Where Does Metamorphism Occur?

Where Does Metamorphism Occur?

kacimo samy   9:12 ص

Where Does Metamorphism Occur? 

So far, we've discussed the nature of changes that occur during metamorphism, the agents of metamorphism (heat, pressure, compression and shear, and hydrothermal fluids), the rock types that form as a result of metamorphism, and the concepts of metamorphic grade and metamorphic facies. With this background, let’s now examine the geologic settings on Earth where metamorphism takes place, as viewed from the perspective of plate tectonics theory.

Because of the wide range of possible metamorphic environments, metamorphism occurs at a wide range of conditions in the Earth. You will see that the conditions under which metamorphism occurs are not the same in all geologic settings. That’s because the geothermal gradient (the relation between temperature and depth), the extent to which rocks endure compression and shear during metamorphism, and the extent to which rocks interact with hydrothermal fluids all depend on the geologic environment.

Thermal or Contact Metamorphism 

Geologic settings of metamorphism.   

Imagine a hot magma that rises from great depth beneath the Earth’s surface and intrudes into cooler rock at a shallow depth. Heat flows from the magma into the wall rock, for heat always flows from hotter to colder materials. As a consequence, the magma cools and solidifies while the wall rock heats up. In addition, hydrothermal fluids circulate through both the intrusion and the wall rock. As a consequence of the heat and hydrothermal fluids, the wall rock undergoes metamorphism, with the highest-grade rocks forming immediately adjacent to the pluton, where the temperatures were highest, and progressively lower-grade rocks forming farther away. The distinct belt of metamorphic rock that forms around an igneous intrusion is called a metamorphic aureole or contact aureole (figure above a). The width of an aureole depends on the size and shape of the intrusion, and on the amount of hydrothermal circulation larger intrusions produce wider aureoles.

The local metamorphism caused by igneous intrusion can be called either thermal metamorphism (see Pottery Making—An Analog  for Thermal Metamorphism), to emphasize that it develops in response to heat without a change in pressure and without differential stress, or contact metamorphism, to emphasize that it develops adjacent to the contact of an intrusion with its wall rock. Because this metamorphism takes place without application of compression or shear, aureoles contain hornfels, a nonfoliated metamorphic rock. 

Contact metamorphism occurs anywhere that the intrusion of plutons occurs. In the context of plate tectonics theory, plutons intrude into the crust at convergent plate boundaries, in rifts, and during the mountain building that takes place where continents collide.

Burial Metamorphism 

As sediment gets buried in a subsiding sedimentary basin, the pressure increases due to the weight of overburden, and the temperature increases due to the geothermal gradient. At depths greater than about 8 to 15 km, depending on the geothermal gradient, temperatures may be great enough for metamorphic reactions to begin, and low-grade metamorphic rocks form. Metamorphism due only to the consequences of very deep burial is called burial metamorphism.

Dynamic Metamorphism 

Faults are surfaces on which one piece of crust slides, or shears, past another. Near the Earth’s surface (in the upper 10 to 15 km) this movement can fracture rock, breaking it into angular fragments or even crushing it to a powder. But at greater depths, rock is so warm that it behaves like soft plastic as shear along the fault takes place. During this process, the minerals in the rock recrystallize. We call this process dynamic metamorphism, because it occurs as a consequence of shearing alone under metamorphic conditions, without requiring a change in temperature or pressure. The resulting rock, a mylonite, has a foliation that roughly parallels the fault (figure above b). Mylonites are very fine-grained, due to processes during dynamic metamorphism that replace larger crystals with a mass of very tiny ones. Dynamic metamorphism takes place anywhere that faulting occurs at depth in the crust. Thus, mylonites can be found at all plate boundaries, in rifts, and in collision zones.

Dynamothermal (Regional) Metamorphism 

During the development of mountain ranges, in response to either convergent-margin tectonics or continental collision, regions of crust are squeezed and large slices of continental crust slip along faults and move up and over other portions of the crust. As a consequence, rock that was once near the Earth’s surface along the margin of a continent ends up at great depth beneath the mountain range (figure above c). In this environment, three changes happen to the protolith: (1) it heats up because of the geothermal gradient and because of igneous activity; (2) it endures greater pressure because of the weight of overburden; and (3) it undergoes compression and shearing. As a result of these changes, the protolith transforms into foliated metamorphic rock. The type of foliated rock that forms depends on the grade of metamorphism slate forms at shallower depths, whereas schist and gneiss form at greater depths. Since the metamorphism we've just described involves not only heat but also compression and shearing, we can call it dynamothermal metamorphism. Typically, such metamorphism affects a large region, so geologists also call it regional metamorphism. 

