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 lava. The magma can be derived
from partial melts of existing rocks in either a planet's mantle or
crust. Typically, the melting is caused by one or more of three
processes: an increase in temperature, a decrease in pressure, or a
change in composition. Solidification into rock occurs either below
the surface as intrusive rocks or on the surface as extrusive rocks.
Igneous rock may form with crystallization to form granular,
crystalline rocks, or without crystallization to form natural glasses.
1 Geological significance
2 Geological setting
3.2 Chemical classification and petrology
3.3 History of classification
4 Mineralogical classification
4.1 Example of classification
5.2 Effects of water and carbon dioxide
7 See also
10 Additional Reading
11 External links
Igneous and metamorphic rocks make up 90–95% of the top 16 km
of the Earth's crust by volume. Igneous rocks form about 15% of the
Earth's current land surface.[note 1] Most of the Earth's oceanic
crust is made of igneous rock.
Igneous rocks are also geologically important because:
their minerals and global chemistry give information about the
composition of the mantle, from which some igneous rocks are
extracted, and the temperature and pressure conditions that allowed
this extraction, and/or of other pre-existing rock that melted;
their absolute ages can be obtained from various forms of radiometric
dating and thus can be compared to adjacent geological strata,
allowing a time sequence of events;
their features are usually characteristic of a specific tectonic
environment, allowing tectonic reconstitutions (see plate tectonics);
in some special circumstances they host important mineral deposits
(ores): for example, tungsten, tin, and uranium are commonly
associated with granites and diorites, whereas ores of chromium and
platinum are commonly associated with gabbros.
Forming of igneous rock
In terms of modes of occurrence, igneous rocks can be either intrusive
(plutonic and hypabyssal) or extrusive (volcanic).
Close-up of granite (an intrusive igneous rock) exposed in Chennai,
Intrusive igneous rocks are formed from magma that cools and
solidifies within the crust of a planet, surrounded by pre-existing
rock (called country rock); the magma cools slowly and, as a result,
these rocks are coarse-grained. The mineral grains in such rocks can
generally be identified with the naked eye. Intrusive rocks can also
be classified according to the shape and size of the intrusive body
and its relation to the other formations into which it intrudes.
Typical intrusive formations are batholiths, stocks, laccoliths, sills
and dikes. When the magma solidifies within the earth's crust, it
cools slowly forming coarse textured rocks, such as granite, gabbro,
The central cores of major mountain ranges consist of intrusive
igneous rocks, usually granite. When exposed by erosion, these cores
(called batholiths) 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 medium-grained.
are less common than plutonic or volcanic rocks and often form dikes,
sills, laccoliths, lopoliths, or phacoliths.
Extrusive igneous rock is made from lava released by volcanoes
Sample of basalt (an extrusive igneous rock), found in Massachusetts
Extrusive igneous rocks, also known as volcanic rocks, are formed at
the crust's surface as a result of the partial melting of rocks within
the mantle and crust. Extrusive igneous rocks cool and solidify
quicker than intrusive igneous rocks. They are 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, solidifies at a
faster rate. Hence such rocks are smooth, crystalline and
Basalt is a 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 in Antrim, Northern
Ireland is an example.
The molten rock, with or without suspended crystals and gas bubbles,
is called magma. It rises because it is less dense than the rock from
which it was created. When magma reaches the surface from beneath
water or air, it is called lava. Eruptions of volcanoes into air are
termed subaerial, whereas those occurring underneath the ocean are
Black smokers and mid-ocean ridge 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
divergent boundary: 73%
convergent boundary (subduction zone): 15%
Magma that erupts from a volcano behaves according to its viscosity,
determined by temperature, composition, crystal content and the amount
of silica. High-temperature magma, most of which is basaltic in
composition, behaves in a manner similar to thick oil and, as it
cools, treacle. Long, thin basalt flows with pahoehoe surfaces are
Intermediate composition magma, such as andesite, tends to
form cinder cones of intermingled ash, tuff and lava, and may have a
viscosity similar to thick, cold molasses or even rubber when erupted.
Felsic magma, such as rhyolite, is usually erupted at low temperature
and is up to 10,000 times as viscous as basalt.
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. Explosively erupted pyroclastic
material is called tephra and includes tuff, agglomerate and
ignimbrite. Fine volcanic ash is also erupted and forms ash tuff
deposits, which can often cover vast areas.
Because lava usually cools and crystallizes rapidly, it is usually
fine-grained. If the cooling has been so rapid as to prevent the
formation of even small crystals after extrusion, the resulting rock
may be mostly glass (such as the rock obsidian). If the cooling of the
lava happened more slowly, the rock would be coarse-grained.
