Igneous rock

[13] The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting.

Extrusive rock is produced in the following proportions:[14] The behaviour of lava depends upon its 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.

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.

[15] 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.

[17] In a simplified compositional classification, igneous rock types are categorized into felsic or mafic based on the abundance of silicate minerals in the Bowen's Series.

[18] An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry.

Texture refers to the size, shape, and arrangement of the mineral grains or crystals of which the rock is composed.

[citation needed] 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.

Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite".

[20] In the great majority of cases, the rock has a more typical mineral composition, with significant quartz, feldspars, or feldspathoids.

In a few cases, such as the diorite-gabbro-anorthite field, additional mineralogical criteria must be applied to determine the final classification.

These are the elements that combine to form the silicate minerals, which account for over ninety percent of all igneous rocks.

[citation needed] 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.

This classification is summarized in the following table: The percentage of alkali metal oxides (Na2O plus K2O) is second only to silica in its importance for chemically classifying volcanic rock.

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.

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.

[33][34] 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.

However, the concept of normative mineralogy has endured, and the work of Cross and his coinvestigators inspired a flurry of new classification schemes.

[citation needed] 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.

Hydrous magmas composed of basalt and andesite are produced directly and indirectly as results of dehydration during the subduction process.

[citation needed] 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.

[43] 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 70 km.

Temperatures can also exceed the solidus of a crustal rock in continental crust thickened by compression at a plate boundary.

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 kilometres within the middle crust along the southern margin of the Tibetan Plateau.

[44] Granite and rhyolite are types of igneous rock commonly interpreted as products of the melting of continental crust because of increases in temperature.

Clinopyroxene thermobarometry is used to determine temperature and pressure conditions at which magma differentiation occurred for specific igneous rocks.

Geologic provinces of the world ( USGS )
Volcanic eruptions of lava are major sources of igneous rocks. ( Mayon volcano in the Philippines, erupting in 2009)
Natural columns of igneous rock separated from each other by columnar joints , in Madeira
Formation of igneous rock
Basic types of intrusions:
Extrusive igneous rock is made from lava released by volcanoes
Sample of basalt (an extrusive igneous rock), found in Massachusetts
Close-up of granite (an intrusive igneous rock)
Gabbro specimen showing phaneritic texture, from Rock Creek Canyon, eastern Sierra Nevada , California
Basic classification scheme for igneous rocks based on their mineral composition. 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, yet in most cases it is a good first guess.
Total alkali versus silica classification scheme (TAS) as proposed in Le Maitre's 2002 Igneous Rocks – A classification and glossary of terms [ 21 ] Blue area is roughly where alkaline rocks plot; yellow area is where subalkaline rocks plot.
AFM ternary diagram showing the relative proportions of Na 2 O + K 2 O (A for Alkali earth metals ), FeO + Fe 2 O 3 (F), and MgO (M) with arrows showing the path of chemical variation in tholeiitic and calc-alkaline series magmas
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.