Igneous minerals form during the cooling and solidification of molten rock, or magma, produced at high temperatures (around 650 to 1200°C) beneath the Earth's surface. Intrusive 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. Test your knowledge by taking this quiz. Igneous rocks are classified according to their mineral content: Ultramafic rocks are dominated by olivine and/or pyroxene. The Average Chemical Composition of Igneous Rocks Frank W. Clarke , Henry S. Washington Proceedings of the National Academy of Sciences May 1922, 8 (5) 108-115; DOI: 10.1073/pnas.8.5.108 The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. The coarser pyroclastic materials accumulate around the erupting volcano, but the finest pyroclasts can be found as thin layers located hundreds of kilometres from the opening. Crystals can form in the mass if cooling happens very slowly, allowing the natural geometrical shapes of the molecules to form. (Denudation is the wearing away of the terrestrial surface by processes including weathering and erosion.) Common igneous rocks comprise 40…77% of silica (SiO2). Classification of the common igneous rocks by means of their chemical composition. Click on any element for further details. Igneous rocks range in SiO 2 content from about 40 to nearly 80 percent, and other constituents increase in amount as SiO 2 decreases. For example, rocks like granite may contain about 70-80% of silica and very little quantity of iron, magnesia, and lime, while on the other hand rocks like peridotite contain only 35-40% of silica and larger quantities of iron, magnesia, and lime. This results in two groups: (1) plutonic intrusive igneous rocks that solidified deep within the crust and (2) volcanic, or extrusive, igneous rocks formed at Earth’s surface. Composition refers to both the types of minerals within a rock and the overall chemical makeup of the rock (the two are obviously related). Other important oxides are alumina (Al2O3), magnesia (MgO), lime (CaO), soda (Na2O), and potash (K2O). In the case of rocks that have excess silica, the silicic rocks will have quartz and magnesium-pyroxene, which are considered saturated minerals, and the rocks that contain them are termed supersaturated. The light-colored silicates include quartz, muscovite and feldspar. Get a Britannica Premium subscription and gain access to exclusive content. Some organization was brought to the continuous variation between these extremes by the Bowen reactions. The compositions of metamorphic rocks are generally similar to the compositions of the rocks that were metamorphosed, and only igneous and sedimentary rock compositions are considered here. Please select which sections you would like to print: Corrections? Because of the dominance of oxygen and silicon in the crust, igneous rocks are mostly made up of silicate minerals.These silicates can be generally divided into light and dark silicates. The key difference between igneous rocks and sedimentary rocks is that igneous rocks are formed from molten liquid minerals called magma, while sedimentary rocks are formed from lithification of existing rocks.. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Generally, the intrusive rocks have cross-cutting contacts with the country rocks that they have invaded, and in many cases the country rocks show evidence of having been baked and thermally metamorphosed at these contacts. For example, one mole of SiO2 is combined with one mole of MgO to make the magnesium-rich pyroxene, MgSiO3 (enstatite): SiO2 + MgO → MgSiO3. Solidification from magma produces great diversity in the mineral compositions which make up the rocks. Most rocks are composed of minerals. Ring in the new year with a Britannica Membership, Classification of volcanic and hypabyssal rocks, Volatile constituents and late magmatic processes, Distribution of igneous rocks on Earth’s surface, https://www.britannica.com/science/igneous-rock, igneous rock - Children's Encyclopedia (Ages 8-11), igneous rock - Student Encyclopedia (Ages 11 and up). When the chemical analysis of an acid rock like granite and of a basic rock like basalt are compared, important differences are seen such as, the greater proportion of silica and alkalies (Na 2 O and K 2 O) in the acid rock and the higher content of lime, magnesia and iron oxide in the basic rock. Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. … Occurrence of igneous rocks can be either intrusive (plutonic) or extrusive (volcanic). 3.4 Classification of Igneous Rock As has already been described, igneous rocks are classified into four categories: felsic, intermediate, mafic, and ultramafic, based on either their chemistry or their mineral composition. The major oxides of the rocks generally correlate well with their silica content: those rocks with low silica content are enriched in magnesium oxide (MgO) and iron oxides (FeO, Fe2O3, and Fe3O4) and are depleted in soda (Na2O) and potash (K2O); those with a large amount of silica are depleted in magnesium oxide and iron oxides but are enriched in soda and potash. Igneous rocks are formed when magma cools, either on Earth’s surface or beneath it, though some may form by fragmentation of solidifying magma. Two moles of SiO2 are needed to be combined with one mole each of CaO and Al2O3 to make the calcium-rich plagioclase, CaAl2Si2O8 (anorthite). Indeed, in 1960 a sodium carbonate (Na2CO3) lava with only 0.05 weight percent