Erosion eventually removes the mountains, exposing a belt of metamorphic rock that once lay at depth. Such belts may be hundreds of kilometres wide and thousands of kilometres long.

Hydrothermal Metamorphism at Mid-Ocean Ridges 

Hot magma rises beneath the axis of mid-ocean ridges, so when cold seawater sinks through cracks down into the oceanic crust along ridges, it heats up and transforms into hydrothermal fluid. This fluid then rises through the crust, near the ridge, causing hydrothermal metamorphism of ocean-floor basalt (figure above d). Eventually, the fluid escapes through vents back into the sea; these vents are called black smokers.

Pottery Making—An Analog  for Thermal Metamorphism

A brick for the wall of an adobe house, an earthenware pot, a stoneware bowl, or a translucent porcelain teacup may all be formed from the same lump of soft clay, scooped from the surface of the Earth and shaped by human hands. This pliable and slimy muck is a mixture of very fine clay minerals and quartz grains formed during the chemical weathering of rock and water. Fine potter’s clay for making white china contains a particular clay mineral called kaolinite, named after the locality in China (called Kauling, meaning high ridge) where it was originally discovered. 

People in arid climates make adobe bricks by forming damp clay into blocks, which they then dry in the sun. Such bricks can be used for construction only in arid climates, because if it rains heavily, the bricks will rehydrate and turn back into sticky muck drying clay in the sun does not change the structure of the clay minerals. 

To make a more durable material, brick makers place clay blocks in a kiln and bake (“fire”) them at high temperatures. This process makes the bricks hard and impervious to water. Potters use the same process to make jugs. In fact, fired clay jugs that were used for storing wine and olive oil have been found intact in sunken Greek and Phoenician ships that have rested on the floor of the Mediterranean Sea for thousands of years! Clearly, the firing of a clay pot fundamentally and permanently changes clay in a way that makes it physically different (see 1st figure a). In other words, firing causes a thermal metamorphic change in the mineral assemblage that composes pottery. The extent of the transformation depends on the kiln temperature, just as the grade of metamorphic rock depends on temperature. Potters usually fire earthenware at about 

1100C and stoneware (which is harder than a knife or fork) at about 1250C. To produce porcelain fine china the clay must partially melt at even higher temperatures up to 1400C. Just as it begins to melt, the potter cools it relatively quickly. Such cooling of the melt creates glass, which gives porcelain its translucent, vitreous (glassy) appearance.

Metamorphism in Subduction Zones 

Blueschist is a relatively rare rock that contains an unusual blue-coloured amphibole. Laboratory experiments indicate that formation of this mineral requires very high pressure but relatively low temperature. Such conditions do not develop in continental crust usually, at the high pressure needed to produce blue amphibole, temperature in continental crust is also high. So to figure out where blueschist forms, we must determine where high pressure can develop at relatively low temperature.

Plate tectonics theory provides the answer to this puzzle. Researchers found that blueschist occurs only in the accretionary prisms that form at subduction zones. They realized that because prisms grow to be over 20 km thick, rock at the base of the prism feels high pressure (due to the weight of overburden). But because the subducted oceanic lithosphere beneath the prism is cool, temperatures at the base of the prism remain relatively low.

Shock Metamorphism 

When large meteorites slam into the Earth, a vast amount of kinetic energy instantly transforms into heat, and a pulse of extreme compression (a shock wave) propagates into the Earth. The heat may be sufficient to melt or even vaporize rock at the impact site, and the extreme compression of the shock wave causes quartz in rocks below the impact site to undergo a phase change and become a more compact mineral called coesite. The changes in rock due to the passage of a shock wave are called shock metamorphism.

Where Do You Find Metamorphic Rocks? 

