Metamorphic Grades Describe “How Much”
- Low-grade metamorphic rocks form at temperatures of about 200 degrees Celsius. ...
- Intermediate grade metamorphic rocks contain mica as well as minerals such as garnet.
- High grade metamorphic rocks form in places that are so hot the rocks almost melt, about 800 degrees Celsius. ...
What is the highest metamorphic grade?
What is a high grade metamorphic rock? Gneiss, the highest grade metamorphic rock, contains bands of easily visible quartz, feldspar, and/or mica. From left to right: slate, phyllite, schist, gneiss. What are the two main categories of metamorphic texture? TEXTURES Textures of metamorphic rocks fall into two broad groups, FOLIATED and NON-FOLIATED.
What are the three types of metamorphism?
What are the four types of metamorphism?
- Type # 1. Contact Metamorphism:
- Type # 2. Regional Metamorphism:
- Type # 3. Hydro-Metamorphism:
- Type # 4. Hydro-Thermo-Metamorphism:
Which metamorphic rock below is the highest grade?
The lowest grade metamorphic rock type is slate, which consists of crystals that are generally too fine to see. The next higher grade rock, phyllite, contains grains that are still too small to see easily, but are coarse enough to catch light and so impart a sheen to the rock. Schist, the next higher grade rock, typically contains easily visible crystals of muscovite or biotite. Gneiss, the highest grade metamorphic rock, contains bands of easily visible quartz, feldspar, and/or mica.
What is a metamorphic grade?
What is the metamorphic grade? Metamorphic grade refers to the range of metamorphic change a rock undergoes, progressing from low (little metamorphic change) grade to high (significant metamorphic change) grade. Geologists use index minerals that form at certain temperatures and pressures to identify metamorphic grade.
How many grades of metamorphism are there?
Metamorphic Rock Types 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.
What is are the types of metamorphic grades?
7.2 Classification of Metamorphic RocksVery Low GradeHigh GradeMudrockslategneissGraniteno changegranite gneissBasaltchlorite schistamphiboliteSandstoneno changequartzite3 more rows
What is a metamorphic grade and how can it be determined?
(Metamorphic grades refer to the degree and intensity of the metamorphism: they are determined by the pressure and temperatures to which the rock has been subjected.) Such areas are generally referred to as metamorphic core complexes.
What are the 4 main types of metamorphism?
Hydro-Thermo-Metamorphism.Type # 1. Contact Metamorphism:Type # 2. Regional Metamorphism:Type # 3. Hydro-Metamorphism:Type # 4. Hydro-Thermo-Metamorphism:
What are the 3 types of metamorphism?
There are three ways that metamorphic rocks can form. The three types of metamorphism are Contact, Regional, and Dynamic metamorphism. Contact Metamorphism occurs when magma comes in contact with an already existing body of rock.
What is low-grade and high-grade metamorphism?
Low-grade metamorphic rocks tend to be fine-grained (the newly formed metamorphic mineral grains that is). High-grade metamorphic rocks tend to be coarse-grained. But grain size is also dependent on the grain size of the protolith.
What is meant by a metamorphic grade?
As the temperature and/or pressure increases on a body of rock we say the rock undergoes prograde metamorphism or that the grade of metamorphism increases. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.
What is the highest metamorphic grade?
GneissGneiss, the highest grade metamorphic rock, contains bands of easily visible quartz, feldspar, and/or mica.
What is metamorphic grade metamorphism?
Metamorphic grade refers to the range of metamorphic change a rock undergoes, progressing from low (little metamorphic change) grade to high (significant metamorphic change) grade. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock conditions.
What are the 5 types of metamorphism?
Observe on the phase diagram above the five kinds of metamorphism: Hydrothermal, Contact, Barrovian (sometimes called " regional" ), Blueschist, and Eclogite. Each is introduced below.
What are the 6 types of metamorphism?
Top 6 Types of Metamorphism | GeologyType # 1. Contact or Thermal Metamorphism:Type # 2. Hydrothermal Metamorphism:Type # 3. Regional Metamorphism:Type # 4. Burial Metamorphism:Type # 5. Plutonic Metamorphism:Type # 6. Impact Metamorphism:
What are the 7 types of regional metamorphism?
Contact Metamorphism.Regional Metamorphism.Shock Metamorphism.High Strain Metamorphism.Generation Types Metamorphism.Exercises Metamorphism.
