
Intrusive rock is formed when
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 natura ...
penetrates existing rock, crystallizes, and solidifies underground to form ''
intrusions'', such as
batholith
A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, su ...
s,
dikes,
sills,
laccolith
A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apar ...
s, and
volcanic necks.
[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 rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or l ...
can form. The other is
extrusion, such as a
volcanic eruption
Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are ...
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 a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a ...
. 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
Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
in batholiths or
stocks
Stocks are feet restraining devices that were used as a form of corporal punishment and public humiliation. The use of stocks is seen as early as Ancient Greece, where they are described as being in use in Solon's law code. The law describing ...
) 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 genre of music which fuses rock and country. 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). 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. ...
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 ...
,
alkali feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
,
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 pro ...
, 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 pri ...
are particularly important in classifying intrusive igneous rocks, and most plutonic rocks are classified by where they fall in the
QAPF diagram.
Dioritic and
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 ...
ic rocks are further distinguished by whether the plagioclase they contain is
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
-rich, and sodium-poor gabbros are classified by their relative contents of various
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
- or
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
-rich minerals (
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 includ ...
minerals) such as
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...
,
hornblende
Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic ro ...
,
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.
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 of
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
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, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-gra ...
s or dolerites. Rare ultramafic hypabyssal rocks called ''
lamprophyre
Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica- undersaturated mafic or ultramafic rocks with high magne ...
s'' have their own classification scheme.
Characteristics
Intrusive rocks are characterized by large
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...
sizes, and as the individual crystals are visible, the rock is called ''
phaneritic''.
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
cavities, 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''.
[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 is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning a ...
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
Euhedral crystals (also known as idiomorphic or automorphic crystals) are those that are well-formed, with sharp, easily recognised faces. The opposite is anhedral (also known as ''xenomorphic'' or ''allotriomorphic''): a rock with an anhedral ...
(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 formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angl ...
, while the higher-temperature polymorph,
sanidine, is more common in volcanic rock. The same distinction holds for
nepheline
Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatit ...
varieties.
Leucite
Leucite is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium tectosilicate KAlSi2O6. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster ...
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 mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage ...
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 and plutonic rocks. They are very commonly porphyritic,
vitreous, 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 () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, su ...
s to thin vein
Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenate ...
like fracture fillings of aplite or pegmatite
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic co ...
.
*Batholith
A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, su ...
: 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, fr ...
: a dome-shaped projection from the top of a large subterranean intrusion
* Dike: a relatively narrow tabular discordant body, often nearly vertical
*Laccolith
A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apar ...
: concordant body with roughly flat base and convex
Convex or convexity may refer to:
Science and technology
* Convex lens, in optics
Mathematics
* Convex set, containing the whole line segment that joins points
** Convex polygon, a polygon which encloses a convex set of points
** Convex polytop ...
top, usually with a feeder pipe below
* Lopolith: concordant body with roughly flat top and a shallow convex base, may have a feeder dike or pipe below
* Phacolith: a concordant lens-shaped pluton that typically occupies the crest of an anticline
In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
or trough of a syncline
*Volcanic pipe
Volcanic pipes or volcanic conduits are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. They are considered to be a type of '' diatreme''. Volcanic pipes are composed of a deep, narrow cone ...
or 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, 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 ...
*Sill
Sill may refer to:
* Sill (dock), a weir at the low water mark retaining water within a dock
* Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma
* Sill (geostatistics)
* Sill (river), a river in Austria
* Sill plate, a ...
: a relatively thin tabular concordant body intruded along bedding planes
*Stock
In finance, stock (also capital stock) consists of all the shares by which ownership of a corporation or company is divided.Longman Business English Dictionary: "stock - ''especially AmE'' one of the shares into which ownership of a company ...
: a smaller irregular discordant intrusive
**Boss: a small stock
See also
* Ellicott City Granodiorite
* Guilford Quartz Monzonite
* Pluton emplacement
* Norbeck Intrusive Suite
* Subvolcanic rock
* Tuolumne Intrusive Suite
* Volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcan ...
* Woodstock Quartz Monzonite
References
{{Authority control
Petrology
Rocks