Felsic and mafic rocks, division of igneous rocks on the basis of their silica content. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. Mafic rock is commonly contrasted with felsic rock, in which light-coloured minerals predominate. Most minerals fall within these two broad groups.…. Mafic rocks derive from basalt protoliths and some volcanogenic sediments…, …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. Geologists traditionally divide the compositions of the igneous rocks into four major groups or … The influence of silica content on the particular minerals that crystallize from a rock magma is a complex interaction of several parameters, and it cannot be assumed that rocks with the same silica content will have the same mineralogy. Corrections? Mafic rock is commonly contrasted with felsic rock, in which light-coloured minerals predominate. Magma often collects in large magma chambers at depth in Earth, but magma is also mobile and can flow through fissures and sometimes reach the surface. Mafic rock, in geology, igneous rock that is dominated by the silicates pyroxene, amphibole, olivine, and mica. basement rock The thick foundation of ancient, and oldest metamorphic and igneous rock that forms the crust of continents, often in the form of granite. Mafic: Felsic: Definition: It is relating to, denoting, or containing a group of dark-colored, mainly ferromagnetic minerals such as pyroxene and olivine. Mafic rock, in geology, igneous rock that is dominated by the silicates pyroxene, amphibole, olivine, and mica. Phaneritic. Chemical analyses of the most abundant components in rocks usually are presented as oxides of the elements; igneous rocks typically consist of approximately 12 major oxides totaling over 99 percent of the rock. The fine-grained or glassy nature of many volcanic rocks makes a chemical classification such as the felsic-mafic taxonomy very useful in distinguishing the different types. Intrusive, crystals easily visible, cooled very slow inside the Earth (Gabbro, granite) Aphanitic. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. Felsic rocks can be produced by metamorphism of both igneous and sedimentary protoliths (e.g., granite and arkose, respectively) and are rich in silicon, sodium (Na), potassium, calcium, aluminum, and lesser amounts of iron and magnesium. Mid-ocean ridges and subduction zones contain most magmas at or near Earth’s surface. Igneous rocks and minerals form from magma, molten rock that originates beneath Earth’s surface. Corrections? The word Mafic comes from (ma) for magnesium and (f) for ferrum or the Latin for iron. Mafic rocks are dark in color. https://www.britannica.com/science/mafic-rock. The term mafic is a combination of the words “magnesium” and “ferric.” The majority of mafic minerals are dark green or greenish black and are medium to coarse-grained. For similar determinations, glasses can also be prepared in the laboratory from crystalline rocks. Let us know if you have suggestions to improve this article (requires login). These minerals are high in magnesium and ferric oxides, and their presence gives mafic rock its characteristic dark colour. Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Save 50% off a Britannica Premium subscription and gain access to exclusive content. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Chemical analyses of the most abundant components in rocks usually are presented as oxides of the elements; igneous rocks typically consist of approximately 12 major oxides totaling over 99 percent of the rock. Felsic. Common mafic rocks include basalt and its coarse-grained intrusive equivalent, gabbro. Rocks containing lots of feldspar and silica. Rocks containing lots of magnesium and ferrum. Our editors will review what you’ve submitted and determine whether to revise the article. In general, the gradation from felsic to mafic corresponds to an increase in colour index (dark-mineral percentage). Premium Membership is now 50% off! Of the oxides, silica (SiO 2) is usually the most abundant. Updates? These minerals are high in magnesium and ferric oxides, and their presence gives mafic rock its characteristic dark colour. Within this scheme, rocks are described as felsic, intermediate, mafic, and ultramafic (in order of decreasing silica content). In a widely accepted silica-content classification scheme, rocks with more than 65 percent silica are called felsic; those with between 55 and 65 percent silica are intermediate; those with between 45 and 55 percent silica are mafic; and those with less than 45 percent are ultramafic. Premium Membership is now 50% off! Because of this abundance and because most igneous minerals are silicates, silica content was used as a basis of early classifications; it remains widely accepted today. Felsic and mafic rocks, division of igneous rocks on the basis of their silica content. Extrusive, crystals are not visible, cooled fast on surface of Earth (Basalt, rhyolite) Porphyritic. Let us know if you have suggestions to improve this article (requires login). Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... …mafic rocks, division of igneous rocks on the basis of their silica content. A fine-grained, mafic igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar. This broad grouping on the basis of mineralogy related to silica content is used in many modern classification schemes. Our editors will review what you’ve submitted and determine whether to revise the article. Chemical analyses of the most abundant components... Mafic and ultramafic rocks (those composed primarily of ferromagnesian—dark-coloured—minerals) 2.7 billion years old within the granite-greenstone terrane are the chief host of the epigenetic gold deposits of Western Australia. Updates? Silica saturation is a classification of minerals and rocks as oversaturated, saturated, or undersaturated with respect to silica. This article was most recently revised and updated by, https://www.britannica.com/science/felsic-rock. Slightly older (2.8 billion years) volcanic ultramafic rocks contain deposits of nickel sulfide.…. The abundant dark-coloured minerals include olivine, pyroxene, amphibole, biotite, garnet, tourmaline, iron oxides, sulfides, and metals. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. 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