What is metamorphic grade metamorphism?
Metamorphic grade refers to the range of metamorphic change a rock undergoes, progressing from low (little metamorphic change) grade to high (significant metamorphic change) grade. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock conditions.
What is meant by a metamorphic grade?
As the temperature and/or pressure increases on a body of rock we say the rock undergoes prograde metamorphism or that the grade of metamorphism increases. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.
What is the highest metamorphic grade?
GneissGneiss, the highest grade metamorphic rock, contains bands of easily visible quartz, feldspar, and/or mica.
What is a metamorphic grade What metamorphic grades are associated with the four main categories of foliated rocks?
The various types of foliated metamorphic rocks, listed in order of the grade or intensity of metamorphism and the type of foliation are: slate , phyllite , schist , and gneiss (Figure 7.2.
What does metamorphism mean?
Meaning of Metamorphism: Any pre-existing type of rock forming a part of the crust may or may not undergo any physical, chemical or structural change for any time after its formation. This depends upon the environment in which the rock exists.
What is the temperature of a metamorphic rock?
It is believed to begin at temperatures around 580°C under pressure of 3.5 kb and continues up to temperature of 800°C and above. A typical example indicative of high-grade metamorphism is provided by the breakdown of muscovite mica in the presence of quartz and plagioclase. Hypersthene is a typical index mineral of high-grade metamorphism and granulites are the common resulting metamorphic rocks.
Why does metamorphism increase with depth?
The grade of metamorphism generally increases with depth for the simple reason that both temperature and pressure factors become strong and stronger at deeper levels within the crust of the earth. This fact has given birth to the concept of Metamorphic Zones that signify the range of metamorphic effects at different depths below the surface. In other words, zones indicate depth-wise extension of particular grades of metamorphism in a general way.
What is the approximate extent of a rock that has been changed due to metamorphism?
The approximate extent or degree – qualitatively speaking – to which an original rock has been changed due to metamorphism is expressed by the term metamorphic grade. Three terms are used to express the grades – low grade, medium grade and high grade. These grades are indicated by the presence of a set of minerals that are called the index minerals. These minerals are stable only within the temperature-pressure range considered characteristic of that particular grade.
How are metamorphic rocks formed?
Metamorphic rocks formed through regional metamorphism occur in the form of extensive mountain belts and also as the core portions of many old eroded mountain systems throughout the world. They bear evidence of formation of new minerals as well as imposition of new textures and structures on an extensive scale.
What is contact metamorphism?
Contact Metamorphism is a common type of thermal metamorphism observed in rocks existing close to the magmatic intrusions, injections and lava flows.
What is the role of pore fluids in metamorphism?
With the rise in temperature, the pore fluids undergo expansion and become very active in disturbing or even breaking the original crystal boundaries of the involved minerals.
What is metamorphism in rocks?
Metamorphism is the change of minerals or geologic texture (distinct arrangement of minerals) in pre-existing rocks ( protoliths ), without the protolith melting into liquid magma (a solid-state change). The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids.
What is regional metamorphism?
Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges , particularly those associated with convergent tectonic plates or the roots of previously eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism. These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects, characterized by paired metamorphic belts. The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane .
How are dynamic metamorphic zones formed?
The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the temperature and confining pressure determine the deformation mechanisms which predominate. Within depths less than 5 km, dynamic metamorphism is not often produced because the confining pressure is too low to produce frictional heat. Instead, a zone of breccia or cataclasite is formed, with the rock milled and broken into random fragments. This generally forms a mélange. At depth, the angular breccias transit into a ductile shear texture and into mylonite zones.
What is the difference between prograde and retrograde metamorphism?
Prograde metamorphism involves the change of mineral assemblages ( paragenesis) with increasing temperature and (usually) pressure conditions. These are solid state dehydration reactions, and involve the loss of volatiles such as water or carbon dioxide. Prograde metamorphism results in rock characteristic of the maximum pressure and temperature experienced. Metamorphic rocks usually do not undergo further change when they are brought back to the surface.
What is the maximum temperature of metamorphic rock?
The maximum temperature for metamorphism is typically 700–900 °C, depending on the pressure and on the composition of the rock. Migmatites are rocks formed at this upper limit, which contains pods and veins of material that has started to melt but has not fully segregated from the refractory residue. Since the 1980s it has been recognized that rocks are rarely dry enough and of a refractory enough composition to record without melting "ultra-high" metamorphic temperatures of 900–1100 °C. The metamorphic process usually occurs at pressures between 100 and 300 MPa, the depth at which these pressures occur depending on which type of rock is applying pressure.
