Igneous rock

Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from either the Earth's mantle or pre-existing rocks made molten by extreme temperature and pressure changes. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. The word "igneous" is derived from the Latin ignis, meaning "fire".

Magma origination

The Earth's crust is about 35 kilometers thick under the continents, but averages only some 7-10 kilometers beneath the oceans. The continental crust is composed primarily of crystalline basement; stable igneous and metamorphic rocks such as granulite, granite and various other intrusive rocks. Oceanic crust is composed primarily of basalt, gabbro and peridotite.

The crust floats on the asthenospheric mantle, which is convecting due to the forces of plate tectonics. The mantle, which extends to a depth of nearly 3,000 kilometers is the source of all magma. Most of the magma which forms igneous rocks is generated within the upper parts of the mantle at temperatures estimated between 600 to 1600 °C.

Melting of rocks requires temperature, water and pressure. The mantle is generally over 1000 to 1200 °c beneath the crust, at depths of between 7 and 70km. However, most magma is generated at depths of between 20 and 50 km. Melting begins because of upwelling of hot mantle from deeper portions of the earth, nearer the Planetary core; because of water driven off subducted oceanic crust at subduction zones (providing water to lower the melting point of the rocks) and because of decompression caused by rifting.

Melting of the continental crust occurs rarely because it is usually dry, and composed of minerals and rocks which are resistant to melting such as pyroxene granulite. However, addition of heat from the mantle or from mantle plumes, subduction related compression and burial as well as some rifting, can prompt the continental crust to melt.

As magma cools, minerals crystallize from the melt at different temperatures (fractional crystallization). There are relatively few minerals which are important in the formation of igneous rocks. This is 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 which combine to form the silicate minerals, which account for over ninety percent of all igneous rocks.

Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma.

Igneous rocks make up approximately ninety five percent of the upper part of the Earth's crust, but their great abundance is hidden on the Earth's surface by a relatively thin but widespread layer of sedimentary and metamorphic rocks.

Igneous rock are geologically important because:

their minerals and global chemistry gives information about the composition of the mantle, from where 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.

Morphology and setting

In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic) or extrusive (volcanic).

Intrusive igneous rocks

Intrusive igneous rocks are formed from magma that cools and solidifies within the earth. 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. The extrusive types usually are called lavas.

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 surface.

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

Extrusive igneous rocks

Extrusive igneous rocks are formed at the Earth's surface as a result of the melting of rocks within the mantle.

The melted rock, called magma rises due to contrasting density with the surrounding mantle. When it reaches the surface, magma extruded onto the surface either beneath water or air, is called lava. Eruptions of volcanoes under the air are termed subaerial whereas those occurring underneath the ocean are termed submarine. Black smokers and mid ocean ridge basalt are examples of submarine volcanic activity.

Magma which erupts from a volcano behaves according to its temperature and composition, which cause a highly different range of viscosity. High temperature magma, which is usually basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. This forms pahoehoe type lava. Intermediate composition magma such as andesite tends to form cinder cones of intermingled ash, tuff and lava, and may have viscosity similar to thick, cold mollasses 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. These volcanoes rarely form lava flows, and usually erupt explosively.

Felsic and intermediate rocks which erupt at surface often do so violently, with explosions driven by release of gases such as carbon dioxide trapped in the magma. Such volcanic deposits are called pyroclastic deposits, and include tuff, agglomerate and ignimbrite. 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 the resulting rock may be a glass (such as the rock obsidian).

Because of this fine grained texture 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.

Classification

Igneous rock are classified according to mode of occurrence, texture, 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. Feldspars, quartz, olivines, pyroxenes, amphiboles, and micas are all important minerals in the formation of igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential (called accessory minerals).

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.

Igneous rocks which 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.

The crystals embedded in fine grained igneous rocks are termed porphyritic. The porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma consolidates into the finer grained uniform material.

Texture

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 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 which are 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 porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic apophyses. Mineralogical classification is used most often to classify plutonic rocks and chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, eg; "olivine-bearing picrite" or "orthoclase-phyric rhyolite".

see also List of rock textures

Chemical classification

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

Chemical - Total alkali - silica content (TAS diagram) for volcanic rock classification used when modal or mineralogic data is unavailable:

acid igneous rocks containing a high silica content, greater than 63% SiO₂ (examples rhyolite and dacite)
intermediate igneous rocks containing between 52 - 63% SiO₂ (example andesite)
basic igneous rocks have low silica 45 - 52% and typically high iron - magnesium content (example basalt)
ultrabasic igneous rocks with less than 45% silica. (examples picrite and komatiite)
alkalic igneous rocks with 5 - 15% alkali (K₂O + Na₂O) content (examples phonolite and trachyte)

Note: the acid-basic terminology is used more broadly in older geological literature.

Chemical classification also extends to differentiating rocks which 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

Mineralogical classification

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

Mineralogic contents - felsic versus mafic

felsic rock, with predominance of quartz, alkali feldspar and/or feldspathoids: the felsic minerals; these rocks (e.g., granite) are usually light coloured, and have low density.
mafic rock, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt) are usually dark coloured, and have higher density than felsic rocks.
ultramafic rock, 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 rock.

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

	Composition
Mode of occurrence	Acid	Intermediate	Basic	Ultrabasic
Intrusive	Granite	Diorite	Gabbro	Peridotite
Extrusive	Rhyolite	Andesite	Basalt	Komatiite

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 with more than 10% of felsic minerals) and phaneritic, subeuhedral texture (minerals are visible for the unaided eye and some of them retain original crystallographic shapes). Granite is the most abundant intrusive rock that can be found in the continents.

Etymology

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.

Reference

Le Maitre, L.E., ed., (2002) Igneous Rocks: A Classification and Glossary of Terms 2nd edition, Cambridge.

External links

Petrology | Igneous rocks | Volcanology

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