When you stand on an outcrop of metamorphic rock, you are standing on material that once lay many kilometers beneath the surface of the Earth. How does metamorphic rock return to the Earth’s surface? Geologists refer to the overall process by which deeply buried rocks end up back at the surface as exhumation. 

Processes that bring metamorphic rock back to Earth’s surface. Three phenomena contribute to exhumation of rocks at depth. Here, the red dot (representing metamorphic rocks formed at the base of a mountain range) gets progressively closer to the surface over time.

To see how exhumation works, let’s look at the specific processes that contribute to bringing high-grade metamorphic rocks from below a collisional mountain range back to the surface (figure above). First, as two continents progressively push together, the rock caught between them squeezes upward, much like dough pressed in a vise; the upward movement takes place by slip on faults and by plastic-like flow of rock. Second, as the mountain range grows, the crust at depth beneath it warms up and becomes softer and weaker. Eventually, the range starts to collapse under its own weight, much like a block of soft cheese placed in the hot sun. As a result of this collapse, the upper crust spreads out laterally. Horizontal stretching of the upper part of the crust causes it to become thinner in the vertical direction, and as the upper part of the crust becomes thinner, the deeper crust ends up closer to the surface. Third, erosion takes place at the surface; weathering, landslides, river flow, and glacial flow together play the role of a giant rasp, stripping away rock at the surface and exposing rock that was once below the surface. 

Examples of rock exposures consisting of Precambrian metamorphic rocks.

Keeping in mind the processes that form metamorphic rock and cause exhumation, let’s ask the question, “Where are metamorphic rocks presently exposed?” You can start your quest to find metamorphic rock outcrops by hiking into a mountain range. As we've seen, the process of mountain building produces and eventually exhumes metamorphic rocks. The towering cliffs in the interior of a mountain range typically reveal schist, gneiss, and quartzite (figure above a). Even after the peaks have eroded away, the record of mountain building remains in the form of a belt of metamorphic rock at the ground surface.

Vast expanses of metamorphic rock crop out in continental shields. A shield is a broad region of long-lived, stable continental crust where Phanerozoic sedimentary cover either was not deposited or has been eroded away so that Precambrian rocks are exposed (figure above b, c). These rocks were metamorphosed during a succession of Precambrian mountain-building events that led to the original growth of continents.

Credits: Stephen Marshak (Essentials of Geology)

Types of Metamorphic Rocks

Types of Metamorphic Rocks

kacimo samy   8:41 ص

Types of Metamorphic Rocks 

Coming up with a way to classify and name the great variety of metamorphic rocks on Earth hasn't been easy. After decades of debate, geologists have found it most convenient to divide metamorphic rocks into two fundamental classes: foliated rocks and non-foliated rocks. Each class contains several rock types. We distinguish foliated rocks from each other partly by their component minerals and partly by the nature of their foliation, whereas we distinguish non-foliated rocks from each other primarily by their component minerals. 

Foliated Metamorphic Rocks 

To understand this class of rocks, we first need to discuss the nature of foliation in more detail. The word comes from the Latin folium, for leaf. Geologists use foliation to refer to the parallel surfaces and/or layers that can occur in a metamorphic rock. Foliation can give metamorphic rocks a striped or streaked appearance in an outcrop, and/or can give them the ability to split into thin sheets. A foliated metamorphic rock has foliation either because it contains inequant mineral crystals that are aligned parallel to one another, defining preferred mineral orientation, and/or because the rock has alternating dark-coloured and light-coloured layers.

Foliated metamorphic rocks can be distinguished from one another according to their composition, their grain size, and the nature of their foliation. The most common types include 

Slate is a foliated metamorphic rock that forms at relatively low temperature and pressure.

• Slate: The finest-grained foliated metamorphic rock, slate, forms by metamorphism of shale or mudstone (rocks composed dominantly of clay) under relatively low pressures and temperatures. Slate contains a type of foliation called slaty cleavage, which allows it to split into thin sheets that make excellent roofing shingles (figure above a). Slaty cleavage develops when pressure solution removes portions of clay flakes that are not perpendicular to the compression direction, while clay flakes that are perpendicular to the compression direction grow. During the process, some flakes passively rotate into parallelism with the cleavage plane, pushed into the new orientation by compression. For example, end-on compression of a sequence of horizontal shale beds produces vertical slaty cleavage (figure above b). Commonly, such compression also causes the layers to bend into curves called folds. 

