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Intrusive rock is formed when
magma Magma () is the molten or semi-molten natural material from which all igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others ...

magma
penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as
batholith A batholith (from Greek ''bathos'', depth + ''lithos'', rock) is a large mass of intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magm ...
s,
dikes Dyke or dike may refer to: General uses * Dyke (slang) The term ''dyke'' is a slang Slang is language (words, phrases, and usages) of an informal register. It also sometimes refer to the language generally exclusive to the members of ...
, sills,
laccolith A laccolith is a sheet-like intrusion (or concordant pluton) that has been injected within or between layers of sedimentary rock Sedimentary rocks are types of Rock (geology), rock that are formed by the accumulation or deposition of mineral o ...

laccolith
s, and
volcanic neck A volcano is a rupture in the Crust (geology), crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and Volcanic gas, gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

volcanic neck
s.Intrusive Rocks
Intrusive rocks
accessdate: March 27, 2017.
Igneous intrusive rocks

accessdate: March 27, 2017.
Britannica.com
intrusive rock , geology , Britannica.com
accessdate: March 27, 2017.
Intrusion is one of the two ways
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock i ...
can form. The other is
extrusion Extrusion is a process used to create objects of a fixed cross section (geometry), cross-sectional profile by pushing material through a Die (manufacturing), die of the desired cross-section. Its two main advantages over other manufacturing pro ...
, such as a
volcanic eruption Several types of volcanic eruptions—during which lava Lava is magma once it has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or ...
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 A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

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 350px, Plates in the crust of Earth Earth's crust is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is the top component of the lithosphere, a division of Earth's layers that includes the Crust (geology), ...
in batholiths or
stocks Stocks are feet restraining devices that were used as a form of and . Form and application The stocks, , and pranger each consist of large wooden boards with hinges; however, the stocks are distinguished by their restraint of the feet. The s ...
) and medium-grained subvolcanic or hypabyssal rock (typically formed higher in the crust in dikes and sills).


Classification

Because the solid
country rock Country rock is a subgenre of , formed from the fusion of and . It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal ...
into which magma intrudes is an excellent insulator, cooling of the magma is extremely slow, and intrusive igneous rock is coarse-grained (
phaneritic A phanerite is an igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rom ...
). 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 or ''hypabyssal''. Plutonic rocks are classified separately from extrusive igneous rocks, generally on the basis of their
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
content. The relative amounts of
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
,
alkali feldspar Feldspars () are a group of rock-forming tectosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. ...
,
plagioclase Plagioclase is a series of tectosilicate Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition ...
, and
feldspathoid The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are not found in rocks containing primary quar ...
are particularly important in classifying intrusive igneous rocks, and most plutonic rocks are classified by where they fall in the
QAPF diagram A QAPF diagram is a double ternary diagram A diagram is a symbolic Depiction, representation of information using Visualization (graphics), visualization techniques. Diagrams have been used since prehistoric times on Cave painting, walls of cave ...
.
Gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusion, intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a crystallinity, holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coa ...

Gabbro
ic rocks are further classified by their relative contents of plagioclase and various
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

iron
- or
magnesium Magnesium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

magnesium
-rich minerals (
mafic A mafic mineral or rock is a silicate mineral Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composit ...
minerals) such as
olivine The mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. R ...

olivine
,
hornblende Hornblende is a complex inosilicate series of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...

hornblende
,
clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming s found in many and . Pyroxenes have the general formula , where X represents (Ca), (Na), (Fe II) or (Mg) and more rarely , or , and Y represents ions of ...
, and
orthopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming Silicate minerals#Inosilicates, inosilicate minerals found in many Igneous rock, igneous and metamorphic rock, metamorphic rock (geology), rocks. Pyroxenes have th ...
, which are the most common mafic minerals in intrusive rock. Rare ultramafic rocks, which contain more than 90% mafic minerals, and
carbonatite Carbonatite lava at Ol Doinyo Lengai volcano, Tanzania Carbonatite () is a type of Intrusive rock, intrusive or extrusive igneous Rock (geology), rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbon ...

carbonatite
rocks, containing over 50% carbonate minerals, have their own special classifications. 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 Dyke or dike may refer to: General uses * Dyke (slang) The term ''dyke'' is a slang Slang is language (words, phrases, and usages) of an informal register. It also sometimes refer to the language generally exclusive to the members of ...
of
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extr ...

basalt
ic composition often show grain sizes intermediate between plutonic and volcanic rock, and are classified as
diabase Diabase (), also called dolerite () or microgabbro, is a mafic A mafic mineral or rock is a silicate mineral Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a so ...

diabase
s or
dolerite Diabase (), also called dolerite () or microgabbro, is a mafic A mafic mineral or rock is a silicate mineral Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγ ...

dolerite
s. Rare ultramafic hypabyssal rocks called ''
lamprophyre Lamprophyres (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million a ...
s'' have their own classification scheme.


Characteristics

Intrusive rocks are characterized by large
crystal A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformatio ...

crystal
sizes, and as the individual crystals are visible, the rock is called ''
phaneritic A phanerite is an igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rom ...
''. 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. Contained gases cannot escape through the overlying strata, and these gases sometimes form
cavitiesA cavity is a hollow in an object. Biology and healthcare *Cavity or Tooth decay, dental caries, damage to the structure of a tooth *Body cavity, a fluid filled space in many animals where organs typically develop **The gastrovascular cavity, which ...
, often lined with large, well-shaped crystals. These are particularly common in granites and their presence is described as ''miarolitic texture''. Because their crystals are of roughly equal size, intrusive rocks are said to be ''
equigranular An equigranular material is composed chiefly of crystals of similar orders of magnitude to one another. Basalt Basalt (, ) is a fine-grained extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and ...
''.rocks and minerals
Geology - rocks and minerals
accessdate: March 28, 2017.
Plutonic rocks are less likely than volcanic rocks to show a pronounced
porphyritic porphyritic rock. The white, square feldspar phenocrysts are much larger than crystals in the surrounding matrix; eastern Sierra Nevada (U.S.), Sierra Nevada, Rock Creek Canyon, California. Porphyritic is an adjective used in geology, specifical ...
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 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 Orthoclase, or orthoclase feldspar (endmember An endmember (also end-member or end member) in mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical ...

orthoclase
, while the higher-temperature polymorph,
sanidine Sanidine is the high temperature form of potassium Potassium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the propert ...

sanidine
, is more common in volcanic rock. The same distinction holds for
nepheline Nepheline, also called nephelite (from Greek: νεφέλη, "cloud"), is a rock forming mineral in the feldspathoid group: a silica-undersaturated aluminosilicate, Na3K Al4Si4 O16, that occurs in intrusive and volcanic rocks with low silica, ...

nepheline
varieties.
Leucite Leucite is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium Aluminium (aluminum in American English, American and Canadian English) is a chemical element with the Symbol (che ...

Leucite
is common in lavas but very rare in plutonic rocks.
Muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'' ...

Muscovite
is confined to intrusions. These differences show the influence of the physical conditions under which crystallization takes place. Hypabyssal rocks show structures intermediate between those of
extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extrusive rock refers to the mode of igneous volcanic ...
and plutonic rocks. They are very commonly
porphyritic porphyritic rock. The white, square feldspar phenocrysts are much larger than crystals in the surrounding matrix; eastern Sierra Nevada (U.S.), Sierra Nevada, Rock Creek Canyon, California. Porphyritic is an adjective used in geology, specifical ...
, , and sometimes even vesicular. In fact, many of them are petrologically indistinguishable from lavas of similar composition.


