![NotchPeak](https://upload.wikimedia.org/wikipedia/commons/2/20/NotchPeak.jpg)
In
geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
, an igneous intrusion (or intrusive body
or simply intrusion
) is a body of
intrusive 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 ...
that forms by crystallization of
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
slowly cooling below the surface of the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
. Intrusions have a wide variety of forms and compositions, illustrated by examples like the
Palisades Sill
The Palisades Sill is a Triassic, 200 Ma diabase intrusion. It extends through portions of New York and New Jersey. It is most noteworthy for The Palisades, the cliffs that rise steeply above the western bank of the Hudson River. The ideal loc ...
of
New York and
New Jersey
New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...
; the
Henry Mountains
The Henry Mountains is a mountain range located in the southeastern portion of the U.S. state of Utah that runs in a generally north-south direction, extending over a distance of about . They were named by Almon Thompson in honor of Joseph Henry ...
of
Utah
Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...
; the
Bushveld Igneous Complex
The Bushveld Igneous Complex (BIC) is the largest layered igneous intrusion within the Earth's crust. It has been tilted and eroded forming the outcrops around what appears to be the edge of a great geological basin: the Transvaal Basin. It i ...
of
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
;
Shiprock
Shiprock ( nv, , "rock with wings" or "winged rock") is a monadnock rising nearly above the high-desert plain of the Navajo Nation in San Juan County, New Mexico, United States. Its peak elevation is above sea level. It is about southwest ...
in
New Mexico
)
, population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano)
, seat = Santa Fe
, LargestCity = Albuquerque
, LargestMetro = Tiguex
, OfficialLang = None
, Languages = English, Spanish ( New Mexican), Navajo, Ker ...
; the
Ardnamurchan
Ardnamurchan (, gd, Àird nam Murchan: headland of the great seas) is a peninsula in the ward management area of Lochaber, Highland, Scotland, noted for being very unspoiled and undisturbed. Its remoteness is accentuated by the main access ...
intrusion in Scotland; and the
Sierra Nevada Batholith of
California
California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...
.
Because the solid
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
A phanerite is an igneous rock whose microstructure is made up of crystals large enough to be distinguished with the unaided human eye. In contrast, the crystals in an aphanitic rock are too fine-grained to be identifiable. Phaneritic texture fo ...
). Intrusive igneous rocks are classified separately from
extrusive
Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contra ...
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. ( ...
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 tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
,
alkali feldspar,
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
, 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 usually not found in rocks containing prim ...
is particularly important in
classifying intrusive igneous rocks.
Intrusions must displace existing country rock to make room for themselves. The question of how this takes place is called the ''room problem'', and it remains a subject of active investigation for many kinds of intrusions.
The term pluton is poorly defined, but has been used to describe an intrusion emplaced at great depth; as a synonym for all igneous intrusions; as a
dustbin category The term "dustbin category" is sometimes used to describe a category that includes people or things that might be heterogeneous, only loosely related or poorly understood. It has been used in discussion of law, linguistics, medicine, sociology and ...
for intrusions whose size or character are not well determined; or as a name for a very large intrusion or for a crystallized
magma chamber
A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it up ...
. A pluton that has intruded and obscured the contact between a
terrane
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust (geology), crust fragment formed on a tectonic plate (or broken off from it) and Accretion (geology), accreted or "Suture (geology), sutured" to crust lying on another pla ...
and adjacent rock is called a stitching pluton.
Classification
![Intrusion types](https://upload.wikimedia.org/wikipedia/commons/6/66/Intrusion_types.svg)
Intrusions are broadly divided into ''discordant intrusions'', which cut across the existing structure of the country rock, and ''concordant intrusions'' that intrude parallel to existing
bedding
Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environm ...
or
fabric
Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not th ...
. These are further classified according to such criteria as size, evident mode of origin, or whether they are tabular in shape.
An ''
intrusive suite'' is a group of intrusions related in time and space.
[Oxford Academic]
Crustal Contamination of Picritic Magmas During Transport Through Dikes: the Expo Intrusive Suite, Cape Smith Fold Belt, New Quebec , Journal of Petrology , Oxford Academic
accessdate: March 27, 2017.[9/28/94]
accessdate: March 27, 2017
Discordant intrusions
Dikes
Dikes are tabular discordant intrusions, taking the form of sheets that cut across existing rock beds. They tend to resist erosion, so that they stand out as natural walls on the landscape. They vary in thickness from millimeter-thick films to over and an individual sheet can have an area of . They also vary widely in composition. Dikes form by hydraulic fracturing of the country rock by magma under pressure, and are more common in regions of
crustal tension.
