Igneous Rock


Igneous rocks are fashioned deep internal Earth’s crust (intrusive rocks) or on the floor (extrusive rocks). As molten rock (called magma) below the surface cools, the liquid cloth starts to solidify into minerals of differing sizes and compositions. Generally, the longer the cooling time, the bigger the mineral crystals can grow. Trapped deep within the Earth, magma is authorized to cool slowly. Examples of intrusive rocks are granite and gabbro. Granite is made from three commonplace minerals – quartz, mica and feldspar – and when viewed from a distance appear tan, pinkish or gray, relying at the concentrations and grain sizes of the 3 minerals. This rock may be very widely used as a constructing material, because of its abundance and electricity. Gabbro has a decrease quartz content material than granite, and is therefore much darker. Liquid rock that reaches the surface cools very quickly. As a result of this immediately cooling, the crystals produced are extraordinarily small. These tiny-grained minerals tend to give these extrusive rocks a rather opaque, homogenous colour. All of the arena’s sea flooring are produced in this way of rapid cooling, and the dark-gray, heavy rock produced is known as basalt. Andesite (named after the Andes Mountains in South America) is a cousin to basalt in that it’s also volcanic in nature, however is lighter-coloured due to the decreased concentration of darker minerals.

Igneous rock formation
Igneous rock formation

Geological setting

Occurrence of igneous rocks can be either intrusive (plutonic) or extrusive (volcanic).


Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet, surrounded by way of pre-present rock (called us of a rock); the magma cools slowly and, as a result, these rocks are coarse-grained. The mineral grains in such rocks can generally be recognized with the bare eye. Intrusive rocks also can be categorized consistent with the shape and size of the intrusive body and its relation to the opposite formations into which it intrudes. Typical intrusive formations are batholiths, stocks, laccoliths, sills and dikes. When the magma solidifies inside the earth’s crust, it cools slowly forming coarse textured rocks, inclusive of granite, gabbro, or diorite.


Extrusive igneous rocks, additionally referred to as volcanic rocks, are fashioned on the crust’s surface due to 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 way of the cooling of molten magma on earth’s surface. The magma, which is delivered to the surface via fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth, crystalline and great-grained. Basalt is a not unusual extrusive igneous rock and bureaucracy 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 instance.

Molten rocks

The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions: divergent boundary: 73% convergent boundary (subduction zone): 15% hotspot: 12%.


Igneous rocks are categorised in line with mode of prevalence, texture, mineralogy, chemical composition, and the geometry of the igneous frame. The type of the various styles of distinct igneous rocks can provide us with essential records approximately the conditions underneath which they fashioned. Two critical variables used for the category of igneous rocks are particle length, which largely depends on the cooling records, and the mineral composition of the rock. Feldspars, quartz or feldspathoids, olivines, pyroxenes, amphiboles, and micas are all essential minerals inside the formation of virtually all igneous rocks, and they may be primary to the type of these rocks. All different minerals gift are regarded as nonessential in nearly all igneous rocks and are referred to as accent minerals. Types of igneous rocks with other critical minerals are very uncommon, and those rare rocks include people with important carbonates.

Classification and nomenclature of plutonic rocks according to their modal mineral contents using theQAPF diagram (

Chemical classification and petrology

Igneous rocks may be classified in keeping with chemical or mineralogical parameters.

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

    • felsic igneous rocks containing a excessive silica content, more than 63% SiO2 (examples granite and rhyolite),
    • intermediate igneous rocks containing among fifty two–63% SiO2 (example andesite and dacite),
    • mafic igneous rocks have low silica forty five–fifty two% and typically high iron – magnesium content material (example gabbro and basalt),
    • ultramafic rock igneous rocks with much less than forty five% 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 extra than 1:6 (examples phonolite and trachyte).

Mineralogical classification

For volcanic rocks, mineralogy is critical in classifying and naming lavas. The maximum crucial criterion is the phenocryst species, followed by way of the groundmass mineralogy. Often, in which the groundmass is aphanitic, chemical category should be used to correctly pick out a volcanic rock.

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 feature low density.
    • Mafic rock, lesser content material of silicon relative to felsic rocks, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (instance, basalt, gabbro) are generally dark coloured, and have a higher density than felsic rocks.
    • Ultramafic rock, lowest content of silicon, with greater than 90% of mafic minerals (e.G., dunite).

For intrusive, plutonic and typically phaneritic igneous rocks (in which all minerals are seen as a minimum via microscope), the mineralogy is used to categorise the rock. This generally happens on ternary diagrams, where the relative proportions of 3 minerals are used to categorise the rock.

Mode of occurrence Felsic Intermediate Mafic Ultramafic
Intrusive Granite Diorite Gabbro Peridotite
Extrusive Rhyolite Andesite Basalt Komatiite