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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 Rome, known as Latium. Through the power of the Roman Republic, it became the ...

Latin
word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being
sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation. Sedimentation Sedimentation is the tendency for particle (ecology), parti ...

sedimentary
and
metamorphic
metamorphic
. Igneous rock is formed through the cooling and solidification of
magma 300px, Hawaii Hawaii ( ; haw, Hawaii or ) is a U.S. state, state in the Western United States, located in the Pacific Ocean about 2,000 miles from the U.S. mainland. It is the only state outside North America, the only state that is ...

magma
or
lava of pāhoehoe lava, Hawaii, United States , Iceland in 1984 Lava is molten Rock (geology), rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon. Magma is generated by the inte ...

lava
. The magma can be derived from partial melts of existing rocks in either a
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 ...
's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ma ...

pressure
, or a change in composition. Solidification into rock occurs either below the surface as
intrusive rock , an igneous ''intrusion'' exposed when the surrounding softer rock eroded away Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, ...
s or on the surface as
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 ro ...
rocks. Igneous rock may form with
crystallization Crystallization or crystallisation is the process by which a solid forms, where the atoms or molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five car ...

crystallization
to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust.


Geological significance

Igneous and metamorphic rocks make up 90–95% of the top of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are also geologically important because: * their
minerals 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. (20 ...

minerals
and global chemistry give information about the composition of the lower crust or upper mantle from which their parent magma was extracted, and the temperature and pressure conditions that allowed this extraction; * their
absolute agesAbsolute dating is the process of determining an age on a specified chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language ...
can be obtained from various forms of
radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the ...
and can be compared to adjacent geological
strata (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half of South America. Sharing the bulk of the Southern Cone with Chile to the west, the country is ...
, thus permitting calibration of the
geological time scale The geologic time scale (GTS) is a system of chronological dating that classifies geological strata (stratigraphy through Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that ...
; * their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstructions (see
plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτον ...
); * in some special circumstances they host important mineral deposits (
ore ore – psilomelane (size: 6.7 × 5.8 × 5.1 cm) ore – galena and anglesite (size: 4.8 × 4.0 × 3.0 cm) ore (size: 7.5 × 6.1 × 4.1 cm) File:OreCartPachuca.JPG, upMinecart on display at the Historic Archive and Museum of Min ...

ore
s): for example,
tungsten Tungsten, or wolfram, is a chemical element with the Symbol (chemistry), symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 17 ...

tungsten
,
tin Tin is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers ...

tin
, and
uranium Uranium is a chemical element with the Symbol (chemistry), symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence elect ...

uranium
are commonly associated with
granite Granite () is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools ...

granite
s and
diorite Orbicular diorite from Corsica (corsite) Diorite ( ) is an intrusive rock, intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metal ...
s, whereas ores of
chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

chromium
and
platinum Platinum is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

platinum
are commonly associated with
gabbro Gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive 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 language ...

gabbro
s.


Geological setting

Igneous rocks can be either
intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magma leaving behind igneous rock * Saltwater intrusion, the movement of saline water into freshwa ...
(
plutonic , an igneous ''intrusion'' exposed when the surrounding softer rock eroded away Intrusive rock is formed when magma 300px, Lava flow on Hawaii (island), Hawaii. Lava is the extrusive equivalent of magma. Magma (from Ancient Greek μάγμα (' ...
and hypabyssal) or
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 ro ...
(
volcanic 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 ar ...

volcanic
).