Where does contact metamorphism occur?
Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock. The area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole or contact aureole.
What is phase change metamorphism?
Phase change metamorphism is the creating of new minerals with the same chemical formula as the protolith. This involves a rearrangement of the atoms in the crystals.
What is metamorphic grade?
Metamorphic grade reflects the pressure and temperature involved in forming a particular metamorphic rock. It is based on the existence of particular minerals, known as index minerals. Because each mineral crystallizes within a limited pressure and temperature range, the presence of particular index minerals indicates the relatively specific set of conditions that existed when the rock formed.
What is regional metamorphism?
Regional metamorphism produces the bulk of Earth ’ s metamorphic rock. The volume of rock affected can be hundreds or even thousands of cubic miles. It is usually associated with mountain building processes.
What type of metamorphism produces green schist?
Regional metamorphism produces greenschist facies (low-grade metamorphism), which contains slate, phyllite and greenschist; amphibolite facies (medium-grade metamorphism) containing schist and/or amphibolite; and granulite facies (high-grade metamorphism), which contains gneiss and/or granulite.
Why do geologists focus on metamorphic facies?
This is because the environment in which metamorphic rocks formed is not easily identified based on a single type of rock.
What is the term for the group of chemical reactions that occur when water is released during metamorphism of rocks?
Metasomatism — A group of chemical reactions that occur when water released during metamorphism of rocks is involved in the creation of new minerals.
Where are metamorphic facies formed?
Two other metamorphic facies are formed on a regional scale and under unique circumstances. The blueschist facies forms in the low-temperature, high-pressure environment in the upper portion of a subduction zone. Land-derived sediments accumulated deep on the ocean floor are driven into an area of high pressure during subduction of an oceanic plate. These rocks often have a blue cast or color.
How does contact metamorphism occur?
Contact metamorphism results mainly from an increase in temperature with little change in pressure. The increase in temperature is caused by injection of molten rock, or magma, into surrounding rock (referred to as country rock). The area of rock altered by the injection of magma is known as an aureole, whereas the body of rock formed from the molten magma is called an intrusion. The rock closest to the source of heat is the most altered; further from the source of increased temperature, less alteration occurs. Eventually, due to the distance from the intrusion, unaltered country rock is encountered.
What are the two types of metamorphism?
There are two major kinds of metamorphism: regional and contact. Regional metamorphism. Most metamorphic rocks are the result of regional metamorphism (also called dynamothermal metamorphism). These rocks were typically exposed to tectonic forces and associated high pressures and temperatures.
What is the zone of metamorphism that surrounds the intrusion called?
The zone of metamorphism that surrounds the intrusion is called the halo (or aureole) and rarely extends more than 100 meters into the country rock. Geostatic pressure is usually a minor factor, since contact metamorphism generally takes place less than 10 kilometers from the surface. Previous Factors Controlling Metamorphism.
What is the lowest facies of metamorphism?
Weathering, diagenesis and metamorphism are all changes that take place in solid rocks due to temperature, pressure, and fluid interactions. The lowest facies of metamorphism is generally considered the zeolite facies .
What is metamorphism in rocks?
Weathering, diagenesis and metamorphism are all changes that take place in solid rocks due to temperature, pressure, and fluid interactions . The lowest facies of metamorphism is generally considered the zeolite facies.
How many metamorphic facies are there?
There are seven widely recognized metamorphic facies, ranging from the zeolite facies at low P and T to eclogite at very high P and T. Geologists determine a facies in the lab after examining many specimens under the microscope and doing bulk chemistry analyses. Metamorphic facies is not obvious in a given field specimen. To sum up, a metamorphic facies is the set of minerals found in a rock of a given composition. That mineral suite is taken as a sign of the pressure and temperature that made it.
Which metamorphic rocks have a layered or banded appearance?
Foliated metamorphic rocks such as gneiss, phyllite, schist, and slate have a layered or banded appearance that is produced by exposure to heat and directed pressure. Non-foliated metamorphic rocks such as hornfels, marble, quartzite, and novaculite do not have a layered or banded appearance.
How does sandstone form?