=Ring dikes and cone sheets
=
Ring dikes and cone sheets are dikes with particular forms that are associated with the formation of
caldera
A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
s.
Volcanic necks
Volcanic necks are feeder pipes for
volcanoes
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates a ...
that have been exposed by
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
. Surface exposures are typically cylindrical, but the intrusion often becomes elliptical or even cloveleaf-shaped at depth. Dikes often radiate from a volcanic neck, suggesting that necks tend to form at intersections of dikes where passage of magma is least obstructed.
Diatremes and breccia pipes
Diatremes and breccia pipes are pipe-like bodies of
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
that are formed by particular kinds of explosive eruptions.
Stocks
A stock is a non-tabular discordant intrusion whose exposure covers less than . Although this seems arbitrary, particularly since the exposure may be only the tip of a larger intrusive body, the classification is meaningful for bodies which do not change much in area with depth and that have other features suggesting a distinctive origin and mode of emplacement.
Batholiths
Batholiths are discordant intrusions with an exposed area greater than . Some are of truly enormous size, and their lower contacts are very rarely exposed. For example, the
Coastal Batholith of Peru is long and wide. They are usually formed from magma rich in
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
, and never from
gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...
or other rock rich in mafic minerals, but some batholiths are composed almost entirely of
anorthosite
Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most ...
.
Concordant intrusions
Sills
A sill is a tabular concordant intrusion, typically taking the form of a sheet parallel to sedimentary beds. They are otherwise similar to dikes. Most are of
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...
composition, relatively low in silica, which gives them the low viscosity necessary to penetrate between sedimentary beds.
Laccoliths
A laccolith is a concordant intrusion with a flat base and domed roof. Laccoliths typically form at shallow depth, less than , and in regions of crustal compression.
Lopoliths and layered intrusions
Lopoliths are concordant intrusions with a saucer shape, somewhat resembling an inverted laccolith, but they can be much larger and form by different processes. Their immense size promotes very slow cooling, and this produces an unusually complete mineral segregation called a
layered intrusion
A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around to over and several hundred metres to ...
.
Formation
The room problem
The ultimate source of
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
is
partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...
of rock in the
upper mantle
The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...
and lower
crust. This produces magma that is less dense than its source rock. For example, a granitic magma, which is high in silica, has a density of 2.4 Mg/m
3, much less than the 2.8 Mg/m
3 of high-grade metamorphic rock. This gives the magma tremendous buoyancy, so that ascent of the magma is inevitable once enough magma has accumulated. However, the question of precisely how large quantities of magma are able to shove aside
country rock to make room for themselves (the ''room problem'') is still a matter of research.
The composition of the magma and country rock and the stresses affecting the country rock strongly influence the kinds of intrusions that take place. For example, where the crust is undergoing extension, magma can easily rise into tensional fractures in the upper crust to form dikes. Where the crust is under compression, magma at shallow depth will tend to form laccoliths instead, with the magma penetrating the least competent beds, such as shale beds.
Ring dikes and cone sheets form only at shallow depth, where a plug of overlying country rock can be raised or lowered. The immense volumes of magma involved in batholiths can force their way upwards only when the magma is highly silicic and buoyant, and are likely do so as
diapir
A diapir (; , ) is a type of igneous intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–T ...
s in the ductile deep crust and through a variety of other mechanisms in the brittle upper crust.
Multiple and composite intrusions
Igneous intrusions may form from a single magmatic event or several incremental events. Recent evidence suggests that incremental formation is more common for large intrusions. For example, the Palisades Sill was never a single body of magma thick, but was formed from multiple injections of magma. An intrusive body is described as ''multiple'' when it forms from repeated injections of magma of similar composition, and as ''composite'' when formed of repeated injections of magma of unlike composition. A composite dike can include rocks as different as
granophyre
Granophyre ( ; from ''granite'' and ''porphyry'') is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image.
The texture is called granophyric. The texture can b ...
and
diabase
Diabase (), also called dolerite () or microgabbro,
is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-graine ...
.
While there is often little visual evidence of multiple injections in the field, there is geochemical evidence.
Zircon
Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...
zoning provides important evidence for determining if a single magmatic event or a series of injections were the methods of emplacement.