Intrusive

Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within the crust of a planet. Bodies of intrusive rock are known as ''
intrusion
intrusion
s'' and are surrounded by pre-existing rock (called ''
country rock Country rock is a subgenre of popular music, formed from the fusion of 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 ...
''). The country rock is an excellent
thermal insulator Insulation, 1600 dpi scan Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulati ...
, so the magma cools slowly, and intrusive rocks are coarse-grained (''
phaneritic Close-up of phaneritic granite exposed in Chennai, India A phanerite is an igneous rock whose Rock microstructure, microstructure is made up of crystals large enough to be distinguished with the unaided human eye, eye. (In contrast, the crystals ...
''). The mineral
grain A grain is a small, hard, dry seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, ...
s in such rocks can generally be identified with the naked eye. Intrusions can be classified according to the shape and size of the intrusive body and its relation to the
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 environmen ...
of the country rock into which it intrudes. Typical intrusive bodies are
batholith Image:Yosemite 20 bg 090404.jpg, upright=1.3, Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada Batholith A batholith (from Greek ''bathos'', depth + ''lithos'', rock) is a large mass of Intrusive rock, intrusiv ...
s,
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 in ...
s,
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, sills and dikes. Common intrusive rocks are
granite Granite () is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools ...

granite
,
gabbro Gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive 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 language ...

gabbro
, or
diorite Orbicular diorite from Corsica (corsite) Diorite ( ) is an intrusive rock, intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metal ...
. The central cores of major mountain ranges consist of intrusive igneous rocks. When exposed by erosion, these cores (called ''
batholith Image:Yosemite 20 bg 090404.jpg, upright=1.3, Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada Batholith A batholith (from Greek ''bathos'', depth + ''lithos'', rock) is a large mass of Intrusive rock, intrusiv ...
s'') may occupy huge areas of the Earth's surface. Intrusive igneous rocks that form at depth within the crust are termed plutonic (or ''abyssal'') rocks and are usually coarse-grained. Intrusive igneous rocks that form near the surface are termed '' subvolcanic'' or ''hypabyssal'' rocks and they are usually much finer-grained, often resembling volcanic rock. Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths,
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 ...
s, or
phacolith
phacolith
s.


Extrusive

Extrusive igneous rock, also known as volcanic rock, is formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or
volcanic eruptions Several types of volcanic eruptions—during which lava of pāhoehoe lava, Hawaii, United States , Iceland in 1984 Lava is molten Rock (geology), rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) ...
, rapidly solidifies. Hence such rocks are fine-grained (
aphanitic is aphanitic. Image:LvMS-Lvv.jpg, An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope Aphanite, or aphanitic as an adjective (from the Greek language, Greek αφανης, "invisible"), is a name ...
) or even glassy.
Basalt Basalt (, ) is a 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 a moon. More than 90% of all ...
is the most common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns. The
Giant's Causeway The Giant's Causeway is an area of about 40,000 interlocking Columnar basalt, basalt columns, the result of an ancient volcano, volcanic fissure vent, fissure eruption. It is located in County Antrim on the north coast of Northern Ireland, about ...

Giant's Causeway
in Antrim, Northern Ireland is an example. The molten rock, which typically contains suspended crystals and dissolved gases, is called
magma 300px, Hawaii Hawaii ( ; haw, Hawaii or ) is a U.S. state, state in the Western United States, located in the Pacific Ocean about 2,000 miles from the U.S. mainland. It is the only state outside North America, the only state that is ...

magma
. It rises because it is less dense than the rock from which it was extracted. When magma reaches the surface, it is called
lava of pāhoehoe lava, Hawaii, United States , Iceland in 1984 Lava is molten Rock (geology), rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon. Magma is generated by the inte ...

lava
. Eruptions of
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 plat ...
into air are termed ''
subaerialIn natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of ...
'', whereas those occurring underneath the ocean are termed ''
submarine upright=1.35, Russian ''Akula''-class submarine of the Northern Fleet A submarine (or sub) is a watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles used in water, including boats, ship A ship is a ...

submarine
''. Black smokers and
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate ...
basalt are examples of submarine volcanic activity. The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions: *
divergent boundary In plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin, tectonicus, from ...
: 73% *
convergent boundary A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction Subduction is a geological process in w ...
(
subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate This is a list of tectonic plates on Earth's surface. Tec ...
): 15% * hotspot: 12%. The behaviour of lava depends upon its
viscosity The viscosity of a fluid is a measure of its drag (physics), resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity can be ...