Because the minerals are mostly fine-grained, 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 sections of the
rock under a microscope, so only an approximate classification can
usually be made in the field.
Igneous rocks are classified according to mode of occurrence, texture,
mineralogy, chemical composition, and the geometry of the igneous
The classification of the many types of different igneous rocks can
provide us with 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. Feldspars, quartz or
feldspathoids, olivines, pyroxenes, amphiboles, and micas 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, and these rare rocks include
those with essential carbonates.
In a simplified classification, igneous rock types are separated on
the basis of the type of feldspar present, the presence or absence of
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 feldspathoids 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; those with crystals too small to be seen
are called aphanitic. 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.
develops when some of the crystals grow to considerable size before
the main mass of the magma crystallizes as finer-grained, uniform
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.
Gabbro specimen showing phaneritic texture; Rock Creek Canyon, eastern
Sierra Nevada, California; scale bar is 2.0 cm.
Main article: Rock microstructure
Texture is an important criterion for the naming of volcanic rocks.
The texture 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, a pyroclastic lava
or a simple 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 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 gaining
structural fabrics. Textural terms can be used to differentiate
different intrusive phases of large plutons, for instance porphyritic
margins to large intrusive bodies, porphyry stocks and subvolcanic
dikes (apophyses). 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".
List of rock textures and Igneous textures
Basic classification scheme for igneous rocks on their mineralogy. If
the approximate volume fractions of minerals in the rock are known,
the rock name and silica content can be read off the diagram. This is
not an exact method, because the classification of igneous rocks also
depends on other components than silica, yet in most cases it is a
good first guess.
Chemical classification and petrology
Total alkali versus silica classification scheme (TAS) as proposed in
Le Maitre's 2002 Igneous Rocks - A classification and glossary of
Igneous rocks can be classified according to chemical or mineralogical
Chemical: total alkali-silica content (TAS diagram) for volcanic rock
classification used when modal or mineralogic data is unavailable:
felsic igneous rocks containing a high silica content, greater than
63% SiO2 (examples granite and rhyolite),
intermediate igneous rocks containing between 52–63% SiO2 (example
andesite and dacite),
mafic igneous rocks have low silica 45–52% and typically high iron
– magnesium content (example gabbro and basalt),
ultramafic rock igneous rocks with less than 45% silica (examples
picrite, komatiite and peridotite),
alkalic igneous rocks with 5–15% alkali (K2O + Na2O) content or with
a molar ratio of alkali to silica greater than 1:6 (examples phonolite
Chemical classification also extends to differentiating rocks that are
chemically similar according to the TAS diagram, for instance:
Ultrapotassic – rocks containing molar K2O/Na2O >3.
Peralkaline – rocks containing molar (K2O + Na2O)/Al2O3 >1.
Peraluminous – rocks containing molar (K2O + Na2O)/Al2O3 <1.
An idealized mineralogy (the normative mineralogy) can be calculated
from the chemical composition, and the calculation is useful for rocks
too fine-grained or too altered for identification of minerals that
crystallized from the melt. For instance, normative quartz classifies
a rock as silica-oversaturated; an example is rhyolite. In an 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 classifies a
rock as silica-undersaturated; an example is nephelinite.
History of classification
In 1902, a group of American petrographers 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
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, 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.
For volcanic rocks, mineralogy is important in classifying and naming
lavas. The most important criterion is the phenocryst species,
followed by the groundmass mineralogy. Often, where the groundmass is
aphanitic, chemical classification must be used to properly identify a
Mineralogic contents – felsic versus mafic
felsic rock, highest content of silicon, with predominance of quartz,
alkali feldspar and/or feldspathoids: the felsic minerals; these rocks
(e.g., granite, rhyolite) are usually light coloured, and have low
mafic rock, lesser content of silicon relative to felsic rocks, with
predominance of mafic minerals pyroxenes, olivines and calcic
plagioclase; these rocks (example, basalt, gabbro) are usually dark
coloured, and have a higher density than felsic rocks.
ultramafic rock, lowest content of silicon, with more than 90% of
mafic minerals (e.g., dunite).
For intrusive, plutonic and usually phaneritic igneous rocks (where
all minerals are visible at least via microscope), the mineralogy is
used to classify the rock. This usually occurs on ternary diagrams,
where the relative proportions of three minerals are used to classify
The following table is a simple subdivision of igneous rocks according
to both their composition and mode of occurrence.
Mode of occurrence
For a more detailed classification see QAPF diagram.
Example of classification
Granite is an igneous intrusive rock (crystallized at depth), with
felsic composition (rich in silica and predominately quartz plus
potassium-rich feldspar plus sodium-rich plagioclase) and phaneritic,
subeuhedral texture (minerals are visible to the unaided eye and
commonly some of them retain original crystallographic shapes).
