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. When magma intrudes pre-existing country rock , the rock will be cooked by the magma.
Contact metamorphism occurs when rocks are exposed to high temperatures and low pressures. For example, when a magma intrudes into pre-existing rocks, or a lava Liquid rock on the surface of the Earth. Barrow noticed and described the metamorphic sequence across a mountain belt showing regional metamorphism , now concluded to represent continental collision.
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. Metamorphism is the process that changes existing rocks called protoliths into new rocks with new minerals and new textures. The geothermal gradient is a measure of the rate in increase in temperature with depth in the Earth's crust. Chemically Reactive Fluids - some of the dissolved constituents of rock move from the fluid to form new minerals.
Differential stress - Plate tectonic forces cause larger stress in one direction than in other directions. This difference in stress is responsible for the deformed nature of metamorphic rocks.
Metamorphic Textures - most metamorphic rocks develop a fabric which is characterized by planar surfaces composed of clay or mica flakes. The orientation of the planar surfaces is controlled by the stress to which the metamorphic rock is subjected.
Foliation - parallel flakes of mica grown during metamorphic reactions produce a planar fabric called foliation. Slaty Cleavage - a pervasive foliation formed within a fine-grained rock such as shale. In these low-grade metamorphic rocks the mica flakes are microscopic.
Slate which is metamorphosed shale is often used as roofing tiles. Schistosity - the foliation that develops when mica grains grow to be visible as the temperature increases. The mica is still pervasive and mixed evenly between quartz and feldspar grains in a rock called a schist. Gneiss - a rock with foliation but in which quartz and feldspar become segregated between layers of mica. Fault zone metamorphism - when rock is subject to great differential stress grains are crushed and shattered, particular along deep fault zones.
The weight of your body applies a differential stress to the soles of your feet. Engineers generally speak of three types: Tension : A force acting perpendicular to and away from a surface. Compression : A force acting perpendicular to and towards a surface. Shear : A force acting parallel to a surface. Strain : The change in a solid's shape caused by the application of a stress. Depending on the solid, a given stress might cause a great or small strain.
Kinds of deformation - Review : We discussed this briefly with regard to earthquakes. Elastic deformation : Generally, when a solid is strained, it will return to its original shape when the stress is removed. This is because its chemical bonds, although stretched, have not broken, as in the illustration above. Beyond that point, however, the change in shape becomes permanent. Ductile deformation : A. This is because it has been strained to where chemical bonds have begun to break.
In the Earth, we see ductile deformation manifested as folds. Brittle deformation : Break enough chemical bonds, and the entire object breaks. In the Earth, we see breaks manifested as faults. The actual breakage event, of course, is manifested as an earthquake. Measurements of Stiffness: Young's modulus is the ratio of stress to strain.
It is a measure of stiffness. It's usually measured using tensile stress. A stiffer solid like steel has a high Young's modulus, whereas a less stiff one like lead has a lower Young's modulus. Measurements of strength : Yield strength : The amount of stress required to cause a solid to deform ductilely. Tensile strength : The amount of stress required to cause a solid to break.
Stiffness and strength are two different things. Anticlines are folds in which each half of the fold dips away from the crest.
Synclines are folds in which each half of the fold dips toward the trough of the fold. Generally, rocks respond to stress in one of two ways: they break, or they bend. When a rock breaks, it is called brittle deformation. Any material that breaks into pieces exhibits brittle behavior.
When rocks bend or flow, like clay, it is called ductile deformation. This is what allows springs to store elastic potential energy. The opposite of elasticity is plasticity; when something is stretched, and it stays stretched, the material is said to be plastic.
When energy goes into changing the shape of some material and it stays changed, that is said to be plastic deformation.
When rocks are pulled in opposite directions, the stress is tensional. When a body of rock is distorted, the stress is shear. Because of isostasy, deformed and thickened crust will undergo regional uplift both during mountain building and for a long period afterward. The stress that squeezes something. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.
Types of deformation Elastic deformation. True stress and strain. Plastic deformation. Deforming force is the force which displaces a body from its original position or configuration either by changing location or shape. Eg: A pencil which is broken by applying force. Restoring force tends to move the place of the body back to its original position or configuration. What is the difference between differential stress and confining pressure?
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