Igneous rocks are commonly classified by their composition and texture. Within Earth’s deep crust the temperatures and pressures are much higher than at its surface; consequently, the hot magma cools slowly and crystallizes completely, leaving no trace of the liquid magma. Texture is the term applied to the overall appearance of a rock based on the size, shape, and arrangement of the interlocking mineral crystals which form it. Updates? Igneous rocks are classified according to their mineral content: Ultramafic rocks are dominated by olivine and/or pyroxene. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. 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). 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. Most of the time, they're simple to tell apart. These groups refer to differing amounts of silica, iron, and magnesium found in the minerals that make up the rocks. Igneous rocks are classified on the basis of texture and chemical composition, usually as reflected in the minerals that from due to crystallization. At the light-colored extreme are rocks made up mainly of quartz and the feldspars, with about 70% silica. Quartz clearly will not be present in these rocks. Whereas sedimentary rocks are produced by processes operating mainly at Earth’s surface by the disintegration of mostly older igneous rocks, igneous—and metamorphic—rocks are formed by internal processes that cannot be directly observed and that necessitate the use of physical-chemical arguments to deduce their origins. The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. The various igneous textures result mainly from the different cooling histories, whereas the mineral composition of an igneous rock is the result of the chemical makeup of the parent magma. Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. Igneous Rocks by Composition. 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). Texture refers to the size and arrangement of the minerals or grains that make up a rock. The silica content also reflects the mineral composition of the rocks. Igneous rocks are classified according to their texture and composition. 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. Because magma is less dense than the surrounding solid rocks, it rises toward the surface. Eight elements make up about 98% by weight of most magmas from which igneous rocks are made. Get a Britannica Premium subscription and gain access to exclusive content. Professor of Geology, University of New Mexico, Albuquerque. They include olivine, pyroxene, amphibole and biotite. 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]). Igneous rocks can be divided into four categories based on their chemical composition: felsic, intermediate, mafic, and ultramafic. 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. Beyond the "big 8", manganese and titanium are present in small concentrations in magma and therefore appear in a number of minerals. Minerals such as magnesium-olivine, nepheline, and leucite are termed undersaturated (with respect to silica), and the subsilicic rocks that contain them are termed undersaturated as well. The composition usually reflects the composition of the magma, and thus provides information on the source of the rock. Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. The dominance of oxygen and silicon in the Earth's crust gaurantees that most igneous rocks are made up of silicate minerals. Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. Our editors will review what you’ve submitted and determine whether to revise the article. Earth is composed predominantly of a large mass of igneous rock with a very thin veneer of weathered material—namely, sedimentary rock. 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. NEUES JAHRB MINERAL MONATSH 1 , 1 – 15 . You will explore the classification of igneous rocks in the laboratory portion of this course. The diagram in Figure 3.16 can be used to help classify igneous rocks by their mineral composition. The chemical composition of the magma determines the minerals that will crystallize and their proportions. Typical occurrences of igneous rock bodies on the surface include lava flows, lava domes, necks and spines, Their emanations have provided the water for the oceans, the gases for the primordial oxygen-free atmosphere, and many valuable mineral deposits. Because of the high temperatures within Earth, the principles of chemical equilibrium are applicable to the study of igneous and metamorphic rocks, with the latter being restricted to those rocks formed without the direct involvement of magma. 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