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Igneous rock (derived from the word ignis meaning fire) is one of the three main , the others being and . Igneous rock is formed through the cooling and of or . Igneous rock may form with or without , either below the surface as () rocks or on the surface as () rocks. This magma can be derived from partial melts of pre-existing rocks in either a planet's or . 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. Over 700 types of igneous rocks have been described, most of them having formed beneath the surface of 's crust.

Geological significance
Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. ξ1

Igneous rocks are 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 and thus can be compared to adjacent geological , allowing a time sequence of events;
  • their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see );
  • in some special circumstances they host important mineral deposits (): for example, , , and are commonly associated with and , whereas ores of and are commonly associated with .

Morphology and setting
In terms of modes of occurrence, igneous rocks can be either (plutonic), () or .

Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock (called ), the magma cools slowly, and as a result these rocks are coarse grained. The mineral in such rocks can generally be identified with the naked eye. rocks can also be classified according to the shape and size of the intrusive body and its relation to the other into which it intrudes. Typical intrusive formations are , stocks, , and . When the magma soildifies within the earth's crust it cools slowly forming coarse textured rocks such as granite, gabbro, or diorite.

The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by erosion, these cores (called ) may occupy huge areas of the Earth's surface.

Coarse grained intrusive igneous rocks which form at depth within the crust are termed as ; intrusive igneous rocks which form near the surface are termed .

Extrusive igneous rocks are formed at the crust's surface as a result of the partial melting of rocks within the and crust. Extrusive igneous rocks cool and solidify quicker than intrusive igneous rocks. Since the rocks cool very quickly, they are fine grained. 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, solidify at a faster rate. Hence such rocks are smooth, crystalline and fine grained. They are also called volcanic rocks. 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. For example, the Giant's Causeway in Antrim, Northern Ireland.

The melted rock, with or without suspended crystals and gas bubbles, is called . 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 . Eruptions of into air are termed , whereas those occurring underneath the ocean are termed . and 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:Fisher, R. V. & Schmincke H.-U., (1984) Pyroclastic Rocks, Berlin, Springer-Verlag

Magma which erupts from a behaves according to its , 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, . Long, thin basalt flows with surfaces are common. Intermediate composition magma such as tends to form cinder cones of intermingled , and lava, and may have viscosity similar to thick, cold or even rubber when erupted. magma such as 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 typically are 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 release of dissolved gases—typically water but also . Explosively erupted material is called and includes , and . Fine volcanic ash is also erupted and forms ash tuff deposits which can often cover vast areas.

Because lava cools and crystallizes rapidly, it is 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 ). If the cooling of the lava happened slowly, the rocks 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 of the rock under a , so only an approximate classification can usually be made in the field.

Hypabyssal igneous rocks are formed at a depth in between the and . These are formed due to cooling and resultant solidification of rising magma just beneath the earth surface. Hypabyssal rocks are less common than plutonic or volcanic rocks and often form , , , , or .

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 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 upon the cooling history, and the mineral composition of the rock. , or , , , , and 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 .

In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of , 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 are silica-undersaturated, because feldspathoids cannot coexist in a stable association with quartz.

Igneous rocks which have crystals large enough to be seen by the naked eye are called ; those with crystals too small to be seen are called . 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 . 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.

We will classify igneous rocks 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 is an important criterion for the naming of volcanic rocks. The 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 , a lava or a simple .

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 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 tend also 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 margins to large intrusive bodies, stocks and (apophyses). Mineralogical classification is used most often 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".

Chemical classification
Igneous rocks can be classified according to chemical or mineralogical parameters:

Chemical: total alkali-silica content () for classification used when modal or mineralogic data is unavailable:

Chemical classification also extends to differentiating rocks which are chemically similar according to the TAS diagram, for instance;

An idealized mineralogy (the ) 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 or acidic where the SiO2 was greater than 66% and the family term quartzolite was applied to the most silicic. A normative classifies a rock as silica-undersaturated; an example is .

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 was much of the existing terminology 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, , akermannite, , , , and so on, and the rocks are divided into groups strictly according to the relative proportion of these minerals to one another.Cross, W. et al. (1903) Quantitative Classification of Igneous Rocks, Chicago, University of Chicago Press

Mineralogical classification
For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important criterion is the species, followed by the groundmass mineralogy. Often, where the groundmass is , chemical classification must be used to properly identify a volcanic rock.

