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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. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Corrections? Felsic and mafic rocks, division of igneous rocks on the basis of their silica content. The latter may be further divided into two groups: mafic,…, …or quartz, giving the term felsic; other felsic minerals are corundum, zircon, muscovite, lepidolite, and calcite. Rocks containing lots of feldspar and silica. 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. https://www.britannica.com/science/mafic-rock. This article was most recently revised and updated by, https://www.britannica.com/science/felsic-rock. 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. Omissions? Let us know if you have suggestions to improve this article (requires login). In general, the gradation from felsic to mafic corresponds to an increase in colour index (dark-mineral percentage). 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. Our editors will review what you’ve submitted and determine whether to revise the article. 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. Black Friday Sale! By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. 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. Premium Membership is now 50% off! 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 is commonly contrasted with felsic rock, in which light-coloured minerals predominate. Common mafic rocks include basalt and its coarse-grained intrusive equivalent, gabbro. Geologists traditionally divide the compositions of the igneous rocks into four major groups or … The word Mafic comes from (ma) for magnesium and (f) for ferrum or the Latin for iron. Omissions? Rocks containing lots of magnesium and ferrum. Mafic rock, in geology, igneous rock that is dominated by the silicates pyroxene, amphibole, olivine, and mica. Most minerals fall within these two broad groups.…. Extrusive, crystals are not visible, cooled fast on surface of Earth (Basalt, rhyolite) Porphyritic. Compilations of many rock analyses show that rhyolite and granite are felsic, with an average silica content of about 72 percent; syenite, diorite, and monzonite are intermediate, with an average silica content of 59 percent; gabbro and basalt are mafic, with an average silica content of 48 percent; and peridotite is an ultramafic rock, with an average of 41 percent silica. 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. Phaneritic. Of the oxides, silica (SiO2) is usually the most abundant. 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. 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. For similar determinations, glasses can also be prepared in the laboratory from crystalline rocks. 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. Let us know if you have suggestions to improve this article (requires login). Updates? Felsic rocks are commonly oversaturated and contain free quartz (SiO2), intermediate rocks contain little or no quartz or feldspathoids (undersaturated minerals), and mafic rocks may contain abundant feldspathoids. A fine-grained, mafic igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar. Updates? Some examples include gabbro, basalt, and scoria. Mafic rock is commonly contrasted with felsic rock, in which light-coloured minerals predominate. Our editors will review what you’ve submitted and determine whether to revise the article. Felsic and mafic rocks, division of igneous rocks on the basis of their silica content. Mafic is a chemical classification for igneous rocks containing 46-85% mafic (minerals rich in iron and magnesium) minerals. Igneous rocks and minerals form from magma, molten rock that originates beneath Earth’s surface. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Premium Membership is now 50% off! Although there are complete gradations between the averages, rocks tend to cluster about the averages. Mafic. 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. Slightly older (2.8 billion years) volcanic ultramafic rocks contain deposits of nickel sulfide.…. 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. Silica saturation is a classification of minerals and rocks as oversaturated, saturated, or undersaturated with respect to silica. Silica content is especially useful because the density and refractive index of natural glasses have been correlated with silica percentage; this makes identification possible in the absence of chemical data. Mafic rocks are dark in color. In geology, mafic is a term which describes an igneous rock or a silicate mineral rich in iron and magnesium. 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. These minerals are high in magnesium and ferric oxides, and their presence gives mafic rock its characteristic dark colour. Mid-ocean ridges and subduction zones contain most magmas at or near Earth’s surface. Of the oxides, silica (SiO 2) is usually the most abundant. The abundant dark-coloured minerals include olivine, pyroxene, amphibole, biotite, garnet, tourmaline, iron oxides, sulfides, and metals. 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. This broad grouping on the basis of mineralogy related to silica content is used in many modern classification schemes. 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. Felsic. 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