viscosity
, which is determined by temperature, composition, and crystal content. High-temperature magma, most of which is basaltic in composition, behaves in a manner similar to thick oil and, as it cools,
treacle Treacle () is any uncrystallised syrup In cooking, a syrup or sirup (from ar, شراب; ''sharāb'', beverage, wine and la, sirupus) is a condiment that is a thick, viscous The viscosity of a fluid In physics, a fluid is a substanc ...
. Long, thin basalt flows with
pahoehoe of pāhoehoe lava, Hawaii, United States , Iceland in 1984 Lava is molten Rock (geology), rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon. Magma is generated by the inte ...

pahoehoe
surfaces are common. Intermediate composition magma, such as
andesite Andesite ( or ) is an extrusive volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being o ...
, tends to form cinder cones of intermingled
ash Ash or ashes are the solid remnants of fire BBQ. Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products. Fire is hot because the ...
,
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

tuff
and lava, and may have a viscosity similar to thick, cold
molasses Molasses () or black treacle (British English) is a viscosity, viscous product resulting from sugar refining, refining sugarcane or sugar beets into sugar. Molasses varies by the amount of sugar, method of extraction, and age of plant. Sugarcane ...

molasses
or even rubber when erupted.
Felsic 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 (geology), rocks of which it is composed, and the processes ...
magma, such as
rhyolite Rhyolite ( ) is the most silica-rich of volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by b ...

rhyolite
, is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows are typically of limited extent and have steep margins because the magma is so viscous. Felsic and intermediate magmas that erupt often do so violently, with explosions driven by the release of dissolved gases—typically water vapour, but also
carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occurs naturally in At ...

carbon dioxide
. Explosively erupted
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material is called
tephra Tephra is fragmental material produced by a volcanic eruption Several types of volcanic eruptions—during which lava, tephra (Volcanic ash, ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a Volcan ...

tephra
and includes
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

tuff
,
agglomerate Agglomerate (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 Rome, known as Latium. Through the power of the ...

agglomerate
and
ignimbrite Ignimbrite is a variety of hardened tuff. Ignimbrites are igneous rocks made up of crystal and rock fragments in a glass-shard Matrix (geology), groundmass, albeit the original Texture (geology), texture of the groundmass might be obliterated due ...

ignimbrite
. Fine volcanic ash is also erupted and forms ash tuff deposits, which can often cover vast areas. Because volcanic rocks are mostly fine-grained or glassy, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of
thin section vein in mica Mica is a group of sheet silicate minerals, not to be confused with Micah. Mica or MICA may also refer to: Companies * Mica DIY, a UK Retailer Co-operative, Symbol group Acronyms * Mahone Islands Conservation Association * ...
s of the rock under a
microscope A microscope (from grc, μικρός ''mikrós'' 'small' and ''skopeîn'' 'to look (at); examine, inspect') is a laboratory instrument used to examine objects that are too small to be seen by the naked eye Naked eye, also called bare ...
, so only an approximate classification can usually be made in the
field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a grassl ...

field
. Although classification by mineral makeup is preferred by the
IUGS The International Union of Geological Sciences (IUGS) is an international non-governmental organization File:Europe in a suitcase - UK.jpg, upright=1.3, alt=A roomful of people, Europe-Georgia Institute head George Melashvili addresses the audi ...
, this is often impractical, and chemical classification is done instead using the
TAS classification The TAS classification can be used to assign names to many common types of volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it dif ...
.


Classification

Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. The classification of the many types of igneous rocks can provide important information about the conditions under which they formed. Two important variables used for the classification of igneous rocks are particle size, which largely depends on the cooling history, and the mineral composition of the rock.
Feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, containing sodium, calcium, potassium or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspars and the ''alkali'' (potas ...
s,
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
or
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 ...
s,
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. Ra ...

olivine
s,
pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approx ...
s,
amphibole Amphibole () is a group of inosilicate 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 350px, Plates in t ...
s, and
mica Micas ( ) are a group of minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect Cleavage (crystal), basal cleavage ...