Mineralogic contents – felsic versus mafic

  • rock, highest content of , with predominance of quartz, alkali and/or feldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light coloured, and have low density.
  • rock, lesser content of silicon relative to felsic rocks, with predominance of mafic minerals , and calcic ; these rocks (example, basalt, gabbro) are usually dark coloured, and have a higher density than felsic rocks.
  • rock, lowest content of silicon, with more than 90% of mafic minerals (e.g., ).

For intrusive, plutonic and usually igneous rocks where all minerals are visible at least via microscope, the mineralogy is used to classify the rock. This usually occurs on , where the relative proportions of three minerals are used to classify the rock.

The following table is a simple subdivision of igneous rocks according both to their composition and mode of occurrence.

! bgcolor="#ffc0c0" colspan="4" Composition

! bgcolor="#ffc0c0" colspan="4" Essential rock forming silicates
Coarse Grained
Medium Grained
Fine Grained
For a more detailed classification see .

Example of classification
is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silica and predominately plus potassium-rich plus sodium-rich ) and phaneritic, texture (minerals are visible to the unaided eye and commonly some of them retain original crystallographic shapes).

Magma origination
The Earth's crust averages about 35 kilometers thick under the , but averages only some 7–10 kilometers beneath the . The continental crust is composed primarily of sedimentary rocks resting on crystalline basement formed of a great variety of metamorphic and igneous rocks including and granite. Oceanic crust is composed primarily of basalt and . Both continental and oceanic crust rest on 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 impact of a meteorite, are less important today, but impacts during of the Earth led to extensive melting, and the outer several hundred kilometers of our early Earth probably was an ocean of magma. Impacts of large meteorites in last few hundred million years have been proposed as one mechanism responsible for the extensive basalt magmatism of several .

Decompression melting occurs because of a decrease in pressure.


The 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 may be hotter than its solidus temperature at some shallower level. If such rock rises during the of solid mantle, it will cool slightly as it expands in an , but the cooling is only about 0.3 °C per kilometer. Experimental studies of appropriate 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 upward movement of solid mantle is critical in the evolution of the Earth.

Decompression melting creates the ocean crust at . It also causes in intraplate regions such as Europe, Africa and the Pacific sea floor. There, it is variously attributed either to the rise of (the "Plume hypothesis") or to intraplate extension (the "Plate hypothesis"). ξ3

Effects of water and carbon dioxide
The change of rock composition most responsible for creation of magma is the addition of water. Water lowers the 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 1500 °C in the absence of water.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 Water is driven out of the oceanic in , and it causes melting in the overlying mantle. Hydrous magmas of basalt and andesite composition are produced directly and indirectly as results of dehydration during the subduction process. Such magmas and those derived from them build up such as those in the . These magmas form rocks of the series, an important part of .

The addition of is relatively a much less important cause of magma formation than addition of water, but genesis of some 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.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 Magmas of rock types such as , , and 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 of 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 of a crustal rock in continental crust thickened by compression at a . The plate boundary between the Indian and Asian continental masses provides a well-studied example, as the just north of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normal continental crust. Studies of electrical deduced from have detected a layer that appears to contain melt and that stretches for at least 1000 kilometers within the middle crust along the southern margin of the Tibetan Plateau.M. J. Unsworth et al. (2005) Crustal rheology of the Himalaya and Southern Tibet inferred from magnetotelluric data. Nature, v. 438, p. 78-81 Granite and are types of igneous rock commonly interpreted as products of melting of continental crust because of increases of temperature. Temperature increases also may contribute to the melting of dragged down in a .

Magma evolution
Most 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 from the melt at different temperatures (). As minerals crystallize, the composition of the residual melt typically changes. If crystals separate from 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 composition if early formed crystals are separated from the magma. Gabbro may have a temperature near 1200 °C, and derivative granite-composition melt may have a liquidus temperature as low as about 700 °C. 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 , a rock type commonly enriched in incompatible elements. 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: , , aluminium, , , , iron, and . These are the elements which combine to form the , 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" typically is 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 1000 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 ignis, meaning "of fire". Volcanic rocks are named after , the name for the god of fire. Intrusive rocks are also called "plutonic" rocks, named after , the Roman god of the underworld.

See also

  • 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

External links

    ^ (2024). 9780716739050, Freeman.
    ^ (1992). 9780521427401, Cambridge University Press. .
    ^ (2024). 9781405161480, Wiley-Blackwell. .

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