Intrusive rock is formed whenmagma penetrates existing rock, crystallizes, and solidifies underground to formintrusions, such asbatholiths,dikes,sills,laccoliths, andvolcanic necks.[1][2][3]
Intrusion is one of the two waysigneous rock can form. The other isextrusion, such as avolcanic eruption or similar event. An intrusion is any body of intrusive igneous rock, formed from magma that cools and solidifies within the crust of theplanet. In contrast, anextrusion consists of extrusive rock, formed above the surface of the crust.
Some geologists use the termplutonic rock synonymously with intrusive rock, but other geologists subdivide intrusive rock, by crystal size, into coarse-grained plutonic rock (typically formed deeper in theEarth's crust in batholiths orstocks) and medium-grainedsubvolcanic or hypabyssal rock (typically formed higher in the crust in dikes and sills).[4]
Because the solidcountry 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 calledabyssal orplutonic while those that form near the surface are calledsubvolcanic orhypabyssal.[4]
Plutonic rocks are classified separately from extrusive igneous rocks, generally on the basis of theirmineral content. The relative amounts ofquartz,alkali feldspar,plagioclase, andfeldspathoid are particularly important in classifying intrusive igneous rocks, and most plutonic rocks are classified by where they fall in theQAPF diagram.Dioritic andgabbroic rocks are further distinguished by whether the plagioclase they contain issodium-rich, and sodium-poor gabbros are classified by their relative contents of variousiron- ormagnesium-rich minerals (mafic minerals) such asolivine,hornblende,clinopyroxene, and orthopyroxene, which are the most common mafic minerals in intrusive rock. Rareultramafic rocks, which contain more than 90% mafic minerals, andcarbonatite rocks, containing over 50% carbonate minerals, have their own special classifications.[5][6]
Hypabyssal rocks resemble volcanic rocks more than they resemble plutonic rocks, being nearly as fine-grained, and are usually assigned volcanic rock names. However,dikes ofbasaltic composition often show grain sizes intermediate between plutonic and volcanic rock, and are classified asdiabases or dolerites. Rare ultramafic hypabyssal rocks calledlamprophyres have their own classification scheme.[7]
Intrusive rocks are characterized by largecrystal sizes, and as the individual crystals are visible, the rock is calledphaneritic.[8] 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.[9] Contained gases cannot escape through the overlying strata, and these gases sometimes formcavities, often lined with large, well-shaped crystals. These are particularly common in granites and their presence is described asmiarolitic texture.[10] Because their crystals are of roughly equal size, intrusive rocks are said to beequigranular.[11]
Plutonic rocks are less likely than volcanic rocks to show a pronouncedporphyritic 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 beidiomorphic (orautomorphic); the latter isxenomorphic.
There are also many other characteristics that serve to distinguish plutonic from volcanic rock. For example, the alkali feldspar in plutonic rocks is typicallyorthoclase, while the higher-temperature polymorph,sanidine, is more common in volcanic rock. The same distinction holds fornepheline 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.[12]
Hypabyssal rocks show structures intermediate between those ofextrusive and plutonic rocks. They are very commonly porphyritic,vitreous, and sometimes evenvesicular. In fact, many of them arepetrologically indistinguishable from lavas of similar composition.[12][7]
Plutonic rocks form 7% of the Earth's current land surface.[13] Intrusions vary widely, from mountain-range-sizedbatholiths to thinveinlikefracture fillings ofaplite orpegmatite.
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Flett, John Smith (1911). "Petrology". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. p. 327. Geologic principles and processes | ||
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