mica
s are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called ''accessory minerals''. Types of igneous rocks with other essential minerals are very rare, but include
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 ...
s, which contain essential
carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline min ...
s. In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence 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
, and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present. Rocks containing quartz (silica in composition) are ''silica-oversaturated''. Rocks with
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 ...
s are ''silica-undersaturated'', because feldspathoids cannot coexist in a stable association with quartz. Igneous rocks that have crystals large enough to be seen by the naked eye are called
phaneritic Close-up of phaneritic granite exposed in Chennai, India A phanerite is an igneous rock whose Rock microstructure, microstructure is made up of crystals large enough to be distinguished with the unaided human eye, eye. (In contrast, the crystals ...
; those with crystals too small to be seen are called
aphanitic is aphanitic. Image:LvMS-Lvv.jpg, An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope Aphanite, or aphanitic as an adjective (from the Greek language, Greek αφανης, "invisible"), is a name ...
. Generally speaking, phaneritic implies an intrusive origin; aphanitic an extrusive one. An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry. Porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material. Igneous rocks are classified on the basis of texture and composition. Texture refers to the size, shape, and arrangement of the mineral grains or crystals of which the rock is composed.


Texture

Texture is an important criterion for the naming of volcanic rocks. The
texture Texture may refer to: Science and technology * Surface texture, the texture means smoothness, roughness, or bumpiness of the surface of an object * Texture (roads), road surface characteristics with waves shorter than road roughness * Texture (co ...
of volcanic rocks, including the size, shape, orientation, and distribution of mineral grains and the intergrain relationships, will determine whether the rock is termed a
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

tuff
, a
pyroclastic 300px, USGS scientist examines Mount_St._Helens.html"_;"title="pumice_blocks_at_the_edge_of_a_pyroclastic_flow_from_Mount_St._Helens">pumice_blocks_at_the_edge_of_a_pyroclastic_flow_from_Mount_St._Helens_ File:Volcanic_Stone_3D.ogg.html" ;"titl ...
lava or a simple
lava of pāhoehoe lava, Hawaii, United States , Iceland in 1984 Lava is molten Rock (geology), rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon. Magma is generated by the inte ...

lava
. However, the texture is only a subordinate part of classifying volcanic rocks, as most often there needs to be chemical information gleaned from rocks with extremely fine-grained
groundmass The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ord ...
or from airfall tuffs, which may be formed from volcanic ash. Textural criteria are less critical in classifying intrusive rocks where the majority of minerals will be visible to the naked eye or at least using a hand lens, magnifying glass or microscope. Plutonic rocks also tend to be less texturally varied and less prone to showing distinctive structural fabrics. Textural terms can be used to differentiate different intrusive phases of large plutons, for instance
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 ...
margins to large intrusive bodies, porphyry stocks and subvolcanic
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 ...
s. Mineralogical classification is most often used to classify plutonic rocks. Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite".


Mineralogical classification

The
IUGS The International Union of Geological Sciences (IUGS) is an international non-governmental organization File:Europe in a suitcase - UK.jpg, upright=1.3, alt=A roomful of people, Europe-Georgia Institute head George Melashvili addresses the audi ...
recommends classifying igneous rocks by their mineral composition whenever possible. This is straightforward for coarse-grained intrusive igneous rock, but may require examination of thin sections under a microscope for fine-grained volcanic rock, and may be impossible for glassy volcanic rock. The rock must then be classified chemically. Mineralogical classification of an intrusive rock begins by determining if the rock is ultramafic, a carbonatite, or a
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 ...
. An ultramafic rock contains more than 90% of iron- and magnesium-rich minerals such as hornblende, pyroxene, or olivine, and such rocks have their own classification scheme. Likewise, rocks containing more than 50% carbonate minerals are classified as carbonatites, while lamprophyres are rare ultrapotassic rocks. Both are further classified based on detailed mineralogy. In the great majority of cases, the rock has a more typical mineral composition, with significant quartz, feldspars, or feldspathoids. Classification is based on the percentages of quartz, alkali feldspar, plagioclase, and feldspathoid out of the total fraction of the rock composed of these minerals, ignoring all other minerals present. These percentages place the rock somewhere on the
QAPF diagram A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym An acronym is a word or name formed from the initial components of a longer name or phrase, usually using ind ...
, which often immediately determines the rock type. In a few cases, such as the diorite-gabbro-anorthite field, additional mineralogical criteria must be applied to determine the final classification. Where the mineralogy of an volcanic rock can be determined, it is classified using the same procedure, but with a modified QAPF diagram whose fields correspond to volcanic rock types.


