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Intrusive rock
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==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=825β833|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=1β52}}</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}}
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