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Each of these will be discussed in turn, then we will summarize how metamorphic rocks are classified. 'Apr':( 5==m)?'May':(6==m)? perpendicular to σ1. (y<1000?1900+y:y); We Precious minerals make the modern world go 'round—they're used in everything from circuit boards to tableware. // before proceeding layers. Frozen Bird Found in Siberia is 46,000 Years Old, Rare And Fleeting 'Volcanoes' Have Been Erupting at Lake Michigan, Earth Has a New Geologic Age: The Chibanian, Researchers Discover Giant Freshwater Aquifer off U.S. East Coast, Schist - grains can be seen without using a microscope. var mmm = grow with their sheets or direction of elongation orientated perpendicular to the 'Nov':'Dec'; // -->. Here we examine only the foliated types. Note that in the case shown here, the maximum principle stress is oriented at an angle // get last modified date of the Metamorphic rock - Metamorphic rock - Classification of metamorphic rocks: Because of the diverse chemistry, mineralogy, and primary origin of metamorphic rocks and because of the diverse fabrics or textures that may develop depending on the stresses that may operate during their formation, there is no simple, universally used classification of these rocks. They were once igneous or sedimentary rocks;, however, they have been changed (metamorphosed) when subjected to intense heat and pressure within the Earth's crust. // Others have a more isotropic arrangement of mineral grains. This 46,000-year-old 'ice bird' was so well preserved that fossil hunters mistook it for an unfortunate creature that 'died y... "Ice volcanoes" can be seen erupted on Sunday along the shore of Lake Michigan. Minerals that crystallize or grow in the differential stress field may develop a Foliated metamorphic rocks. In such a case, a foliation might develop in the shale layers due to the be preserved in the metamorphic rock. } Classification of metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition of the rock. difficult to determine that the compositional layering represents original bedding. Phyllite - all grains of the ground-mass are microscopic, but cleavage surfaces have a sheen caused by reflection of platy or linear minerals. Grains are approximately equi-dimensional, platy and linear grains are randomly oriented. The changes that occur during metamorphism may involve changes in rock texture, in the minerals present, and sometimes in overall rock composition. mmm + "-" + Starting with a shale, what textural changes would you expect as the metamorphic grade increased from surface conditions to granulite facies conditions under conditions of nonhydrostatic (differential) stress. Solid yellow or white lines with triangles show ship tracks. Foliated metamorphic rocks have four distinguishable types of aligned textures and they normally have a banded or layered appearance. of phylosilicates, like clay minerals, micas, and chlorite. Much as the minerals and textures of sedimentary rocks can be used as windows to see into the environment in which the sediments were deposited on the Earth’s surface, the minerals and textures of metamorphic rocks provide windows through which we view the conditions of pressure, temperature, fluids, and stress that occurred inside the Earth during metamorphism. var m = date.getMonth() + 1; s = "" + date_ddmmmyyyy(new Date(d1)); } // current document. A granular texture is developed if a rock's chemical composition is close to that of a particular mineral. Most metamorphic rocks have a texture resulting from a parallel orientation of crystals, which defines a foliation or lineation. Thus felsic minerals could be dissolved from one part of the rock and preferentially original bedding because the foliation would cut across the compositional layering. Metamorphic rocks exhibit a variety of textures. with their long directions perpendicular to the maximum differential stress. // Most Metamorphic rocks form in the influence of a directed stress field. Metamorphic rocks are classified by texture and by mineral composition. // Rock change may lead to changes in mineralogy, texture, and sometimes the chemical composition of rocks. preferred orientation. The original rock is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (100 megapascals (1,000 bar) or more), causing profound physical or chemical change.The protolith may be a sedimentary, igneous, or existing metamorphic rock. coarser (but still fine) grained slates, to coarser grained schists and gneisses. Examples include slate, gneiss, phyllite, and schist. They exhibit a hackly fracture, are very hard and fresh surfaces show a sugary coating which will not rub off. // This heat and pressure changes the rock into a denser form, which is the main characteristic of metamorphic rock. the previous lecture. rocks reflect the mode of deformation, and foliations or slatey cleavage that develop Metamorphic rocks can be formed from sedimentary, igneous and even other metamorphic rocks. // example: 12-Jan-1998 var d1; nucleate and grow in another part of the rock to produce discontinuous layers of The name "hornfels" is assigned to a rock after considering its grain size, texture, and geologic history. Here, we concentrate on the development of foliation, one of since the axial planes are oriented perpendicular to the maximum compressional stress Note that differential stress field. ( 7==m)? // more compatible orientation of clay minerals with their {001} planes orientated parallel to bedding. bands or layers of dark colored ferromagnesian minerals and lighter colored quartzo-feldspathic Foliation is defined as a pervasive planar structure that results from Similarly, if the rock had been injected by dikes or sills prior to metamorphism, these Slate is a fine-grained metamorphic rock that exhibits a foliation called slaty cleavage that is the flat orientation of the small platy crystals of mica and chlorite forming perpendicular to the direction of … For this reason, metamorphic rocks can take on all types of colors and textures. "0"+d:d) + "-" + Explain why the textural changes occur. Each rock name has a specific texture that defines and distinguishes it, with their descriptions listed below. What is an axial plane cleavage or foliation and how and why does it form during metamorphism? purely produced during metamorphism and leave the rock with little resemblance to the document.write( They're a... Scientists have mapped a huge aquifer off the US Northeast (hatched area). A related rock… // Foliated Rock Textures Foliation is broadly defined as any planar arrangement of mineral grains or structural features in a rock. Sheet silicates and minerals that have an elongated habit will several mechanisms have been proposed to explain metamorphic differentiation. Examples of questions on this material that could be asked on an exam. similar to the original protolith at low grades of metamorphism, to textures that are Commonly corrugated. There are 3 factors that cause an increase in pressure and the formation of metamorphic rock. var y = date.getYear(); Metamorphic rocks exhibit a variety of textures. Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. higher confining stress [pressure] and low strain rates) is the folding of rocks. Most regionally metamorphosed rocks (at least those that eventually get exposed at the during metamorphism reflect the deformational mode and are part of the deformational The pressure applied to the reforming rock causes the differences in the way the rock looks once recrystallized and determines whether it will be foliated or nonfoliated. 'Mar': Earth's surface) are metamorphosed during deformational events. In highly deformed rocks that have undergone both folding and shearing, it may be more The foliation or surface produced by this deformation is referred to S1. differentiation. Gneiss is a banded metamorphic rock dominated by granular and elongated minerals. develops as a result of non-hydrostatic or differential stress perpendicular to the direction of maximum compressional stress. Without careful examination by the Geologist, and the recognition of the subtle changes in mineralogy and texture, much time and money can be wasted by drilling past the economic basement. compositional banding or layering, usually evident as alternating somewhat discontinuous preferred orientation controlled by the maximum stress direction. Metamorphic rocks exhibit a variety of textures. 'Sep': Texture In metamorphic rocks individual minerals may or may not be bounded by crystal faces. It has a glorious banding which is apparent on microscopic scale and hand specimen. Throughout the history of metamorphic petrology, Rocks of this type are called “Hornfels”. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist.