Crushed quartzite is sometimes placed under railroad tracks because it is very hard and durable. Marbles may have bands of different colors which were deformed into convoluted folds while the rock was ductile. Define the characteristics of a metamorphic rock. Practice. Let’s see what these rocks are like and how they’re formed. Metamorphic Rocks and Minerals •Sometimes rock are subject to pressure and heat but do not melt. Gneiss is a high-grade metamorphic rock. Schist often contains more than just micas among its minerals, such as quartz, feldspars, and garnet. The type of rock that a metamorphic rock used to be, prior to metamorphism, is called the protolith. As rocks are compressed more and more by other layers of rocks, the lower rocks may change due to the weight of the upper layers. A schist is coarser grained than phyllite or slate and has aligned minerals that can be identified with the naked eye. Metamorphic rocks are the product of “metamorphism”, which is the partial or complete recrystallization of rocks. Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are formed during regional metamorphism. Igneous rocks are those that solidify from a molten or partially molten state. In oceanic basalts that are part of a subducting plate, the high-P, low-T conditions create a distinctive set of metamorphic minerals including a type of amphibole, called glaucophane, that has a blue color. The magma is sweated out, or injected, as layers between foliation planes in the rock. gneiss—like the word schist, the word gneiss is originated from the German language; it is pronounced “nice.” As metamorphic grade continue to increase, sheet silicates become unstable and dark minerals such as hornblende or pyroxene start to grow. Click the posters on the wall to get specific information about the metamorphic rocks. Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". By drawing lines around the areas where each type of index mineral occurs, the geologist delineates the zones of different metamorphic grades in the region. These rocks are changed when heat or pressure alters the existing rock’s physical or chemical make up. Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form. For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for. marble—marble is a metamorphic rock made up almost entirely of either calcite or dolomite, for which the protolith was either limestone or dolostone, respectively. It is also common for the differential stresses under which phyllite forms to have produced a set of folds in the rock, making the foliation surfaces wavy or irregular, in contrast to the often perfectly flat surfaces of slaty cleavage. The new rock is completely different from the original. © 2020 Houghton Mifflin Harcourt. Subduction takes the rocks to great depth in the Earth relatively quickly. Metamorphism can be caused by burial, tectonic stress, heating by magma, or alteration by fluids. Amphibolites are poorly foliated to unfoliated and form at medium to medium-high grades of metamorphism from basalt or gabbro. A geologist working with metamorphic rocks collects the rocks in the field and looks for the patterns the rocks form in outcrops as well as how those outcrops are related to other types of rock with which they are in contact. Field evidence is often required to know for sure whether rocks are products of regional metamorphism, contact metamorphism, or some other type of metamorphism. Most commonly, if there is a fluid phase in a rock during metamorphism, it will be a hydrous fluid, consisting of water and things dissolved in the water. This gives the surfaces of phyllite a satiny luster, much brighter than the surface of a piece of slate. Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids. If it can be determined that a muscovite-biotite schist formed at around 350ºC temperature and 400 MPa pressure, it can be stated that the rock formed in the greenschist facies, even though the rock is not itself a greenschist. Magma intrusion subjects nearby rock to higher temperature with no increase in depth or pressure. Quartzite is very hard and is often crushed and used in building railroad tracks (see figure 4). quartzite—quartzite is a metamorphic rock made almost entirely of quartz, for which the protolith was quartz arenite. Some varieties of schist are mica, garnet‐mica, biotite, kyanite, and talc schist. This is the rock name to remember when you find a hard, nondescript rock that looks like it … Identifying Rocks : Identifying Metamorphic Rocks. This will be especially apparent for micas or other sheet silicates that grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. schist—the size of mineral crystals tends to grow larger with increasing metamorphic grade. Yet another way a rock in the Earth’s crust can have its temperature greatly increased is by the intrusion of magma nearby. Differential stress can flatten pre-existing grains in the rock, as shown in the diagram below. Medium-grade metamorphism takes place at approximately at 320–450 ºC and at moderate pressures. This video discusses how to identify a metamorphic rocks: Answer the question(s) below to see how well you understand the topics covered in the previous section. Marble is used for decorative items and in art. The hydrothermal fluid may originate from a magma that intruded nearby and caused fluid to circulate in the nearby crust, from circulating hot groundwater, or from ocean water. The knowledge of temperatures and pressures at which particular types of metamorphic rocks form led to the concept of metamorphic facies. Metamorphic rocks form gradually from existing rocks in solid state, so they remain rocks throughout the process. If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. However, a more complete name of each particular type of foliated metamorphic rock includes the main minerals that the rock comprises, such as biotite-garnet schist rather than just schist. Remove the magnifying glass from the rock … 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. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. Many types of gneiss look somewhat like granite, except that the gneiss has dark and light stripes whereas in granite randomly oriented and distributed minerals with no stripes or layers. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. migmatite—a combination of high-grade regional metamorphic rock – usually gneiss or schist – and granitic igneous rock. When heat and pressure change the environment of a rock, the crystals may respond by rearranging their structure. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, … Low grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may grow. The presence of a fluid phase is a major factor during metamorphism because it helps determine which metamorphic reactions will occur and how fast they will occur. It typically contains ab… There are two major subdivisions of metamorphic rocks. Figure 4. Metamorphic rocks start off as igneous, sedimentary, or other metamorphic rocks. The last type of rock is metamorphic rocks. Garnet is an example of a mineral which may form porphyroblasts, metamorphic mineral grains that are larger in size and more equant in shape (about the same diameter in all directions), thus standing out among the smaller, flatter, or more elongate minerals. Metamorphic rocks are classified by texture and by mineral composition. Figure 7.7 shows an example of this effect. Typical Pressure Range For Common Metamorphic Rocks = 2-8 Kb GEOTHERMAL GRADIENT-- Controls Heat flow at Surface Two Major Aspects of Geothermal Gradient: • Conduction of heat from Mantle >> Limited effect in continents, Controlled by thickness of lithosphere (but note areas with thin lithosphere, e.g., Great Basin) New minerals such as hornblende will form, which is stable at higher temperatures. Schist. Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. 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