Examples of foliated metamorphic rocks formed at high temperatures and pressures.

• Phyllite: Phyllite is a fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine grained white mica. The word comes from the Greek word phyllon, meaning leaf, as does the word phyllo, the flaky dough in Greek pastry. The parallelism of translucent fine-grained mica gives phyllite a silky sheen known as phyllitic luster (figure above a). Phyllite forms by the metamorphism of slate at a temperature high enough to cause neocrystallization of white mica. 
• Metaconglomerate: Under the metamorphic conditions that produce slate or phyllite, a protolith of conglomerate becomes metaconglomerate. Specifically, pressure solution and plastic deformation flatten pebbles and cobbles into pancake-like shapes. The alignment of inequant clasts defines a foliation (figure above b). 
• Schist: Schist is a medium- to coarse-grained metamorphic rock that possesses a type of foliation, called schistosity, defined by the preferred orientation of large mica flakes (muscovite and/or biotite; figure above c). Schist forms at a higher temperature than does phyllite. 

The formation of gneiss, which takes place at very high temperatures and pressures.

• Gneiss: Gneiss is a compositionally layered metamorphic rock, typically composed of alternating dark-coloured and light-coloured layers that range in thickness from millimetres to meters. This compositional layering, or gneissic banding, gives gneiss a striped appearance (figure above a). How does the banding in gneiss form? Some evolved directly from the original bedding in a rock. For example, metamorphism of a protolith consisting of alternating beds of sandstone and shale produces a gneiss consisting of alternating beds of quartzite and mica. Gneissic banding can also form when the protolith undergoes an extreme amount of shearing under conditions in which the rock can flow like soft plastic  (figure above b). Such flow stretches, folds, and smears out pre-existing compositional contrasts in the rock and transforms them into aligned sheets. Finally, banding in some gneisses can develop by an incompletely understood process called metamorphic differentiation. During differentiation, chemical reactions segregate different minerals into different layers (figure above c). 
• Migmatite: Under certain conditions, gneiss may begin to melt, producing felsic magma and residual, still solid, mafic rock. If the melt freezes again before flowing out of the source area, a mixture of igneous rock and relict metamorphic rock forms. This mixture is called migmatite. In effect, a migmatite is part metamorphic and part igneous.

Nonfoliated Metamorphic Rocks 

Nonfoliated metamorphic rocks contain minerals that recrystallized or grew during metamorphism, but have no foliation. The lack of foliation means either that metamorphism occurred in the absence of compression and shear, or that most of the new crystals can only grow in an equant form. We list below some of the rock types that can occur without foliation.

Examples of quartzite and marble typically, but not always, these are non-foliated.

• Hornfels: Hornfels is a fine-grained nonfoliated rock that contains a variety of metamorphic minerals. The specific mineral assemblage in a hornfels depends on the composition of the protolith and on the temperature and pressure of metamorphism. 
• Quartzite: Quartzite forms by the metamorphism of pure quartz sandstone. During metamorphism, pre-existing quartz grains recrystallize, creating new, larger grains. In the process, the distinction between cement and grains disappears, open pore space disappears, and the grains become interlocking. When quartzite cracks, the fracture cuts across grain boundaries in contrast, fractures in sandstone curve around grains. Quartzite looks glassier than sandstone and does not have the grainy, sandpaper-like surface characteristic of sandstone (figure above a). Depending on the impurities it contains, quartzite can vary in colour from white to gray, purple, or green. 
• Marble: The metamorphism of limestone yields marble. During the formation of marble, calcite composing the protolith recrystallizes, so fossil shells, pore space, and the distinction between grains and cement disappear. Thus, marble typically consists of a fairly uniform mass of interlocking calcite crystals. 

Sculptors love to work with marble because the rock is relatively soft and has a uniform texture that gives it the cohesiveness and homogeneity needed to fashion large, smooth, highly detailed sculptures. Marble comes in a variety of colours white, pink, green, and black depending on the impurities it contains. Michelangelo, one of the great  Italian Renaissance artists, sought large, unbroken blocks of creamy white marble from quarries in the Italian Alps for his masterpieces (figure above b). 

