Editing Intrusive rock (section)

== 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}}