How can rock get metamorphosed

Today it is not very advantageous to use this rock because of its weight and the splitting and cracking over time. Schist is a medium grade metamorphic rock. This means that it has been subjected to more heat and pressure than slate, which is a low grade metamorphic rock. As you can see in the photo above schist is a more coarse grained rock. The individual grains of minerals can be seen by the naked eye. Many of the original minerals have been altered into flakes. Because it has been squeezed harder than slate it is often found folded and crumpled.

Schists are usually named by the main minerals that they are formed from. Bitotite mica schist, hornblende schist, garnet mica schist, and talc schist are some examples of this. Gneiss is a high grade metamorphic rock. This means that gneiss has been subjected to more heat and pressure than schist. Gneiss is coarser than schist and has distinct banding. This banding has alternating layers that are composed of different minerals.

The minerals that compose gneiss are the same as granite. Feldspar is the most important mineral that makes up gneiss along with mica and quartz. Gneiss can be formed from a sedimentary rock such as sandstone or shale, or it can be formed from the metamorphism of the igneouse rock grantite.

Gneiss can be used by man as paving and building stone. Non-Foliates are metamorphic rocks that have no cleavage at all. Quartzite and marble are two examples of non-foliates that we are going to study.

Quartzite is composed of sandstone that has been metamorphosed. Quartzite is much harder than the parent rock sandstone. It forms from sandstone that has come into contact with deeply buried magmas. Quartzite looks similar to its parent rock. The best way to tell quartzite from sandstone is to break the rocks. Sandstone will shatter into many individual grains of sand while quartzite will break across the grains. Marble is metamorphosed limestone or dolomite.

Both limestone and dolomite have a large concentration of calcium carbonate CaCO3. Marble has many different sizes of crystals. It is defined as the force per unit area acting on the surface, in a direction perpendicular to the surface.

Lithostatic pressure is the pressure exerted on a rock by all the surrounding rock. The source of the pressure is the weight of all the rocks above. Lithostatic pressure increases as depth within the Earth increases and is a uniform stress— the pressure applies equally in all directions on the rock. If pressure does not apply equally in all directions, differential stress occurs. There are two types of differential stress. Normal stress compresses pushes together rock in one direction, the direction of maximum stress.

At the same time, in a perpendicular direction, the rock undergoes tension stretching , in the direction of minimum stress.

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. Differential stress has a major influence on the the appearance of a metamorphic rock. Differential stress can flatten pre-existing grains in the rock, as shown in the diagram below.

Metamorphic minerals that grow under differential stress will have a preferred orientation if the minerals have atomic structures that tend to make them form either flat or elongate crystals. This will be especially apparent for micas or other sheet silicates that grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. 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.

This results in a rock that can be easily broken along the parallel mineral sheets. Such a rock is said to be foliated, or to have foliation. Any open space between the mineral grains in a rock, however microscopic, may contain a fluid phase. 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.

Less commonly, it may be a carbon dioxide fluid or some other fluid. 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. The fluid phase can also influence the rate at which mineral crystals deform or change shape. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase.

If during metamorphism enough ions are introduced to or removed from the rock via the fluid to change the bulk chemical composition of the rock, the rock is said to have undergone metasomatism.

However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. Most metamorphism of rocks takes place slowly inside the Earth. Regional metamorphism takes place on a timescale of millions of years. Metamorphism usually involves slow changes to rocks in the solid state, as atoms or ions diffuse out of unstable minerals that are breaking down in the given pressure and temperature conditions and migrate into new minerals that are stable in those conditions.

This type of chemical reaction takes a long time. Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism.

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. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure.

Examples of low grade hydrous minerals include clay, serpentine, and chlorite. Under low grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope. Low grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may grow.

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. Micas tend to break down. New minerals such as hornblende will form, which is stable at higher temperatures. However, as metamorphic grade increases to even higher grade, all hydrous minerals, which includes hornblende, may break down and be replaced by other, higher-temperature, non-hydrous minerals such as pyroxene.

Index minerals, which are indicators of metamorphic grade. 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. 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.

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. 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.

Experiments suggest that the time involved is tens of millions of years. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. Low-grade metamorphism takes place at temperatures between about to o C, and relatively low pressure. Low grade metamorphic rocks are characterized by an abundance of hydrous minerals minerals that contain water, H 2 O, in their crystal structure.