Occurrences

Plutonic rocks form 7% of the Earth's current land surface. Intrusions vary widely, from mountain-range-sized
batholith A batholith (from Greek ''bathos'', depth + ''lithos'', rock) is a large mass of intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magm ...
s to thin
vein Veins are blood vessels The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A b ...
like
fracture Fracture is the separation of an object or material into two or more pieces under the action of stress (physics), stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the ...
fillings of
aplite Aplite () is an in which the mineral composition is the same as , but in which the grains are much finer, under 1 mm across. and are the dominant s. The term 'aplite' or 'aplitic' is often used as a textural term to describe veins of ...
or
pegmatite A pegmatite is an igneous rock showing a very coarse Texture (geology), texure, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a si ...
. :
Batholith A batholith (from Greek ''bathos'', depth + ''lithos'', rock) is a large mass of intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magm ...
: a large irregular discordant intrusion : Chonolith: an irregularly-shaped intrusion with a demonstrable base :
Cupola In architecture, a cupola () is a relatively small, most often dome-like, tall structure on top of a building. Often used to provide a lookout or to admit light and air, it usually crowns a larger roof or dome. The word derives, via Italian lan ...
: a dome-shaped projection from the top of a large subterranean intrusion :
Dike Dyke or dike may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), the Greek goddess of moral justice * Dikes, diagonal pliers, diag ...
: a relatively narrow tabular discordant body, often nearly vertical :
Laccolith A laccolith is a sheet-like intrusion (or concordant pluton) that has been injected within or between layers of sedimentary rock Sedimentary rocks are types of Rock (geology), rock that are formed by the accumulation or deposition of mineral o ...

Laccolith
: concordant body with roughly flat base and
convex Convex means curving outwards like a sphere, and is the opposite of concave. Convex or convexity may refer to: Science and technology * Convex lens A lens is a transmissive optics, optical device which focuses or disperses a light beam by me ...
top, usually with a feeder pipe below :
Lopolith Diagram showing the shape of a lopolith (7 on the diagram) A lopolith is a large igneous intrusion which is Lenticular (geology), lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded stratum, str ...
: concordant body with roughly flat top and a shallow convex base, may have a feeder dike or pipe below :: a concordant lens-shaped pluton that typically occupies the crest of an
anticline In structural geology, an anticline is a type of Fold (geology), fold that is an arch-like shape and has its oldest Bed (geology), beds at its core, whereas a syncline is the inverse of a anticline. A typical anticline is convex curve, conv ...

anticline
or trough of a
syncline In structural geology Structural geology is the study of the three-dimensional distribution of Rock (geology), rock units with respect to their Deformation (geology), deformational histories. The primary goal of structural geology is to use meas ...

syncline
:
Volcanic pipe Volcanic pipes or volcanic conduits are subterranean geological Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geolo ...
or
volcanic neck A volcano is a rupture in the Crust (geology), crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and Volcanic gas, gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

volcanic neck
: tubular roughly vertical body that may have been a feeder vent for a
volcano A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet a ...

volcano
: Sill: a relatively thin tabular concordant body intruded along bedding planes :
Stock In finance, stock (also capital stock) consists of all of the shares In financial markets A financial market is a market in which people trade financial securities and derivatives at low transaction costs. Some of the securities i ...
: a smaller irregular discordant intrusive :
Boss Boss may refer to: * Supervisor A supervisor, or also known as foreman, boss, overseer, facilitator, monitor, area coordinator, or sometimes gaffer, is the job title of a low level management position that is primarily based on authority over a ...
: a small stock


See also

* * * Methods of pluton emplacement *
Norbeck Intrusive Suite The Norbeck Intrusive Suite is an Ordovician granitic pluton in Montgomery County, Maryland, Montgomery County, Maryland. The intrusive suite was originally mapped as the Norbeck Quartz Diorite by Hopson, and is shown as such on the Geologic Map of ...
*
Subvolcanic rock A subvolcanic rock, also known as a hypabyssal rock, is an intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magma leaving behind igneous ...
* Tuolumne Intrusive Suite *
Volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a formed from erupted from a . In other words, it differs from other by being of origin. Like all rock types, the concept of volcanic rock is artificial, and in nature vo ...

Volcanic rock
* Woodstock Quartz Monzonite


References

{{Authority control Petrology Rocks