notes on metamorphic rocks

Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. 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. 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. 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. A rock undergoing metamorphism remains a solid rock during the process. Ground up marble is also a component of toothpaste, plastics, and paper. The original rock (called the “protolith”) is either an igneous or sedimentary rock. Both rock types consist of metamorphic minerals that do not have flat or elongate shapes and thus cannot become layered even if they are produced under differential stress. These rocks are changed when heat or pressure alters the existing rock’s physical or chemical make up. Let’s see what these rocks are like and how they’re formed. The last type of rock is metamorphic rocks. Igneous rock is formed through the … They will form new minerals that are more stable in the new environment. 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. amphibolite—a poorly foliated to unfoliated mafic metamorphic rock, usually consisting largely of the common black amphibole known as hornblende, plus plagioclase, plus or minus biotite and possibly other minerals; it usually does not contain any quartz. The way temperature changes with depth inside the Earth is called the geothermal gradient, geotherm for short. Foliated metamorphic rocks. This gives the surfaces of phyllite a satiny luster, much brighter than the surface of a piece of slate. A geologist maps and collects rock samples across the region and marks the geologic map with the location of each rock sample and the type of index mineral it contains. A mineral assemblage stable at low temperatures and pressures may not be stable at elevated temperatures and pressures. The foliated rocks like slate, gneiss and schist are used as roofing material tabletops, staircases, etc. Hornfels is shown in table 1. However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. It is composed primarily of hornblende (amphibole) and plagioclase, usually with very little quartz. Rocks that have their pressure and temperature conditions increased along such a geotherm will metamorphose in the hornfels facies and, if it gets hot enough, in the granulite facies. 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. Metamorphism • Takes place when preexisting rock is changed when subjected to temperatures and pressures unlike those which it was originally formed. Gneiss is a high-grade metamorphic rock. An example of the categories a shale would pass through as temperatures and pressures increase (from low grade to high grade) is as follows: shale/slate/phyllite/mica schist/gneiss/migmatite. METAMORPHIC ROCKS NOTES. Burial metamorphism is the lowest grade of metamorphism. Quartzite is very hard and is often crushed and used in building railroad tracks (see figure 4). Where intrusions of magma occur at shallow levels of the crust, the zone of contact metamorphism around the intrusion is relatively narrow, sometimes only a few m (a few feet) thick, ranging up to contact metamorphic zones over 1000 m (over 3000 feet) across around larger intrusions that released more heat into the adjacent crust. Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are formed during regional metamorphism. Less commonly, it may be a carbon dioxide fluid or some other fluid. The type of rock that a metamorphic rock used to be, prior to metamorphism, is called the protolith. Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism. The difference in composition between the existing rock and the invading fluid drives the chemical reactions. Schist often contains more than just micas among its minerals, such as quartz, feldspars, and garnet. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. Marble is beautiful for statues and decorative items such as vases (see an example in figure 3). The most common conditions in the Earth are found along geotherms between those two extremes. Rocks do not melt during most conditions of metamorphism. A hornfels rock is characterized by evenly distributed, very fine‐grained mica crystals that give it a more massive, equigranular appearance. During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle. Ions may move between minerals to create minerals of different chemical composition. Metamorphic rocks are the product of “metamorphism”, which is the partial or complete recrystallization of rocks. 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 rock fall into two categories, foliated and unfoliated. Igneous Rock. The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole. Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids. For example, in rocks made of metamorphosed shale, metamorphism may prograde through the following index minerals: Index minerals are used by geologists to map metamorphic grade in regions of metamorphic rock. Yet another way a rock in the Earth’s crust can have its temperature greatly increased is by the intrusion of magma nearby. neomineralization/neocrystallization: formation of new minerals (e.g., the appearance of garnet in a rock that lacked garnet) Figure 4. This temperature is about 200ºC (approximately 400ºF). Learn vocabulary, terms, and more with flashcards, games, and other study tools. There are two major subdivisions of metamorphic rocks. Foliated – These have a planar foliation caused by the preferred orientation (alignment) of minerals and formed under differential stress. At the maximum pressures and temperatures the rocks may encounter within the Earth in this range of geotherms, they will enter either the granulite or eclogite facies. This will be especially apparent for micas or other sheet silicates that grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. Barrovian metamorphic zonesare defined by reactions that result in the appearance or disappearance of minerals and can be mapped as isograds chl —> bi —> gar —> st —> ky —> sill —> sill + or This development of metamorphic mineral assemblages corresponds to this P-T path: A metamorphosed limestone is called a marble. Gneiss. Parent rock: Shale. Temperature depends on the heat flow, which varies from location to location. Some unfoliated metamorphic rocks, such as hornfels, originate only by contact metamorphism, but others can originate either by contact metamorphism or by regional metamorphism. Class Notes - Metamorphism Introduction. Amphibolite forms at medium-high metamorphic grades. 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). There are two types of differential stress. 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. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. It typically contains ab… The most important non-foliated rock is marble. Some slate breaks into such extensively flat sheets of rock that it is used as the base of pool tables, beneath a layer of rubber and felt. At the same time, in a perpendicular direction, the rock undergoes tension (stretching), in the direction of minimum stress. Covers metamorphic rocks, which form from previous rocks exposed to heat and/or pressure. A fluid phase may introduce or remove chemical substances into or out of the rock during metamorphism, but in most metamorphic rock, most of the atoms in the protolith are be present in the metamorphic rock after metamorphism; the atoms will likely be rearranged into new mineral forms within the rock. Metamorphic rocks form gradually from existing rocks in solid state, so they remain rocks throughout the process. MEMORY METER. Tectonic processes are another way rocks can be moved deeper along the geotherm. Metamorphic rocks and processes • Metamorphism comes from the Greek words “Meta” - change “Morphe” - form • Metamorphic rocks form by solid-state (no melting) transformation of preexisting rock by processes that take place beneath Earth’s surface. Most metamorphic rocks are evolved from either sedimentary of igneous rocks. Therefore, if rocks are simply buried deep enough enough sediment, they will experience temperatures high enough to cause metamorphism. The source of the pressure is the weight of all the rocks above. They are classified by texture and by chemical and mineral assemblage. The word metamorphism comes from ancient Greek words for “change” (meta) and “form” (morph). One ways rocks may change during metamorphism is by rearrangement of their mineral crystals. As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions.,, Metamorphism of slate, but under greater heat and pressure than slate, Often derived from metamorphism of claystone or shale; metamorphosed under more heat and pressure than phyllite, Metamorphism of various different rocks, under extreme conditions of heat and pressure, Contact metamorphism of various different rock types, biotite, muscovite, quartz, garnet, plagioclase, plagioclase, orthoclase, quartz, biotite, amphibole, pyroxene. This results in a rock that can be easily broken along the parallel mineral sheets. Figure 7.7 shows an example of this effect. Examples of low grade hydrous minerals include clay, serpentine, and chlorite. The metamorphic rocks are extensively used as building stones. The two main types of metamorphism are both related to heat within Earth: The reason rocks undergo metamorphism is that the minerals in a rock are only stable under a limited range of pressure, temperature, and chemical conditions. Temperature is another major factor of metamorphism. Extreme pressure may also lead to the form… 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. Contact metamorphism occurs to solid rock next to an igneous intrusion and is caused by the heat from the nearby body of magma. Schist. 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. Argillite. Any open space between the mineral grains in a rock, however microscopic, may contain a fluid phase. In migmatite you can see metamorphic rock that has reached the limits of metamorphism and begun transitioning into the igneous stage of the rock cycle by melting to form magma. This is not far beyond the conditions in which sediments get lithified into sedimentary rocks, and it is common for a low-grade metamorphic rock to look somewhat like its protolith. Example: Shale is a parent rock that can become the metamorphic rocks slate, phyllite, schist, and gneiss. A metamorphic rock used to be some other type of rock, but it was changed inside the Earth to become a new type of rock. The type of rock undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. This occurs due to pressure, volume and temperature changes. 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. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes. Most metamorphism of rocks takes place slowly inside the Earth. Metamorphic Rocks: Rocks, which under tremendous heat and pressure are completely changed or metamorphosed from their original form, are called metamorphic rocks. 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. Figure 3. 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. Some marble, which is considered better quality stone for carving into statues, lacks color bands. Ocean water that penetrates hot, cracked oceanic crust and circulates as hydrothermal fluid in ocean floor basalts produces extensive hydrothermal metamorphism adjacent to mid-ocean spreading ridges and other ocean-floor volcanic zones. The protolith is subjected to a change, over time, in the physical and High-pressure, low-temperature geotherms occurs in subduction zones. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. The specimen shown above is about two inches (five centimeters) across. Index minerals, which are indicators of metamorphic grade. Schist and slate are sometimes used as building and landscape materials. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Because contact metamorphism is not caused by changes in pressure or by differential stress, contact metamorphic rocks do not become foliated. Low-grade metamorphism takes place at approximately 200–320 ºC and relatively low pressure. amphibolite—amphibolites are dark-colored rocks with amphibole, usually the common black amphibole known as hornblende, as their most abundant mineral, along with plagioclase and possibly other minerals, though usually no quartz. Note that not all minerals listed in the mineralogy column will be present in every rock of that type and that some rocks may have minerals not listed here. The need for stability may cause the structure of minerals to rearrange and form new minerals. These rocks are characterized as either extrusive or intrusive. Depending on the composition of the rock and the temperature reached, minerals indicative of high metamorphic grade such as pyroxene may occur in some hornfels, though many hornfels have minerals indicating medium grade metamorphism. Lithostatic pressure increases as depth within the Earth increases and is a uniform stress—the pressure applies equally in all directions on the rock. The changed rock is called the metamorphic rock and it will be stable under the new set of conditions till there is a further change in those conditions. Quartzite and marble are commonly used for building materials and artwork. Blueschist facies and hornfels facies are associated with unusual geothermal gradients.

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