The Earth's crust averages about 35 kilometers thick under the
continents, but averages only some 7–10 kilometers beneath the
oceans. The continental crust is composed primarily of sedimentary
rocks resting on a crystalline basement formed of a great variety of
metamorphic and igneous rocks, including granulite and granite.
Oceanic crust is composed primarily of basalt and 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 meteorite impact, are less
important today, but impacts during the 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 melting occurs because of a decrease in pressure.
The 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 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
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 zones, 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
The addition of carbon dioxide is relatively a much less important
cause of magma formation than the addition of water, but genesis of
some 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
Magmas of rock types such as nephelinite, carbonatite,
and kimberlite are among those that may be generated following an
influx of carbon dioxide into mantle at depths greater than about
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 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 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
Schematic diagrams showing the principles behind fractional
crystallisation in a magma. While cooling, the magma evolves in
composition because different minerals crystallize from the melt. 1:
olivine crystallizes; 2: olivine and pyroxene crystallize; 3: pyroxene
and plagioclase crystallize; 4: plagioclase crystallizes. At the
bottom of the magma reservoir, a cumulate rock forms.
Main article: Igneous differentiation
Most magmas only entirely melt 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). 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 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
Bowen's reaction series is
important for understanding the idealised sequence of fractional
crystallisation of a magma.
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.
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, oxygen,
aluminium, sodium, potassium, calcium, iron, and magnesium. These are
the elements that combine to form the silicate minerals, 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 word "igneous" is derived from the
Latin ignis, meaning "of fire".
Volcanic rocks are named after Vulcan, the Roman name for the god of
fire. Intrusive rocks are also called "plutonic" rocks, named after
Pluto, the Roman god of the underworld.
Large igneous province
List of rock types
^ 15% is the arithmetic sum of the area for intrusive plutonic rock
(7%) plus the area for extrusive volcanic rock (8%).
^ Prothero, Donald R.; Schwab, Fred (2004). Sedimentary geology :
an introduction to sedimentary rocks and stratigraphy (2nd ed.). New
York: Freeman. p. 12. ISBN 978-0-7167-3905-0.
^ Wilkinson, Bruce H.; McElroy, Brandon J.; Kesler, Stephen E.;
Peters, Shanan E.; Rothman, Edward D. (2008). "Global geologic maps
are tectonic speedometers—Rates of rock cycling from area-age
frequencies". Geological Society of America Bulletin. 121 (5–6):
^ Fisher, R. V. & Schmincke H.-U., (1984)
^ Ridley, W. I., 2012,
Petrology of Igneous Rocks, Volcanogenic
Massive Sulfide Occurrence Model, USGS Scientific Report 2010-5070-C,
^ Cross, W. et al. (1903) Quantitative Classification of Igneous
Rocks, Chicago, University of Chicago Press
^ One or more of the preceding sentences incorporates text
from a publication now in the public domain: Chisholm, Hugh, ed.
(1911). "Petrology". Encyclopædia Britannica. 21 (11th ed.).
Cambridge University Press. pp. 323–333.
^ Geoff C. Brown; C. J. Hawkesworth; R. C. L. Wilson (1992).
Understanding the Earth (2nd ed.). Cambridge University Press.
p. 93. ISBN 0-521-42740-1.
^ Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy.
Wiley-Blackwell. ISBN 978-1-4051-6148-0.
^ T. L. Grove, N. Chatterjee, S. W. Parman, and E. Medard, (2006)The
influence of H2O on mantle wedge melting. Earth and Planetary Science
Letters, v. 249, p. 74-89
^ R. Dasgupta and M. M. Hirschmann (2007) Effect of variable carbonate
concentration on the solidus of mantle peridotite. American
Mineralogist, v. 92, p. 370-379
^ M. J. Unsworth et al. (2005) Crustal rheology of the Himalaya and
Southern Tibet inferred from magnetotelluric data. Nature, v. 438, p.
R. W. Le Maitre (editor) (2002) Igneous Rocks: A Classification and
Glossary of Terms, Recommendations of the International Union of
Geological Sciences, Subcommission of the Systematics of Igneous
Rocks., Cambridge, Cambridge University Press ISBN 0-521-66215-X
The Wikibook Historical Geology has a page on the topic of: Igneous
rocks and stratigraphy
The Wikibook Historical Geology has a page on the topic of: Igneous
USGS Igneous Rocks
Igneous rock classification flowchart
Igneous Rocks Tour, an introduction to Igneous Rocks
The IUGS systematics of igneous rocks
Wikimedia Commons has media related to Igneous rocks.
Common igneous rocks classified by silicon dioxide content