Metamorphic rocks
Metamorphism
It takes place deep in the bowels of the Earth. Any rock can be metamorphosed. It may be a sedimentary rock, igneous or even an existing metamorphic rock. Depending on the nature of the starting rock there are:
Para-metamorphism: this is a sedimentary rock that is metamorphosed
Ortho-metamorphism: it is an igneous rock which is metamorphosed
Poly-metamorphism: this is a metamorphic rock that is metamorphosed
General principle
More one sinks underground, the higher the ambient temperature increases. An average increase of 3 ° C every 100 meters, the average geothermal gradient. Similarly the pressure increases with depth. If the surface temperature of 1000 ° C is enough to melt most rocks at depth, this value will be much higher. Indeed the pressure will oppose the merger.
When a rock descends, it first undergoes the processes of diagenesis, then gradually as the temperature and pressure increase, ion rearrangements disrupt the structure of certain minerals. There then metamorphism.
Metamorphism corresponds to the gap between sediment diagenesis (low temperature and low pressure) and fusion of rock (by anatexie). The transition between diagenesis and metamorphism is called Anchimétamorphisme.
Thus metamorphism involves only solid rock. Despite the mineralogical and structural transformations undergone by the rock, it remains in the solid state.
external liquid inputs can however take place, resulting in the modification of the chemical composition of the rock metasomatically.
The different types of metamorphisms
We can distinguish three types of metamorphism:
The impact metamorphism: are formed when a meteorite impact.
It includes a form of silica of very high pressure, the Coesite, as well as showing a melting glassy phases.
Contact metamorphism: Rocks are metamorphosed into contact with an intrusive granite (or discordant). It is mainly the temperature which is involved here, there is little pressure-related deformation. The intrusion of magma, pushing land already in place, however, may induce cleavage. There is often a rearrangement without license exchange with other bodies that the original rock (metamorphic isochemical).
It's the heat of the magma that is responsible for the transformation of the rocks that surround it. The metamorphosed zone is reduced and draws a halo of metamorphism around the cooled magma.
Example: The granite flamanville: sedimentary shales observed it, then spotted schists (appearance of cordierite), nodular and micaceous schists (there is loss of oriented rock structure and appearance of andalusite) and then corneal (no preferred orientation of mica and andalusite) in contact with the granite.
Regional metamorphism: it corresponds to zones of metamorphosed over 10 km. One can observe a land succession increasingly metamorphosed as well as cleavage of increasing thrust. This can lead to an onset of melting (Migmatite) or even complete melting of the rock (Anatectite). The granite obtained is then matched (there is no limit frank with the country rock).
The main cause of this type of metamorphism is of tectonic origin. Therefore the minerals of these metamorphic rocks are often flattened and oriented along the foliation planes.
Pressure
The increase in pressure can have different causes:
lithostatic: it is due to the weight of the rocks accumulated sediment subsidence, or by subduction thrust and overlap. It leads to compaction and diagenesis. Lithostatic pressure sediment (2, 5 kilos in a column of 10 m on 1cm2) and tectonic phenomena allows the penetration of the rocks in the crust.
Hydrostatic: It is the pressure of fluids (CO2, H2O). It is mainly involved in their release.
constraining pressure: These are the pressures directed by tectonic phenomena
Chemical factors
Metamorphism is generally isochemical: minerals that appear are formed from the same composition from those of the original rock (it ignores fluid loss). The rocks formed in this way are called ectinites.
If métasomatose (replacement of elements by others), this is most often water and CO2 involved.
Triggering factors
Metamorphism is not uniform in a rock, some areas may not suffer (they allow besides to serve as controls). Indeed the minerals remain in metastable equilibrium throughout the metamorphism and only those areas where there has been destabilization were transformed. For low metamorphism, low temperature, deformation enough to destabilize, to a high temperature metamorphic rocks are retained in their original state if there is no fluids.
Metamorphic rocks
Structure of metamorphic rocks
Metamorphic rocks often undergo deformations. These constraints lead to the onset of particular structures in the rock. We can distinguish 3 types one after another with the intensity of metamorphism:
Stratification that is derived sedimentation phenomena. It is perpendicular to the forces at play (lithostatic pressure). It relates the flow rate of the rock.