Chemical classification and petrology

When it is impractical to classify a volcanic rock by mineralogy, the rock must be classified chemically. There are relatively few minerals that are important in the formation of common igneous rocks, because the magma from which the minerals crystallize is rich in only certain elements:
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...

silicon
,
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

oxygen
, aluminium,
sodium Sodium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...

sodium
,
potassium Potassium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

potassium
,
calcium Calcium is a chemical element with the Symbol (chemistry), symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties a ...

calcium
, iron, and
magnesium Magnesium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

magnesium
. These are the elements that combine to form the
silicate minerals 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 a specific crystal structure that occurs na ...
, which account for over ninety percent of all igneous rocks. The chemistry of igneous rocks is expressed differently for major and minor elements and for trace elements. Contents of major and minor elements are conventionally expressed as weight percent oxides (e.g., 51% SiO2, and 1.50% TiO2). Abundances of trace elements are conventionally expressed as parts per million by weight (e.g., 420 ppm Ni, and 5.1 ppm Sm). The term "trace element" is typically used for elements present in most rocks at abundances less than 100 ppm or so, but some trace elements may be present in some rocks at abundances exceeding 1,000 ppm. The diversity of rock compositions has been defined by a huge mass of analytical data—over 230,000 rock analyses can be accessed on the web through a site sponsored by the U. S. National Science Foundation (see the External Link to EarthChem). The single most important component is silica, SiO2, whether occurring as quartz or combined with other oxides as feldspars or other minerals. Both intrusive and volcanic rocks are grouped chemically by total silica content into broad categories. * ''
Felsic 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 (geology), rocks of which it is composed, and the processes ...
'' rocks have the highest content of silica, and are predominantly composed of the ''felsic minerals'' quartz and feldspar. These rocks (granite, rhyolite) are usually light coloured, and have a relatively low density. * '' Intermediate'' rocks have a moderate content of silica, and are predominantly composed of feldspars. These rocks (diorite, andesite) are typically darker in colour than felsic rocks and somewhat more dense. * ''
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 Amphibole () is a group of Silic ...
'' rocks have a relatively low silica content and are composed mostly of
pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approx ...
s,
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. Ra ...

olivine
s and calcic
plagioclase . (unknown scale) Plagioclase is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagiocla ...
. These rocks (basalt, gabbro) are usually dark coloured, and have a higher density than felsic rocks. * ''
Ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch o ...
'' rock is very low in silica, with more than 90% of mafic minerals (komatiite,
dunite Dunite ( or ) (also known as olivinite, not to be confused with the mineral olivenite) is an igneous, plutonic , an igneous ''intrusion'' exposed when the surrounding softer rock eroded away Intrusive rock is formed when magma penetrates exi ...
). This classification is summarized in the following table: The percentage of
alkali metal oxide The alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origi ...
s ( Na2O plus K2O) is second only to silica in its importance for chemically classifying volcanic rock. The silica and alkali metal oxide percentages are used to place volcanic rock on the TAS diagram, which is sufficient to immediately classify most volcanic rocks. Rocks in some fields, such as the trachyandesite field, are further classified by the ratio of potassium to sodium (so that potassic trachyandesites are latites and sodic trachyandesites are benmoreites). Some of the more mafic fields are further subdivided or defined by
normative mineralogy Normative mineralogy is a calculation of the composition of a rock sample that estimates the ''idealised mineralogy'' of a rock based on a quantitative chemical analysis according to the principles of geochemistry. Normative mineral calculations ...
, in which an idealized mineral composition is calculated for the rock based on its chemical composition. For example,
basanite Basanite () is an igneous, volcanic ( extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoid The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structur ...
is distinguished from
tephrite Leucite tephrite from Mayen, Eifel, Germany Tephrite is an igneous rock, igneous, volcanic (extrusive) rock (geology), rock, with aphanitic to porphyritic Texture (crystalline), texture. Mineral content is usually abundant feldspathoids (leucite ...