silica (SiO2) was erupted from Ol Doinyo Lengai, a volcano in northern Tanzania, Africa. Minerals are defined by geologists as naturally occurring inorganic solids that have a crystalline structure and a distinct chemical composition. Chemical Composition. Such rocks are called granitic rock. Alumina in rocks that contain more than 45 percent silica is generally above approximately 14 weight percent, with the greatest abundance occurring at an intermediate silica content of about 56 weight percent. The diagram in Figure 3.16 can be used to help classify igneous rocks by their mineral composition. There are two major types of igneous rocks: Extrusive, fine grained, and intrusive, fine grained. Beyond the "big 8", manganese and titanium are present in small concentrations in magma and therefore appear in a number of minerals. Both intrusive and extrusive magmas have played a vital role in the spreading of the ocean basin, in the formation of the oceanic crust, and in the formation of the continental margins. The chemical composition of the magma determines the minerals that will crystallize and their proportions. The terms mafic (from magnesium and ferrous iron) and felsic (feldspar and silica) are used interchangeably with femic and sialic. Mafic rocks are dominated by plagioclase and pyroxene (even if you can't see them with the naked eye) and smaller amounts of olivine. The deep-seated plutonic rocks can be exposed at the surface for study only after a long period of denudation or by some tectonic forces that push the crust upward or by a combination of the two conditions. NAMING IGNEOUS ROCKS Geologists use both the minerals and texture to classify and name igneous rocks. These groups refer to differing amounts of silica, iron, and magnesium found in the minerals that make up the rocks. Composition Chemical components. Because of the dominance of oxygen and silicon in the crust, igneous rocks are mostly made up of silicate minerals. The latter may be further divided into two groups: mafic, rocks with 45 to 55 percent silica and ultramafic, those containing less than 45 percent. 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. The diagram of Bowen’s reaction series ( Figure 7.6 ) shows that differences in chemical composition correspond to differences in the types of minerals within an igneous rock. How solid is your knowledge of all things geological? Igneous rocks are classified according to their texture and composition. The former case usually occurs in subsilicic rocks that characteristically will have silicate minerals like magnesium-olivine, sodium-nepheline (NaAlSiO4, which requires only one mole of silicon for every mole of sodium [Na]), and leucite (KAlSi2O6, which requires only two moles of silicon to one mole of potassium [K]). As a result, the rock is either composed of minerals that can be seen only with the aid of a microscope (called aphanitic, from the Greek aphanēs, meaning “invisible”) or contains no minerals at all (in the latter case, the rock is composed of glass, which is a highly viscous liquid). The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Intrusive rocks also can be categorized consistent with the shape and size of the intrusive body and its relation t… Most lava flows do not travel far from the volcano, but some low-viscosity flows that erupted from long fissures have accumulated in thick (hundreds of metres) sequences, forming the great plateaus of the world (e.g., the Columbia River plateau of Washington and Oregon and the Deccan plateau in India). On the other hand, a silicic magma may have excess silica such that some will be left after all the silicate minerals were formed from the combination of the oxides; the remaining “free” silica crystallizes as quartz or its polymorphs. They include olivine, pyroxene, amphibole and biotite. Typical occurrences of igneous rock bodies on the surface include lava flows, lava domes, necks and spines, The subsilicic rocks, enriched as they are in iron (Fe) and magnesium (Mg), are termed femic (from ferrous iron and magnesium), whereas the silicic rocks are referred to as sialic (from silica and aluminum, with which they are enriched) or salic (from silica and aluminum). For igneous rock, the composition is divided into four groups: felsic, intermediate, mafic, and ultramafic. Quartz clearly will not be present in these rocks. Articles from Britannica Encyclopedias for elementary and high school students. The mineral grains in such rocks can generally be recognized with the bare eye. Some intrusive rocks, known as subvolcanic, were not formed at great depth but were instead injected near the surface where lower temperatures result in a more rapid cooling process; these tend to be aphanitic and are referred to as hypabyssal intrusive rocks. The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Extrusive rocks occur in two forms: (1) as lava flows that flood the land surface much like a river and (2) as fragmented pieces of magma of various sizes (pyroclastic materials), which often are blown through the atmosphere and blanket Earth’s surface upon settling. The major mineralogical components of igneous rocks can be divided into two groups: felsic (from fel dspar and si lica) and mafic (from ma gnesium and f errous iron). Because of the limited occurrence of such carbonate-rich igneous rocks, however, the following discussion will consider the chemistry of silicate rocks only.