Because they lack foliation, these rocks are named entirely on the basis of their mineralogy. The platy layers in this large outcrop of metamorphic rock show the effects of pressure on rocks during metamorphism. This large boulder has bedding still visible as dark and light bands sloping steeply down to the right. Shear stress pushes one side of the rock in a direction parallel to the side, while at the same time, the other side of the rock is being pushed in the opposite direction. The original rock (called the “protolith”) is either an igneous or sedimentary rock. Metamorphic rocks form gradually from existing rocks in solid state, so they remain rocks throughout the process. Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. Therefore, not only does the protolith determine the initial chemistry of the metamorphic rock, most metamorphic rocks do not change their bulk (overall) chemical compositions very much during metamorphism. The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole. Class Notes - Metamorphism Introduction. Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. If a rock is not foliated, its name is derived from its chemical composition. % Progress . Crushed quartzite is sometimes placed under railroad tracks because it is very hard and durable. Low-grade metamorphism takes place at approximately 200â320 ºC and relatively low pressure. Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. This gives the geologist literally “inside information” on what occurs within the Earth during such processes as the formation of new mountain ranges, the collision of continents, the subduction of oceanic plates, and the circulation of sea water into hot oceanic crust. Metamorphic Rocks: Rocks, which under tremendous heat and pressure are completely changed or metamorphosed from their original form, are called metamorphic rocks. Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism. In a given rock type, which starts with a particular chemical composition, lower-grade index minerals are replaced by higher-grade index minerals in a sequence of chemical reactions that proceeds as the rock undergoes prograde metamorphism. Medium-grade metamorphism takes place at approximately at 320â450 ºC and at moderate pressures. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. Instead, the quartz grains recrystallize into a denser, harder rock than the original sandstone. Such a rock is said to be foliated, or to have foliation. Contact metamorphism occurs to solid rock next to an igneous intrusion and is caused by the heat from the nearby body of magma. As sedimentation is favoured by water, so they are mostly formed under water. Extrusive igneous rocks typically have a fine-grained texture (individual minerals are not visible unless magnified) because the lava cools rapidly when exposed to the atmosphere, preventing crystal growth. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes. Any open space between the mineral grains in a rock, however microscopic, may contain a fluid phase. Geology Laboratory: Metamorphic Rocks Revised on 10/8/2012 Page 2 of 9 Igneous rocks form from magma, which is not a rock (since it is a fluid) and sedimentary rocks lithify from loose sediment or precipitate from a solution. Slates are generally fineâgrained, darkâcolored, metamorphosed sedimentary rocks that split easily along slaty foliations and were formed under lowâgrade temperature and pressure conditions. A quartzârich rock such a sandstone, for example, is called a quartzite when it has been metamorphosed. New minerals such as hornblende will form, which is stable at higher temperatures. slateâslates form at low metamorphic grade by the growth of fine-grained chlorite and clay minerals. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, … chlorite characterizes the lowest regional metamorphic grade, biotite replaces chlorite at the next metamorphic grade, which could be considered medium-low grade, garnet appears at the next metamorphic grade, medium grade, staurolite marks the next metamorphic grade, which is medium-high grade, sillimanite is a characteristic mineral of high grade metamorphic rocks. Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other. Schist and slate are sometimes used as building and landscape materials. The diagram below shows folds forming during an early stage of regional metamorphism, along with development of foliation, in response to normal stress. The type of rock undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. The rock also has a strong slaty foliation, which is horizontal in this view, and has developed because the rock was being squeezed during metamorphism. High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a volcanically active area. If the fluid introduces substantal amounts of ions into the rock and removes substantial amounts of ions from it, the fluid has metasomatized the rockâchanged its chemical composition. In most subduction zones the subducting plate is relatively cold compared with the high temperature it had when first formed at a mid-ocean spreading ridge. If pressure does not apply equally in all directions, differential stress occurs. As the rocks become heated at depth in the Earth during regional metamorphism they become ductile, which means they are relatively soft even though they are still solid. Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. The granitic rock in migmatite probably originated from partial melting of some of the metamorphic rock, though in some migmatites the granite may have intruded the rock from deeper in the crust. Covers metamorphic rocks, which form from previous rocks exposed to heat and/or pressure. Heat from the interior of the earth may sometimes help to change the rock. If the minerals are segregated into alternating lightâcolored and darkâcolored layers, the rock is called a gneiss. The foliated rocks like slate, gneiss and schist are used as roofing material tabletops, staircases, etc. However, if the protolith is shale, a muscovite-biotite schist, which is not green, will form instead. If any of these flat minerals are growing under normal stress, they will grow with their sheets oriented perpendicular to the direction of maximum compression. Metamorphic rocks are formed as a result of temperature and/or pressure action on existing rocks causing changes in the composition and resulting in the appearance of minerals in rocks. Gneiss. They are classified by texture and by chemical and mineral assemblage. It is composed primarily of hornblende (amphibole) and plagioclase, usually with very little quartz. Regionally metamorphosed rocks that contain hydrous fluids will begin to melt before they pass beyond the amphibolite facies. Remove the magnifying glass from the rock … Quartz and marble are prime examples of unfoliated that can be produced by either regional or contact metamorphism. In schist, the sheets of mica are usually arranged in irregular planes rather than perfectly flat planes, giving the rock a schistose foliation (or simply schistosity). •Instead the mineral composition, texture, or chemical composition of the rock changes •Metamorphic rock – rocks that changes form while remaining solid •Metamorphic mineral – … For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for. Figure 7.7 shows an example of this effect. The type of rock that a metamorphic rock used to be, prior to metamorphism, is called the protolith. Removing #book# Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form. The three geotherms represent different geological settings in the Earth. 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