tephrite
by having a high normative olivine content. Other refinements to the basic TAS classification include: *
UltrapotassicUltrapotassic igneous rocks are a class of rare, volumetrically minor and generally ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous Igneous rock (derived from the Latin ...
– rocks containing molar K2O/Na2O >3. *
PeralkalinePeralkaline rocks include those 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 ...
– rocks containing molar (K2O + Na2O)/Al2O3 >1. * Peraluminous – rocks containing molar (K2O + Na2O + CaO)/Al2O3 <1. In older terminology, silica oversaturated rocks were called ''silicic'' or ''acidic'' where the SiO2 was greater than 66% and the family term ''quartzolite'' was applied to the most silicic. A normative
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 ...
classifies a rock as silica-undersaturated; an example is nephelinite. Image:AFM diagram -.svg, thumbnail, AFM ternary diagram showing the relative proportions of Na2O + K2O (A for Alkali earth metals), FeO + Fe2O3 (F), and MgO (M) with arrows showing the path of chemical variation in tholeiitic and calc-alkaline series magmas Magmas are further divided into three series: * The ''Tholeiitic magma series, tholeiitic'' series – basaltic andesites and
andesite Andesite ( or ) is an extrusive volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being o ...
s. * The Calc-alkaline magma series, calc-alkaline series – andesites. * The Alkaline magma series, alkaline series – subgroups of alkaline basalts and the rare, very high potassium-bearing (i.e. Shoshonite, shoshonitic) lavas. The alkaline series is distinguishable from the other two on the TAS diagram, being higher in total alkali oxides for a given silica content, but the tholeiitic and calc-alkaline series occupy approximately the same part of the TAS diagram. They are distinguished by comparing total alkali with iron and magnesium content. These three magma series occur in a range of plate tectonic settings. Tholeiitic magma series rocks are found, for example, at mid-ocean ridges, back-arc basins, oceanic islands formed by hotspots, island arcs and continental large igneous provinces. All three series are found in relatively close proximity to each other at subduction zones where their distribution is related to depth and the age of the subduction zone. The tholeiitic magma series is well represented above young subduction zones formed by magma from relatively shallow depth. The calc-alkaline and alkaline series are seen in mature subduction zones, and are related to magma of greater depths. Andesite and basaltic andesite are the most abundant volcanic rock in island arc which is indicative of the calc-alkaline magmas. Some island arcs have distributed volcanic series as can be seen in the Japanese island arc system where the volcanic rocks change from tholeiite—calc-alkaline—alkaline with increasing distance from the trench.


History of classification

Some igneous rock names date to before the modern era of geology. For example, ''basalt'' as a description of a particular composition of lava-derived rock dates to Georgius Agricola in 1546 in his work ''De Natura Fossilium''. The word ''granite'' goes back at least to the 1640s and is derived either from French ''granit'' or Italian ''granito'', meaning simply "granulate rock". The term ''rhyolite'' was introduced in 1860 by the German traveler and geologist Ferdinand von Richthofen The naming of new rock types accelerated in the 19th century and peaked in the early 20th century. Much of the early classification of igneous rocks was based on the geological age and occurrence of the rocks. However, in 1902, the American petrologists Charles Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry Stephens Washington proposed that all existing classifications of igneous rocks should be discarded and replaced by a "quantitative" classification based on chemical analysis. They showed how vague, and often unscientific, much of the existing terminology was and argued that as the chemical composition of an igneous rock was its most fundamental characteristic, it should be elevated to prime position. Geological occurrence, structure, mineralogical constitution—the hitherto accepted criteria for the discrimination of rock species—were relegated to the background. The completed rock analysis is first to be interpreted in terms of the rock-forming minerals which might be expected to be formed when the magma crystallizes, e.g., quartz feldspars,
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. Ra ...

olivine
, akermannite, Feldspathoids, magnetite, corundum, and so on, and the rocks are divided into groups strictly according to the relative proportion of these minerals to one another. This new classification scheme created a sensation, but was criticized for its lack of utility in fieldwork, and the classification scheme was abandoned by the 1960s. However, the concept of normative mineralogy has endured, and the work of Cross and his coinvestigators inspired a flurry of new classification schemes. Among these was the classification scheme of M.A. Peacock, which divided igneous rocks into four series: the alkalic, the alkali-calcic, the calc-alkali, and the calcic series. His definition of the alkali series, and the term calc-alkali, continue in use as part of the widely used Irvine-Barager classification, along with W.Q. Kennedy's tholeiitic series. By 1958, there were some 12 separate classification schemes and at least 1637 rock type names in use. In that year, Albert Streckeisen wrote a review article on igneous rock classification that ultimately led to the formation of the IUGG Subcommission of the Systematics of Igneous Rocks. By 1989 a single system of classification had been agreed upon, which was further revised in 2005. The number of recommended rock names was reduced to 316. These included a number of new names promulgated by the Subcommission.


Origin of magmas

The Earth's crust averages about thick under the Continental crust, continents, but averages only some beneath the Oceanic crust, oceans. The continental crust is composed primarily of sedimentary rocks resting on a crystalline ''Basement (geology), basement'' formed of a great variety of metamorphic and igneous rocks, including granulite and granite. Oceanic crust is composed primarily of basalt and
gabbro Gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive 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 language ...

gabbro
. Both continental and oceanic crust rest on peridotite of the mantle. Rocks may melt in response to a decrease in pressure, to a change in composition (such as an addition of water), to an increase in temperature, or to a combination of these processes. Other mechanisms, such as melting from a Impact event, meteorite impact, are less important today, but impacts during the accretion (geology), accretion of the Earth led to extensive melting, and the outer several hundred kilometers of our early Earth was probably an ocean of magma. Impacts of large meteorites in the last few hundred million years have been proposed as one mechanism responsible for the extensive basalt magmatism of several large igneous provinces.


Decompression

Decompression melting occurs because of a decrease in pressure. The solidus (chemistry), solidus temperatures of most rocks (the temperatures below which they are completely solid) increase with increasing pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level. If such rock rises during the mantle convection, convection of solid mantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about 0.3 °C per kilometer. Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by 3 °C to 4 °C per kilometer. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from the upward movement of solid mantle is critical in the evolution of the Earth. Decompression melting creates the ocean crust at mid-ocean ridges. It also causes volcanism in intraplate regions, such as Europe, Africa and the Pacific sea floor. There, it is variously attributed either to the rise of mantle plumes (the "Plume hypothesis") or to intraplate extension (the "Plate hypothesis").


Effects of water and carbon dioxide

The change of rock composition most responsible for the creation of magma is the addition of water. Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800 °C in the presence of excess water, but near or above about 1,500 °C in the absence of water. Water is driven out of the oceanic lithosphere in
subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate This is a list of tectonic plates on Earth's surface. Tec ...
s, and it causes melting in the overlying mantle. Hydrous magmas composed of basalt and andesite are produced directly and indirectly as results of dehydration during the subduction process. Such magmas, and those derived from them, build up island arcs such as those in the Pacific Ring of Fire. These magmas form rocks of the calc-alkaline series, an important part of the continental crust. The addition of
carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occurs naturally in At ...

carbon dioxide
is relatively a much less important cause of magma formation than the addition of water, but genesis of some normative mineralogy, silica-undersaturated magmas has been attributed to the dominance of carbon dioxide over water in their mantle source regions. In the presence of carbon dioxide, experiments document that the peridotite solidus temperature decreases by about 200 °C in a narrow pressure interval at pressures corresponding to a depth of about 70 km. At greater depths, carbon dioxide can have more effect: at depths to about 200 km, the temperatures of initial melting of a carbonated peridotite composition were determined to be 450 °C to 600 °C lower than for the same composition with no carbon dioxide. Magmas of rock types such as nephelinite,
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 ...
, and kimberlite are among those that may be generated following an influx of carbon dioxide into mantle at depths greater than about 70 km.