Examples of hydrous minerals that occur in low grade metamorphic rocks: Clay Minerals Serpentine Chlorite High-grade metamorphism takes place at temperatures greater than o C and relatively high pressure. As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H 2 O and non-hydrous minerals become more common. Examples of less hydrous minerals and non-hydrous minerals that characterize high grade metamorphic rocks: Muscovite - hydrous mineral that eventually disappears at the highest grade of metamorphism Biotite - a hydrous mineral that is stable to very high grades of metamorphism.

Pyroxene - a non hydrous mineral. Garnet - a non hydrous mineral. Retrograde Metamorphism As temperature and pressure fall due to erosion of overlying rock or due to tectonic uplift, one might expect metamorphism to a follow a reverse path and eventually return the rocks to their original unmetamorphosed state.

Metamorphic Rock Types There are two major subdivisions of metamorphic rocks. Non-foliated Metamorphic Rocks Non-foliated rocks lack a planar fabric. Absence of foliation possible for several reasons: Rock not subjected to differential stress.

Dominance of equant minerals like quartz, feldspar, and garnet. Absence of platy minerals sheet silicates. Protolith Composition Although textures and structures of the protolith are usually destroyed by metamorphism, we can still get an idea about the original rock from the minerals present in the metamorphic rock. General terms used to describe the chemical composition of both the protolith and the resulting metamorphic rock are: Pelitic Alumina rich rocks, usually shales or mudstones.

Types of Metamorphism Metamorphism can take place in several different environments where special conditions exist in terms of pressure, temperature, stress, conditions, or chemical environments. Contact Metamorphism also called thermal metamorphism - Occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion.

Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to a zone surrounding the intrusion, called a metamorphic aureole.

Outside of the contact aureole, the rocks are unmetamorphosed. The grade of metamorphism increases in all directions toward the intrusion.

Because temperature differences between the surrounding rock and the intruded magma are larger at shallow levels in the crust, contact metamorphism is usually referred to as high temperature, low pressure metamorphism. The rock produced is often a fine-grained rock that shows no foliation, called a hornfels. Burial Metamorphism - When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than o C may develop in the absence of differential stress.

New minerals grow, but the rock does not appear to be metamorphosed. The main minerals produced are the Zeolites. Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase. Dynamic Metamorphism - This type of metamorphism is due to mechanical deformation, like when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along the zone, and the rocks tend to crushed and pulverized due to the sliding.

Dynamic metamorphism is not very common and is restricted to a narrow zone along which the sliding occurred. The rock that is produced is called a mylonite. Regional Metamorphism - This type of metamorphism occurs over large areas that were subjected to high degrees of deformation under differential stress. Thus, it usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gneisses.

The differential stress usually results from tectonic forces that produce a compression of the rocks, such as when two continental masses collide with one another. Thus, regionally metamorphosed rocks occur in the cores of mountain ranges or in eroded mountain ranges. Compressive stresses result in folding of the rock, as shown here, and results in thickening of the crust which tends to push rocks down to deeper levels where they are subjected to higher temperatures and pressures See Figure 8.

Compressional stresses acting in the subduction zone create the differential stress necessary to form schists and thus the resulting metamorphic rocks are called blueschist Shock Metamorphism - When a large meteorite collides with the Earth, the kinetic energy is converted to heat and a high pressure shock wave that propagates into the rock at the impact site.

Metamorphic Facies In general, metamorphic rocks do not undergo significant changes in chemical composition during metamorphism. If a low geothermal gradient was present, such the one labeled "C" in the diagram, then rocks would progress from zeolite facies to blueschist facies to eclogite facies. Thus, if we know the facies of metamorphic rocks in the region, we can determine what the geothermal gradient must have been like at the time the metamorphism occurred.

The Rock Cycle Before moving on to the rest of the course, you should read Interlude C in your textbook pages The rock cycle involves cycling of elements between various types of rocks, and thus mostly involves the lithosphere. The rock cycle involves the three types of rocks as reservoirs 1 igneous, 2 sedimentary, and 3 metamorphic. Chemical elements can reside in each type of rock, and geologic processes move these elements into another type of rock.