Sedimentary deposits of sand develop into sandSTONE through the process of diagenesis: burial and formation of a cement from interstitial water (typically). The cement is - in a quartz-dominant sand (and most continentally derived sediments of sand size grains are typically quartz dominant) - typically high in silica.
Is quartzite a phyllite?
More common instances, the quartzite is interbedded with phyllites and their mineralogy includes minerals that can be used to pinpoint a grade corresponding to an amphibolite facies. Chlorite-bearing quartzites occur, that did not get cooked past the greenschist facies, etc.
Is sandstone a broken rock?
Basically - I learned ‘sandstone’ as a somewhat friable (easily broken) rock, orthoquartzite as simply a very strongly cemented sandstone, and metaquartzite as the metamorphosed equivalent.
Overview
Types
Regional metamorphism is a general term for metamorphism that affects entire regions of the Earth's crust. It most often refers to dynamothermal metamorphism, which takes place in orogenic belts (regions where mountain building is taking place), but also includes burial metamorphism, which results simply from rock being buried to great depths below the Earth's surface in a su…
Metamorphic processes
Metamorphism is the set of processes by which existing rock is transformed physically or chemically at elevated temperature, without actually melting to any great degree. The importance of heating in the formation of metamorphic rock was first recognized by the pioneering Scottish naturalist, James Hutton, who is often described as the father of modern geology. Hutton wrote in 1795 that so…
Classification of metamorphic rocks
Metamorphic rocks are classified by their protolith, if this can be determined from the properties of the rock itself. For example, if examination of a metamorphic rock shows that its protolith was basalt, it will be described as a metabasalt. When the protolith cannot be determined, the rock is classified by its mineral composition or its degree of foliation.
Metamorphic grade is an informal indication of the amount or degree of metamorphism.
Equilibrium mineral assemblages
Metamorphic processes act to bring the protolith closer to thermodynamic equilibrium, which is its state of maximum stability. For example, shear stress (nonhydrodynamic stress) is incompatible with thermodynamic equilibrium, so sheared rock will tend to deform in ways that relieve the shear stress. The most stable assemblage of minerals for a rock of a given composition is that which …
See also
• Geothermobarometry – History of rock pressure and temperature
• Metamorphosis of snow
• Ultra-high-temperature metamorphism – Crustal metamorphism with temperatures exceeding 900 °C
Footnotes
1. ^ Marshak 2009, p. 177.
2. ^ Vernon 2008, p. 1.
3. ^ Yardley 1989, pp. 1–5.
4. ^ Yardley 1989, p. 5.
5. ^ Yardley 1989, pp. 29–30.
Further reading
• Winter J.D., 2001, An Introduction to Igneous and Metamorphic Petrology, Prentice-Hall ISBN 0-13-240342-0.
Metamorphic Rocks and Facies
- Metamorphic rocks form below the ground surface, beyond the reach of near-surface sedimentary processes of disintegration and consolidation. Pressure and temperature increase as depth below the ground surface increases. For each 1 mi (1.6 km) increase in depth below Earth’s surface, pressure typically increases by 0.56 kilobars, while temperature increases an average of 70°F (4…
Types of Metamorphic Facies
- Common metamorphic facies include: 1. Hornfels facies: low- to high-grade metamorphism. 2. Zeolite facies: low-grade metamorphism. 3. Greenschist facies: low-grade metamorphism. 4. Amphibolite facies: medium-grade metamorphism. 5. Granulite facies: high-grade metamorphism. 6. Blueschist facies: low-temperature/high-pressure metamorphism. 7. Eclogit...
Types of Metamorphism
- Contact metamorphism
Contact metamorphism results mainly from an increase in temperature with little change in pressure. The increase in temperature is caused by injection of molten rock, or magma, into surrounding rock (referred to as country rock). The area of rock altered by the injection of magm… - KEY TERMS
Asthenosphere—Flowing layer of plastic rock situated below the lithosphere. Hornfels—A metamorphic rock containing micas, quartz and garnets and that is formed from clay-rich rocks. Kilobar— A unit of measure used to express the high pressures found within Earth’s interior. It is …
Resources
- BOOKS
Blatt, H., R. Tracy, and B. Owens. Petrology: Igneous, Sedimentary, and Metamorphic. New York: Freeman, 2005. Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology.Upper Saddle River, NJ: Prentice Hall, 2004. Monica Anderson