Not all marble is non-foliated. If the original protolith contained layers with different impurities, and shear caused the marble to flow plastically, the resulting marble has colour banding that makes it a prized decorative stone (figure above c).

Defining Metamorphic Intensity 

Intensity of metamorphism is indicated by metamorphic grade

Not all metamorphism takes place under the same physical conditions. For example, rocks carried to a great depth beneath a mountain range undergo more intense metamorphism than do rocks closer to the surface. Geologists use the term metamorphic grade in a somewhat informal way to indicate the intensity of metamorphism, meaning the amount or degree of metamorphic change. (To provide a more complete indication of the intensity of metamorphism, geologists use the concept of metamorphic facies; see Metamorphic Facies) Classification of metamorphic grade depends primarily on temperature, because temperature plays the dominant role in determining the extent of recrystallization and neocrystallization during metamorphism. Metamorphic rocks that form at relatively low temperatures (between about 250C and 400C) are lowgrade rocks, and metamorphic rocks that form at relatively high temperatures (over about 600C) are high-grade rocks.  Intermediate-grade rocks form at temperatures between these two extremes (figure above a). 

Different grades of metamorphism yield different metamorphic mineral assemblages. As grade increases, recrystallization and neocrystallization tend to produce coarser grains and new mineral assemblages that are stable at higher temperatures and pressures (figure above b). 

Geologists discovered that the presence of certain minerals, known as index minerals, in a rock indicates the approximate metamorphic grade of the rock. The line on a map along which an index mineral first appears is called an isograd (from the Greek iso, meaning equal). All points along an isograd have approximately the same metamorphic grade. Metamorphic zones are regions between two isograds; zones are named after an index mineral that was not present in the previous, lower-grade zone. To compare rocks of different grades, you could take a hike from central New York State eastward into central Massachusetts in the eastern United States. Your path starts in a region where rocks were not metamorphosed, and it takes you into the internal part of the Appalachian Mountain belt, where rocks were intensely metamorphosed. As a consequence, you cross several metamorphic zones (figure above c).

Metamorphic Facies

In the early years of the 20th century, geologists working in Scandinavia, where erosion by glaciers has left beautiful, nearly unweathered exposures of rocks once buried very deeply in the crust, came to realize that metamorphic rocks, in general, do not consist of a hodgepodge of minerals formed at different times and in different places, but rather consist of a distinct set of minerals that grew in association with each other at a certain pressure and temperature. It seemed that such mineral assemblages more or less represent a condition of chemical equilibrium, meaning that the chemicals making up the rock had organized into a group of mineral grains that were to anthropomorphize a bit comfortable with each other and their surroundings, and thus did not feel the need to change further. The geologists also determined that the specific mineral assemblage in a rock depends on pressure and temperature conditions, and on the composition of the protolith. 

This discovery led the geologists to  propose the concept of metamorphic facies. A metamorphic facies is a set of metamorphic mineral assemblages indicative of a certain range of pressure and temperature. Each specific assemblage in a facies reflects the original protolith composition. According to this definition, a given metamorphic facies includes several different kinds of rocks that differ from each other in terms of chemical composition and, therefore, mineral content but all the rocks of a given facies formed under roughly the same temperature and pressure conditions. Geologists recognize several facies, of which the major ones are zeolite, hornfels, greenschist, amphibolite, blueschist, eclogite, and granulite. The names of the different facies are based on a distinctive feature or mineral found in some of the rocks of the facies. 

The common metamorphic facies. The boundaries between the facies are depicted as wide bands because they are gradational and approximate. Note that some amphibolite-facies rocks and all granulite-facies rocks form only if the protolith is dry. The relatively rare P-P (”prehnite-pumpellyite”) facies, is named for two metamorphic minerals.

We can represent the approximate conditions under which metamorphic facies formed by using a pressure temperature graph (figure above). Each area on the graph, labeled with a facies name, represents the approximate range of temperatures and pressures in which mineral assemblages characteristic of that particular facies form. For example, a rock subjected to the pressure and temperature at Point A (4.5 kbar and 400C) develops a mineral assemblage characteristic of the greenschist facies. As the graph implies, the pressure and temperature conditions defining boundaries between facies cannot be precisely determined, and the transitions between facies are gradual.

