One theory is the overriding rock gets shouldered aside, displaced by the increased volume of magma. It is commonly vesicular and aphanitic. Pyro, meaning fire, refers to the igneous source of the tephra and clastic refers to the rock fragments. The Mafic Igneous rocks have less silica content around … Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. Diorite is a coarse-crystalline intermediate intrusive igneous rock. The fine-grained texture indicates the quickly cooling lava did not have time to grow large crystals. Intermediate magmas are somewhat more viscous than the mafic magmas. in Developments in Precambrian Geology (ed. Felsic rocks are light in color and are composed of feldspars and silicates. This category includes primarily feldspathic rocks with small admixtures of such ferromagnesian minerals as pyroxene, hornblende, and, more rarely, biotite. Rock formed from large deposits of tephra fragments is called tuff. Residual molten material expelled from igneous intrusions may form veins or masses containing very large crystals of minerals like feldspar, quartz, beryl, tourmaline, and mica. Felsic magma such as rhyolite is usually erupted at low temperature and is up to 10,000 times as [?] Johannsen, A., 1931, 1937, A Descriptive Petrography of the Igneous Rocks, Vols. Granite is a good approximation for the continental crust, both in density and composition. 3. The intermediate igneous rock in Figure 3.3B does have a few visible phenocrysts; this odd texture will be covered later in this chapter. As the magma rises to the surface, the drop in pressure causes the dissolved volatiles to come bubbling out of solution, like the fizz in an opened bottle of soda. This book is for geoscience students taking introductory or intermediate–level courses in igneous petrology, to help develop key skills (and confidence) in identifying igneous minerals, interpreting and allocating appropriate names to unknown rocks presented to them. Plus, they are, by definition, exposed to the elements of erosion immediately. If the intrusive rock cooled underground but near the surface, it is called subvolcanic or hypabyssal, and often has visible, but tiny mineral grains. A rock that chiefly consists of pegmatitic texture is known as a pegmatite. If the fragments accumulate while still hot, the heat may deform the crystals and weld the mass together, forming a welded tuff. Batholiths and stocks are discordant intrusions that cut across and through surrounding country rock. It is found in its namesake, the Andes Mountains as well as the Henry and Abajo mountains of Utah. ash, tuff and lava, and may have viscosity similar to thick, cold molasses or even rubber when erupted. Two Types of Igneous Rocks ... - Diorite: from intermediate magma - Gabbro: from mafic magma. In igneous petrology an intermediate composition refers to the chemical composition of a rock that has 52-63 wt% SiO 2 being an intermediate between felsic and mafic compositions. Rev. A magma chamber is a large underground reservoir of molten rock. Igneous rocks made from magma form underneath the Earth’s surface and are called intrusive igneous rocks When the melted material is on or above the Earth’s surface, it is called lava. Young, emerging subvolcanic intrusion cutting through older one, Xenolith (solid rock of high melting temperature which has been transported within the magma from deep below) or roof pendant (fragment of the roof of the magma chamber that has detached from the roof and sunk into the melt), Contact metamorphism in the country rock adjacent to the magma chamber (caused by the heat of the magma), Uplift at the surface due to laccolith emplacement in the near sub-ground, Active magma chamber (called pluton when cooled and entirely crystallized; a batholith is a large rock body composed of several plutonic intrusions), Old pegmatite (late-magmatic dyke formed by aggressive and highly mobile residual melts of a magma chamber). Plutons can have irregular shapes, but can often be somewhat round. Rosenbusch, H., 1898, Elemente der Gesteinslehre, 1st edn. Watch the recordings here on Youtube! There is one important exception to the dark = mafic rule, and that is aphanitic, vesicle-free, black or very dark-gray glass that is called obsidian. I. Frequency-distribution of the major oxides in analyses of igneous rocks, Mineral. 1. 2. Peacock, M. A. Rhyolite is a fine-crystalline felsic extrusive rock. As with dikes, sills are younger than the surrounding layers and may be radioactively dated to study the age of sedimentary strata. Felsic, intermediate and mafic igneous rocks differ in their mineral composition. Rhyolite refers to the volcanic and felsic igneous rocks and granite refer to intrusive and felsic igneous rocks. Because of this, dikes are often vertical or at an angle relative to the pre-existing rock layers that they intersect. in. Typical intermediate rocks include andesite, dacite and trachyandesite among volcanic rocks and diorite and granodiorite among plutonic rocks. Peacock, M. A. These groups refer to differing amounts of silica, iron, and magnesium found in the minerals that make up the rocks. All magmas contain gases dissolved in a solution called volatiles. Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intermediate is a composition between felsic and mafic. Diorite is identifiable by it’s Dalmatian-like appearance of black hornblende and biotite and white plagioclase feldspar. Mag. The solid parts, called tephra, settle back to earth and cool into rocks with pyroclastic textures. This relates to the cooling history of the molten magma from which it came. (b) Intermediate igneous rocks: These are those rocks in which the Silica presence is in between 55 and 66%. Classification of Igneous Rock Series. They are formed when magma exploits a weakness between these layers, shouldering them apart and squeezing between them. Figure 3.4 | Examples of igneous rocks from the mafic (A), intermediate (B), and felsic (C) rock compositions. Classification of Igneous Rocks; COMPOSITION; TEXTURE Felsic Intermediate Mafic Ultramafic ; Phaneritic: Granite: Diorite: Gabbro: Peridotite: Aphanitic : Rhyolite: Andesite: Basalt: Vesicular: Pumice: Scoria: Scoria: Glassy: Obsidian: PART I - Volcanic (Extrusive) Rocks. Andesite is a fine crystalline intermediate extrusive rock. Missed the LibreFest? ... Intermediate composition magma such as andesite tends to form cinder cones of [?] Gabbro is a major component of the lower oceanic crust. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Intrusive rocks, forming underground with larger, stronger crystals, are more likely to last. This service is more advanced with JavaScript available, Over 10 million scientific documents at your fingertips. Additionally, they contain somewhat more gas than do the mafic magmas, but not quite as much as the felsic magmas (see below). When many plutons merge together in an extensive single feature, it is called a batholith. Arndt, N. T. Chapter 1 Archean Komatiites. Igneous rocks are formed from the solidification of molten rock material. 1. Richardson, W. A. and G. Sneesuby, 1922, The frequency-distribution of igneous rocks. © 2020 Springer Nature Switzerland AG. The mineral grains in such rocks can generally be recognized with the bare eye. Forty-five mafic-intermediate igneous rocks in South Laos with mg-number = 42–69 have SiO 2 = 53.04–61.66 wt%, Al 2 O 3 = 12.13–20.92 wt% and MgO = 2.73–9.55 wt%, and are characterized by enrichment in LILEs and depletion in HFSEs with Nb Ta negative anomalies. They are typically more than 100 km2 in area, associated with subduction zones, and mostly felsic in composition. Mafic rocks are darker and are composed of magnesium and iron. These points are: (1) degree of crystallization. Legal. It has a composition that is intermediate between rhyolite and andesite. Igneous rocks—classification and nomenclature. Notice the difference in appearance between these rocks and those in Figure 3.2.