Energy for the parts of the crustal cycle near the Earth's surface is solar and gravitational energy which control erosion and weathering , whereas energy that drives processes beneath the surface is geothermal and gravitational energy which control uplift, subsidence, melting, and metamorphism. Questions on this material that might be asked on an exam Define the following: a geothermal gradient, b metamorphism, c differential stress, d prograde metamorphism, e metasomatism f protolith, g foliation, i metamorphic aureole, j isograd, k greenstone, l blueschist.

Starting with a shale, describe the textural changes that would occur to the rock during prograde metamorphism with differential stress conditions present. Why is retrograde metamorphism uncommon? What is polymorphism among minerals? Polymorphism is minerals with the same chemical formula having different crystal structures therefore being different minerals.

Index minerals are special minerals that only form at certain temperatures and pressures and therefore can be used to identify the degree of metamorphism to which the rocks have been exposed. As with igneous processes, metamorphic rocks form at different zones of pressure depth and temperature as shown on the pressure- temperature P-T diagram.

The term facies is an objective description of a rock. In metamorphic rocks facies are groups of minerals called mineral assemblages. The names of metamorphic facies on the pressure- temperature diagram reflect minerals and mineral assemblages that are stable at these pressures and temperatures and provide information about the metamorphic processes that have affected the rocks. This is useful when interpreting the history of a metamorphic rock. In the late s, British geologist George Barrow mapped zones of index minerals in different metamorphic zones of an area that underwent regional metamorphism.

The first of the Barrovian sequence has a mineral group that is commonly found in the metamorphic greenschist facies. Greenschist rocks form under relatively low pressure and temperatures and represent the fringes of regional metamorphism. Many different styles of metamorphic facies are recognized, tied to different geologic and tectonic processes.

Recognizing these facies is the most direct way to interpret the metamorphic history of a rock. A simplified list of major metamorphic facies is given below. Burial metamorphism occurs when rocks are deeply buried, at depths of more than meters 1.

Burial metamorphism commonly occurs in sedimentary basins , where rocks are buried deeply by overlying sediments. As an extension of diagenesis , a process that occurs during lithification Chapter 5 , burial metamorphism can cause clay minerals , such as smectite, in shales to change to another clay mineral illite. Or it can cause quartz sandstone to metamorphose into the quartzite such the Big Cottonwood Formation in the Wasatch Range of Utah.

This formation was deposited as ancient near- shore sands in the late Proterozoic see Chapter 7 , deeply buried and metamorphosed to quartzite , folded, and later exposed at the surface in the Wasatch Range today. Increase of temperature with depth in combination with an increase of confining pressure produces low- grade metamorphic rocks with a mineral assemblages indicative of a zeolite facies.

Contact metamorphism occurs in rock exposed to high temperature and low pressure, as might happen when hot magma intrudes into or lava Liquid rock on the surface of the Earth. This combination of high temperature and low pressure produces numerous metamorphic facies.

The lowest pressure conditions produce hornfels facies , while higher pressure creates greenschist, amphibolite, or granulite facies. As with all metamorphic rock , the parent rock texture and chemistry are major factors in determining the final outcome of the metamorphic process, including what index minerals are present.

Fine-grained shale and basalt , which happen to be chemically similar, characteristically recrystallize to produce hornfels. Sandstone silica surrounding an igneous intrusion becomes quartzite via contact metamorphism , and limestone carbonate becomes marble. Contact metamorphism in outcrop. When contact metamorphism occurs deeper in the Earth, metamorphism can be seen as rings of facies around the intrusion, resulting in aureoles. These differences in metamorphism appear as distinct bands surrounding the intrusion, as can be seen around the Alta Stock in Little Cottonwood Canyon, Utah.

The Alta Stock is a granite intrusion surrounded first by rings of the index minerals amphibole tremolite and olivine forsterite , with a ring of talc dolostone located further away. Regional metamorphism occurs when parent rock is subjected to increased temperature and pressure over a large area, and is often located in mountain ranges created by converging continental crustal plates. This is the setting for the Barrovian sequence of rock facies , with the lowest grade of metamorphism occurring on the flanks of the mountains and highest grade near the core The innermost chemical layer of the Earth, made chiefly of iron and nickel.