Temperature increase

Increase in temperature is the most typical mechanism for formation of magma within continental crust. Such temperature increases can occur because of the upward intrusion of magma from the mantle. Temperatures can also exceed the solidus of a crustal rock in continental crust thickened by compression at a plate boundary. The plate boundary between the Indian and Asian continental masses provides a well-studied example, as the Tibetan Plateau just north of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normal continental crust. Studies of electrical resistivity deduced from Magnetotellurics, magnetotelluric data have detected a layer that appears to contain silicate melt and that stretches for at least 1,000 kilometers within the middle crust along the southern margin of the Tibetan Plateau. Granite and
rhyolite Rhyolite ( ) is the most silica-rich of volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by b ...

rhyolite
are types of igneous rock commonly interpreted as products of the melting of continental crust because of increases in temperature. Temperature increases also may contribute to the melting of lithosphere dragged down in a subduction zone.


Magma evolution

: Most magmas are fully melted only for small parts of their histories. More typically, they are mixes of melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually have different densities, and so they can separate as magmas evolve. As magma cools, minerals typically crystallize from the melt at different temperatures (Fractional crystallization (geology), fractional crystallization). As minerals crystallize, the composition of the residual melt typically changes. If crystals separate from the melt, then the residual melt will differ in composition from the parent magma. For instance, a magma of gabbroic composition can produce a residual melt of granite, granitic composition if early formed crystals are separated from the magma. Gabbro may have a liquidus temperature near 1,200 °C, and the derivative granite-composition melt may have a liquidus temperature as low as about 700 °C. Incompatible elements are concentrated in the last residues of magma during fractional crystallization and in the first melts produced during partial melting: either process can form the magma that crystallizes to pegmatite, a rock type commonly enriched in incompatible elements. Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma. Clinopyroxene thermobarometry is used to determine temperature and pressure conditions at which magma differentiation occurred for specific igneous rocks. Magma composition can be determined by processes other than partial melting and fractional crystallization. For instance, magmas commonly interact with rocks they intrude, both by melting those rocks and by reacting with them. Magmas of different compositions can mix with one another. In rare cases, melts can separate into two immiscible melts of contrasting compositions.


Etymology

The word ''wikt:igneous, igneous'' is 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 Rome, known as Latium. Through the power of the Roman Republic, it became the ...

Latin
''ignis'', meaning "of fire". Volcanic rocks are named after Vulcan (mythology), Vulcan, the Ancient Rome, Roman name for the god of fire. Intrusive rocks are also called "plutonic" rocks, named after Pluto (god), Pluto, the Roman god of the underworld.


Gallery

File:Kanaga Volcano (22432739869).jpg, Mount Kanaga, Kanaga volcano in the Aleutian Islands with a 1906 lava flow in the foreground File:Molten Lava in Pahoehoe Skylight.jpg, A "skylight" hole, about across, in a solidified lava crust reveals molten lava below (flowing towards the top right) in an eruption of Kīlauea in Hawaii File:"This means something. This is important" (19967592275).jpg, Devils Tower, an eroded laccolith in the Black Hills of Wyoming File:Happy Anniversary Hawaii (14802198589).jpg, A cascade of molten lava flowing into Aloi Crater during the 1969-1971 Mauna Ulu eruption of Kilauea volcano File:Columnar jointing in the Alcantara Gorge, Sicily.jpg, Columnar jointing in the Alcantara Gorge, Sicily File:Cretaceous sedimentary rocks intruded by a Miocene granite laccolith.jpg, A laccolith of granite (light-coloured) that was intruded into older sedimentary rocks (dark-coloured) at Cuernos del Paine, Torres del Paine National Park, Chile File:Multiple Igneous Intrusion Phases Kosterhavet Sweden.jpg, An igneous intrusion cut by a pegmatite dike, which in turn is cut by a dolerite dike


See also

* * * * *


Notes


References


External links


USGS Igneous Rocks





The IUGS systematics of igneous rocks
{{DEFAULTSORT:Igneous Rock Igneous rocks, 01 Igneous petrology Volcanology