The chemical composition of the magma determines the minerals that will crystallize and their proportions. 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. The top 20–25 km of the Earth's crust consists of approximately 95% igneous rocks or their metamorphic equivalents, 4% shale, 0.75% sandstone, and 0.25% limestone. Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. Articles from Britannica Encyclopedias for elementary and high school students. The table below summarizes the common classifications. Because of the dominance of oxygen and silicon in the crust, igneous rocks are mostly made up of silicate minerals. There are three great categories of rocks: igneous, sedimentary, and metamorphic. The composition usually reflects the composition of the magma, and thus provides information on the source of the rock. The exposed intrusive rocks are found in a variety of sizes, from small veinlike injections to massive dome-shaped batholiths, which extend for more than 100 square kilometres (40 square miles) and make up the cores of the great mountain ranges. (Denudation is the wearing away of the terrestrial surface by processes including weathering and erosion.) How solid is your knowledge of all things geological? Because of the limited occurrence of such carbonate-rich igneous rocks, however, the following discussion will consider the chemistry of silicate rocks only. 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. NAMING IGNEOUS ROCKS Geologists use both the minerals and texture to classify and name igneous rocks. Crystals can form in the mass if cooling happens very slowly, allowing the natural geometrical shapes of the molecules to form. Igneous rocks are crystalline solids which cool from magma: the liquid phase of solid rock. Igneous rocks are classified according to their texture and composition. The main differences in the composition of igneous rocks are the variations in the other six elements. The classification and nomenclature of igneous rocks are treated in the final section. Omissions? These groups refer to differing amounts of silica, iron, and magnesium found in the minerals that make up the rocks. Professor of Geology, University of New Mexico, Albuquerque. Igneous rock is formed through the cooling and solidification of magma or lava. Occurrence of igneous rocks can be either intrusive (plutonic) or extrusive (volcanic). Quartz clearly will not be present in these rocks. Solidification into rock occurs either below the surface Such rocks are called granitic rock. Sediments composed of weathered rock lithify to form sedimentary rock, which then becomes metamorphic rock under the pressure of Earth's crust. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. The average chemical composition of common rocks is given in Table 1. Most rocks are composed of minerals. Igneous rocks constitute one of the three principal classes of rocks, the others being metamorphic and sedimentary. Let us know if you have suggestions to improve this article (requires login). 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. The term geology refers, according to Britannica, the fields of study concerned with the solid Earth. Common igneous rocks comprise 40…77% of silica (SiO2). Igneous rocks are classified on the basis of texture and chemical composition, usually as reflected in the minerals that from due to crystallization. 4.1.2: Composition. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. The light-colored silicates include quartz, muscovite and feldspar. They include olivine, pyroxene, amphibole and biotite. Igneous rocks can be divided into four categories based on their chemical composition: felsic, intermediate, mafic, and ultramafic. Igneous processes have been active since the onset of the formation of Earth some 4.6 billion years ago. Texture refers to the size and arrangement of the minerals or grains that make up a rock. The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Typical occurrences of igneous rock bodies on the surface include lava flows, lava domes, necks and spines, Because of the importance of silica content, it has become common practice to use this feature of igneous rocks as a basis for subdividing them into the following groups: silicic or felsic (or acid, an old and discredited but unfortunately entrenched term), rocks having more than 66 percent silica; intermediate, rocks with 55 to 66 percent silica; and subsilicic, rocks containing less than 55 percent silica. The most important chemical elements are oxygen and silicon. Updates? Igneous rocks are commonly classified by their composition and texture. It is evident that SiO 2 and Al 2 … 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. 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. Composition refers to both the types of minerals within a rock and the overall chemical makeup of the rock (the two are obviously related). 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). 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. Click on any element for further details. As the magma cools and begins to crystallize, silica is taken from the magma to be combined with the other cationic oxides to form the silicate minerals. This can also form, therefore, a sound basis for classifying the igneous rocks. 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). There are two major types of igneous rocks: Extrusive, fine grained, and intrusive, fine grained. The chemical composition of an igneous rock tells us about the origin of the magma, beginning with which type of rock melted within the earth to form the magma in the first place, and how deep in the earth the melting occurred. The chemical composition of igneous rocks varies widely. 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 study of the igneous rocks has hitherto largely consisted in an analysis of their mineralogical and chemical composition, with the special intent to produce a satisfactory nomenclature and classification of the rocks as they occur throughout the world. Igneous rocks are classified according to their mineral content: Ultramafic rocks are dominated by olivine and/or pyroxene. Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. 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. Both calcium oxide (CaO) and alumina (Al2O3) are depleted in the rocks that have a silica content of less than about 45 weight percent, but, above 45 percent, calcium oxide can be as high as 10 percent; this amount decreases gradually as the silica increases. 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. Felsic, intermediate, mafic, and magnesium found in the final.! 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