Intrusive rocks also can be categorized consistent with the shape and size of the intrusive body and its relation t… The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Mafic rocks are denser and darker in color, so at least as far as their chemical composition, mafic rocks and felsic rocks can be thought to be quite different from each other. Igneous rock is formed through the cooling and solidification of magma or lava. 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. Igneous Rocks by Composition. Professor of Geology, University of New Mexico, Albuquerque. Beyond the "big 8", manganese and titanium are present in small concentrations in magma and therefore appear in a number of minerals. The great majority of the igneous rocks are composed of silicate minerals (meaning that the basic building blocks for the magmas that formed them are made of silicon [Si] and oxygen [O]), but minor occurrences of carbonate-rich igneous rocks are found as well. Earth is composed predominantly of a large mass of igneous rock with a very thin veneer of weathered material—namely, sedimentary rock. 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. Chemical composition of igneous rocks is the most distinguishing feature. The bulk chemical composition of igneous rocks is silicate, and depending or the type, the rocks contain varying amounts of quartz, feldspar, and dark ferromagnesian minerals. Our editors will review what you’ve submitted and determine whether to revise the article. However, in a case where magma does not have enough silica relative to the magnesium oxide to produce the pyroxene, the magma will compensate by making a magnesium-olivine (forsterite; Mg2SiO4), along with the pyroxene, since the olivine requires only one-half as much silica for every mole of magnesium oxide. 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). 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. Rocks formed from the cooling and solidification of magma deep within the crust are distinct from those erupted at the surface mainly owing to the differences in physical and chemical conditions prevalent in the two environments. Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. 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. The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. These three minerals substitute in part for enstatite, albite (NaAlSi3O8, requiring three moles of silicon for one mole of sodium), and orthoclase feldspar (KAlSi3O8, requiring three moles of silicon for one mole of potassium), respectively. 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