It has both liquid and solid components. An example of an old regional metamorphic environment is visible in the northern Appalachian Mountains while driving east from New York state through Vermont and into New Hampshire. Along this route the degree of metamorphism gradually increases from sedimentary parent rock , to low- grade metamorphic rock , then higher- grade metamorphic rock , and eventually the igneous core The innermost chemical layer of the Earth, made chiefly of iron and nickel.

The rock sequence is sedimentary rock , slate , phyllite , schist , gneiss , migmatite , and granite. In fact, New Hampshire is nicknamed the Granite State. The reverse sequence can be seen heading east, from eastern New Hampshire to the coast. Subduction zone metamorphism is a type of regional metamorphism that occurs when a slab Name given to the subducting plate, where volatiles are driven out at depth, causing volcanism. Because rock is a good insulator, the temperature of the descending oceanic slab Name given to the subducting plate, where volatiles are driven out at depth, causing volcanism.

Glaucophane, which has a distinctive blue color, is an index mineral found in blueschist facies see metamorphic facies diagram. The California Coast Range near San Francisco has blueschist - facies rocks created by subduction -zone metamorphism , which include rocks made of blueschist , greenstone, and red chert.

Greenstone, which is metamorphized basalt , gets its color from the index mineral chlorite. There are a range of metamorphic rocks made along faults. Near the surface, rocks are involved in repeated brittle faulting produce a material called rock flour, which is rock ground up to the particle size of flour used for food.

At lower depths, faulting create cataclastites , chaotically-crushed mixes of rock material with little internal texture. At depths below cataclasites , where strain becomes ductile , mylonites are formed.

Mylonites are metamorphic rocks created by dynamic recrystallization through directed shear forces , generally resulting in a reduction of grain size. When larger, stronger crystals like feldspar , quartz , garnet embedded in a metamorphic matrix are sheared into an asymmetrical eye-shaped crystal, an augen is formed.

Shock lamellae in a quartz grain. Shock also known as impact metamorphism is metamorphism resulting from meteor or other bolide impacts, or from a similar high-pressure shock event. Shock metamorphism is the result of very high pressures and higher, but less extreme temperatures delivered relatively rapidly. Shock metamorphism produces planar deformation features, tektites, shatter cones, and quartz polymorphs.

Shock metamorphism produces planar deformation features shock laminae , which are narrow planes of glassy material with distinct orientations found in silicate mineral grains.

Shocked quartz has planar deformation features. Shatter cone. Shatter cones are cone-shaped pieces of rock created by dynamic branching fractures caused by impacts.

While not strictly a metamorphic structure, they are common around shock metamorphism. Their diameter can range from microscopic to several meters. Fine-grained rocks with shatter cones show a distinctive horsetail pattern. Shock metamorphism can also produce index minerals , though they are typically only found via microscopic analysis.

The quartz polymorphs coesite and stishovite are indicative of impact metamorphism. As discussed in chapter 3, polymorphs are minerals with the same composition but different crystal structures. Tektites Shock metamorphism can also produce glass. Tektites are gravel-size glass grains ejected during an impact event. They resemble volcanic glass but, unlike volcanic glass, tektites contain no water or phenocrysts , and have a different bulk and isotopic chemistry. Tektites contain partially melted inclusions of shocked mineral grains.

Although all are melt glasses, tektites are also chemically distinct from trinitite, which is produced from thermonuclear detonations , and fulgurites, which are produced by lightning strikes. All geologic glasses not derived from volcanoes can be called with the general term pseudotachylytes , a name which can also be applied to glasses created by faulting.

Metamorphic facies are characterized by rock properties or assemblages groups of index minerals. Which metamorphic facies is associated with subduction zones? By analyzing facies on the Metamorphic PT diagram, blueschist is shown on the left at a low temperature but at a high pressure. The PT diagram indicates that high pressure and low temperature minerals are found at subduction zones.

The core The innermost chemical layer of the Earth, made chiefly of iron and nickel. Regional metamorphism occurs when temperatures and pressures are exerted on a rock over a large geographic area.

This is often associated with mountain belts from converging continental tectonic plates. Increasing metamorphic grade can be observed as one travels from the edge of a mountain belt into its high- grade core The innermost chemical layer of the Earth, made chiefly of iron and nickel. Barrow noticed and described the metamorphic sequence across a mountain belt showing regional metamorphism , now concluded to represent continental collision.

When magma intrudes pre-existing country rock , the rock will be cooked by the magma.