], while the higher-temperature polymorph, [[sanidine]], is more common in volcanic rock. 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 crystallization takes place.<ref name=EB1911>{{EB1911|wstitle=Petrology |volume=21 |page=327 |inline=1 |first=John Smith |last=Flett}}</ref>

Hypabyssal rocks show structures intermediate between those of [[extrusive rock|extrusive]] and plutonic rocks. They are very commonly porphyritic, [[Vitrification|vitreous]], and sometimes even [[Vesicular texture|vesicular]]. In fact, many of them are [[petrology|petrologically]] indistinguishable from lavas of similar composition.<ref name=EB1911/>{{sfn|Philpotts|Ague|2009|p=139}}

==Occurrences==
{{main|Igneous intrusion}}
Plutonic rocks form 7% of the Earth's current land surface.<ref name="Wilkinson2008">{{cite journal | title=Global geologic maps are tectonic speedometers—Rates of rock cycling from area-age frequencies | first1=Bruce H. | last1=Wilkinson | first2=Brandon J. | last2=McElroy | first3=Stephen E. | last3=Kesler | first4=Shanan E. | last4=Peters | first5=Edward D. | last5=Rothman | journal=Geological Society of America Bulletin | year=2008 | volume=121 | issue=5–6 | pages=760–779 | doi=10.1130/B26457.1}}</ref> Intrusions vary widely, from mountain-range-sized [[batholith]]s to thin [[vein (geology)|vein]]like [[Fracture (geology)|fracture]] fillings of [[aplite]] or [[pegmatite]].

[[File:Volcanosed.svg|thumb|Diagram showing various types of igneous intrusion]]
[[File:Geological Dike Cross-Island Trail Alaska.jpg|thumb|Dark dikes intruded into the [[country rock (geology)|country rock]], [[Baranof Island]], [[Alaska]], United States]]
*[[Batholith]]: a large irregular discordant intrusion
*[[Chonolith]]: an irregularly-shaped intrusion with a demonstrable base
*[[Cupola (geology)|Cupola]]: a dome-shaped projection from the top of a large subterranean intrusion
*[[Dike (geology)|Dike]]: a relatively narrow tabular discordant body, often nearly vertical
*[[Laccolith]]: concordant body with roughly flat base and [[Wikt:convex|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 (geology)|Sill]]: a relatively thin tabular concordant body intruded along bedding planes
*[[Stock (geology)|Stock]]: a smaller irregular discordant intrusive
**Boss: a small stock

==See also==
* [[Ellicott City Granodiorite]]
* [[Guilford Quartz Monzonite]]
* [[Pluton emplacement]]
* [[Norbeck Intrusive Suite]]
* [[Subvolcanic rock]]
* [[Tuolumne Intrusive Suite]]
* [[Volcanic rock]]
* [[Woodstock Quartz Monzonite]]

== References ==
{{Reflist}}

{{Geologic Principles}}
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[[Category:Petrology]]
[[Category:Rocks]]