Large felsic intrusions likely form from melting of lower crust that has been heated by an intrusion of mafic magma from the upper mantle. The different densities of felsic and mafic magma limit mixing, so that the silicic magma floats on the mafic magma. Such limited mixing as takes place results in the small inclusions of mafic rock commonly found in granites and granodiorites.
Cooling
![Thermal diffusion at contact](https://upload.wikimedia.org/wikipedia/commons/b/b3/Thermal_diffusion_at_contact.jpg)
An intrusion of magma loses heat to the surrounding country rock through heat conduction. Near the contact of hot material with cold material, if the hot material is initially uniform in temperature, the temperature profile across the contact is given by the relationship
where
is the initial temperature of the hot material, ''k'' is the thermal diffusivity (typically close to 10
−6 m
2 s
−1 for most geologic materials), x is the distance from the contact, and t is the time since intrusion. This formula suggests that the magma close to the contact will be rapidly chilled while the country rock close to the contact is rapidly heated, while material further from the contact will be much slower to cool or heat. Thus a ''
chilled margin A chilled margin is a shallow intrusive or volcanic rock texture characterised by a glassy or fine-grained zone along the margin where the magma or lava has contacted air, water, or particularly much cooler rock. This is caused by rapid crystalliz ...
'' is often found on the intrusion side of the contact, while a ''
contact aureole
Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...
'' is found on the country rock side. The chilled margin is much finer grained than most of the intrusion, and may be different in composition, reflecting the initial composition of the intrusion before fractional crystallization, assimilation of country rock, or further magmatic injections modified the composition of the rest of the intrusion. Isotherms (surfaces of constant temperature) propagate away from the margin according to a square root law, so that if the outermost meter of the magma takes ten years to cool to a given temperature, the next inward meter will take 40 years, the next will take 90 years, and so on.
This is an idealization, and such processes as magma convection (where cooled magma next to the contact sinks to the bottom of the magma chamber and hotter magma takes its place) can alter the cooling process, reducing the thickness of chilled margins while hastening cooling of the intrusion as a whole. However, it is clear that thin dikes will cool much faster than larger intrusions, which explains why small intrusions near the surface (where the country rock is initially cold) are often nearly as fine-grained as volcanic rock.
Structural features of the contact between intrusion and country rock give clues to the conditions under which the intrusion took place. ''Catazonal intrusions'' have a thick aureole that grades into the intrusive body with no sharp margin, indicating considerable chemical reaction between intrusion and country rock, and often have broad
migmatite
Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock th ...
zones.
Foliations in the intrusion and the surrounding country rock are roughly parallel, with indications of extreme deformation in the country rock. Such intrusions are interpreted as taking placed at great depth. ''Mesozonal intrusions'' have a much lower degree of metamorphism in their contact aureoles, and the contact between country rock and intrusion is clearly discernible. Migmatites are rare and deformation of country rock is moderate. Such intrusions are interpreted as occurring at medium depth. ''Epizonal intrusions'' are discordant with country rock and have sharp contacts with chilled margins, with only limited metamorphism in a contact aureole, and often contain xenolithic fragments of country rock suggesting brittle fracturing. Such intrusions are interpreted as occurring at shallow depth, and are commonly associated with volcanic rocks and collapse structures.
Cumulates
An intrusion does not crystallize all minerals at once; rather, there is a sequence of crystallization that is reflected in the
Bowen reaction series. Crystals formed early in cooling are generally denser than the remaining magma and can settle to the bottom of a large intrusive body. This forms a ''cumulate layer'' with distinctive texture and composition. Such cumulate layers may contain valuable ore deposits of
chromite. The vast
Bushveld Igneous Complex
The Bushveld Igneous Complex (BIC) is the largest layered igneous intrusion within the Earth's crust. It has been tilted and eroded forming the outcrops around what appears to be the edge of a great geological basin: the Transvaal Basin. It i ...
of
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
includes cumulate layers of the rare rock type, chromitite, composed of 90% chromite,
[Guilbert, John M., and Park, Charles F., Jr. (1986) ''The Geology of Ore Deposits,'' Freeman, ]
See also
References
Further reading
* Best, Myron G. (1982). ''Igneous and Metamorphic Petrology''. San Francisco: W. H. Freeman & Company. pp. 119 ff. .
* Young, Davis A. (2003). ''Mind Over Magma: the Story of Igneous Petrology''. Princeton University Press. .
External links
{{Authority control
Igneous petrology
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