We can also portray the geothermal gradients of different crustal regions on the graph. Beneath mountain ranges, for example, the geothermal gradient passes through the zeolite, greenschist, amphibolite, and granulite facies. In contrast, in the accretionary prism that forms at a subduction zone, temperature increases slowly with increasing depth, so blueschist assemblages can form.

Credit: Stephen Marshak (Essentials of Geology)

Consequences and Causes of Metamorphism

Consequences and Causes of Metamorphism

kacimo samy   10:24 ص

What Is a Metamorphic Rock? 

If someone were to put a rock on a table in front of you, how would you know that it is metamorphic? First, metamorphic rocks can possess metamorphic minerals, new minerals that grow in place within the solid rock only under metamorphic temperatures and pressures. In fact, metamorphism can produce a group of minerals that together make up what geologists call a “metamorphic mineral assemblage.” And second, metamorphic rocks can have metamorphic texture defined by distinctive arrangements of mineral grains not found in other rock types. Commonly, the texture results in metamorphic foliation, due to the parallel alignment of platy minerals (such as mica) and/ or the presence of alternating light-coloured and dark-coloured layers. When metamorphic minerals and/or textures develop, a metamorphic rock becomes as different from its protolith as a butterfly is from a caterpillar. For example, metamorphism of red shale can yield a metamorphic rock consisting of aligned mica flakes and brilliant garnet crystals (a in figure above), and metamorphism of a limestone composed of cemented-together fossil fragments can yield a metamorphic rock consisting of large interlocking crystals of calcite (b in figure above). The process of forming metamorphic minerals and textures takes place very slowly it may take thousands to millions of years and it involves several processes, which sometimes occur alone and sometimes together. The most common processes are: 

• Recrystallization, which changes the shape and size of grains without changing the identity of the mineral making up the grains (a in figure above). 
• Phase change, which transforms one mineral into another mineral with the same composition but a different crystal structure. On an atomic scale, phase change involves the rearrangement of atoms. 
• Metamorphic reaction, or neocrystallization (from the Greek neos, for new), which results in growth of new mineral crystals that differ from those of the protolith (b in figure above). During neocrystallization, chemical reactions digest minerals of the protolith to produce new minerals of the metamorphic rock. 
• Pressure solution, which happens when a wet rock is squeezed more strongly in one direction than in others. Mineral grains dissolve where their surfaces are pressed against other grains, producing ions that migrate through the water to precipitate elsewhere (c in figure above). 
• Plastic deformation, which happens when a rock is squeezed or sheared at elevated temperatures and pressures. Under these conditions, grains behave like soft plastic and change shape without breaking (d in figure above). 

Caterpillars undergo metamorphosis because of hormonal changes in their bodies. Rocks undergo metamorphism when they are subjected to heat, pressure, compression and shear, and/or very hot water. Let’s now consider the details of how these agents of metamorphism operate.

Metamorphism Due to Heating 

When you heat cake batter, the batter transforms into a new material cake. Similarly, when you heat a rock, its ingredients transform into a new material metamorphic rock. Why? Think about what happens to atoms in a mineral grain as the grain warms. Heat causes the atoms to vibrate rapidly, stretching and bending chemical bonds that lock atoms to their neighbours. If bonds stretch too far and break, atoms detach from their original neighbours, move slightly, and form new bonds with other atoms. Repetition of this process leads to rearrangement of atoms within grains, or to migration of atoms into and out of grains, a process called solid-state diffusion. As a consequence, recrystallization and/or neo-crystallization take place, enabling a metamorphic mineral  assemblage to grow in solid rock. Metamorphism takes place at temperatures between those at which diagenesis occurs and those that cause melting. Roughly speaking, this means that most metamorphic rocks you find in outcrops on continents formed at temperatures of between 250C and 850C.

Metamorphism Due to Pressure 

As you swim underwater in a swimming pool, water squeezes against you equally from all sides in other words, your body feels pressure. Pressure can cause a material to collapse inward. For example, if you pull an air-filled balloon down to a depth of 10 m in a lake, the balloon becomes significantly smaller. Pressure can have the same effect on minerals. Near the Earth’s surface, minerals with relatively open crystal structures can be stable. However, if you subject these minerals to extreme pressure, the atoms pack more closely together and denser minerals tend to form. Such transformations involve phase changes and/or neo-crystallization.

Changing Both Pressure and Temperature 

So far, we've considered changes in pressure and temperature as separate phenomena. But in the Earth, pressure and temperature change together with increasing depth. For example, at a depth of 8 km, temperature in the crust reaches about 200C and pressure reaches about 2.3 kbar. If a rock slowly becomes buried to a depth of 20 km, as can happen during mountain building, temperature in the rock increases to more than 500C, and pressure to 5.5 kbar. Experiments and calculations show that the “stability” of certain minerals (the ability of a mineral to form and survive) depends on both pressure and temperature. When pressure and temperature increase, the original mineral assemblage in a rock becomes unstable, and a new assemblage forms out of minerals that are stable. Thus, a metamorphic rock formed at 8 km does not contain the same minerals as one formed at 20 km.

Compression, Shear, and Development  of Preferred Orientation 

Imagine that you have just built a house of cards and, being in a destructive mood, you step on it. The structure collapses because the downward push you apply with your foot exceeds the push provided by air in other directions. We can say that we have subjected the cards to compression (a in figure above). Compression flattens a material (b in figure above). Shear, in contrast, moves one part of a material sideways, relative to another. If, for example, you place a deck of cards on a table, then set your hand on top of the deck and move your hand parallel to the table, you shear the deck (c in figure above). When rocks are subjected to compression and shear at elevated temperatures and pressures, they can change shape without breaking. As it changes shape, the internal texture of a rock also changes. For example, platy (pancake-shaped) grains become parallel to one another, and elongate (cigar shaped) grains align in the same direction. Both platy and elongate grains are inequant grains, meaning that the dimension of a grain is not the same in all directions; in contrast, equant grains have roughly the same dimensions in all directions (d in figure above). The alignment of inequant minerals in a rock results in a preferred orientation (e in figure above).

The Role of Hydrothermal Fluids 

Metamorphic reactions commonly take place in the presence of hydrothermal fluids (very hot-water solutions). Where does the water in hydrothermal fluids come from? Some of it was originally bonded to minerals in the protolith, for metamorphic reactions can release such water into its surroundings. Some of it may seep up into the protolith from a nearby igneous intrusion, or down from overlying groundwater reservoirs. Notably, under extremely high pressures and temperatures, the water of hydrothermal fluids is in neither gas nor liquid state, but rather is in a “supercritical” state, meaning that it has characteristics of both gas and liquid. Such hydrothermal fluids chemically react with rock; they accelerate metamorphic reactions, because atoms involved in the reactions can migrate faster through a fluid than they can through a solid, and hydrothermal fluids provide water that can be absorbed by minerals during metamorphic reactions. Finally, fluids passing through a rock may pick up some dissolved ions and drop off others, as a bus picks up and drops off passengers, and thus can change the overall chemical composition of a rock during metamorphism. The process of changing a rock’s chemical composition by reactions with hydrothermal fluids is called metasomatism.

Metamorphic rocks

Metamorphic rocks

kacimo samy   2:38 ص

Metamorphic rocks are formed by the heating of pre-existing rocks. The heat provide to a rock changes it mineralogical and physical changes which are called metamorphic rocks.

These rocks forms mostly where magma chamber is available to heat enough for mineralogical changes occurrence. These rocks have multiple features in distinguishing like schistosic, gneissic and slaty texures.

Texture is the physical character or a pattern.

Slaty texture

Slate are formed by the metamorphism of shale. Sheets are formed in the slates where it can broke  into sheets. This help in determining the slates.

Schistose

Schistose is formed after metamorphism of slate where it rearrange in forming irregular sheets like character which is schistosic texture.

Gneissic

Gneiss is the high grade metamorphic rock of shale which is distinguished by regular interval of dark and light bands present in it. These are called gneissic bands which is the recognizable character.

Metamorphism and its changes are following

Shale- Slate- Schist - Gneiss

Shale is sedimentary rock when metamorphose produces slate and slate to schist and schist to gneiss with increasing metamorphic grades. In the same way marble is produced by the metamorphism of limestone.