Schistosity where the rock is cut up into sheets of the same mineralogical composition. This provision appears from 5 km deep. It can occur during diagenesis (lithostatic pressure) but is often linked to the tectonic stress. Most often the cleavage is perpendicular or oblique to the forces in play.
Foliation where some rock minerals are transformed. The new minerals that appear flatten and are moving in the direction of the cleavage. They can be grouped as bed The foliation front is located around 10 km deep. (Mica schist, gneiss).
During metamorphism, the same rock undergoes mineralogical changes. Some minerals appear, others disappear. Gold minerals appear only under certain conditions of temperature and pressure, this is called their stability range. To avoid errors of interpretation by studying only a single mineral, it has defined assemblages. In fact, not a mineral is observed, but a mineral association or paragenesis.
It takes place deep in the bowels of the Earth. Any rock can be metamorphosed. It may be a sedimentary rock, igneous or even an existing metamorphic rock. Depending on the nature of the starting rock there are:
Para-metamorphism: this is a sedimentary rock that is metamorphosed
Ortho-metamorphism: it is an igneous rock which is metamorphosed
Poly-metamorphism: this is a metamorphic rock that is metamorphosed
General principle
More one sinks underground, the higher the ambient temperature increases. An average increase of 3 ° C every 100 meters, the average geothermal gradient. Similarly the pressure increases with depth. If the surface temperature of 1000 ° C is enough to melt most rocks at depth, this value will be much higher. Indeed the pressure will oppose the merger.
When a rock descends, it first undergoes the processes of diagenesis, then gradually as the temperature and pressure increase, ion rearrangements disrupt the structure of certain minerals. There then metamorphism.
Metamorphism corresponds to the gap between sediment diagenesis (low temperature and low pressure) and fusion of rock (by anatexie). The transition between diagenesis and metamorphism is called Anchimétamorphisme.
Thus metamorphism involves only solid rock. Despite the mineralogical and structural transformations undergone by the rock, it remains in the solid state.
external liquid inputs can however take place, resulting in the modification of the chemical composition of the rock metasomatically.
The different types of metamorphisms
We can distinguish three types of metamorphism:
The impact metamorphism: are formed when a meteorite impact.
It includes a form of silica of very high pressure, the Coesite, as well as showing a melting glassy phases.
Contact metamorphism: Rocks are metamorphosed into contact with an intrusive granite (or discordant). It is mainly the temperature which is involved here, there is little pressure-related deformation. The intrusion of magma, pushing land already in place, however, may induce cleavage. There is often a rearrangement without license exchange with other bodies that the original rock (metamorphic isochemical).
It's the heat of the magma that is responsible for the transformation of the rocks that surround it. The metamorphosed zone is reduced and draws a halo of metamorphism around the cooled magma.
Example: The granite flamanville: sedimentary shales observed it, then spotted schists (appearance of cordierite), nodular and micaceous schists (there is loss of oriented rock structure and appearance of andalusite) and then corneal (no preferred orientation of mica and andalusite) in contact with the granite.
Regional metamorphism: it corresponds to zones of metamorphosed over 10 km. One can observe a land succession increasingly metamorphosed as well as cleavage of increasing thrust. This can lead to an onset of melting (Migmatite) or even complete melting of the rock (Anatectite). The granite obtained is then matched (there is no limit frank with the country rock).
The main cause of this type of metamorphism is of tectonic origin. Therefore the minerals of these metamorphic rocks are often flattened and oriented along the foliation planes.
Pressure
The increase in pressure can have different causes:
lithostatic: it is due to the weight of the rocks accumulated sediment subsidence, or by subduction thrust and overlap. It leads to compaction and diagenesis. Lithostatic pressure sediment (2, 5 kilos in a column of 10 m on 1cm2) and tectonic phenomena allows the penetration of the rocks in the crust.
Hydrostatic: It is the pressure of fluids (CO2, H2O). It is mainly involved in their release.
constraining pressure: These are the pressures directed by tectonic phenomena
Chemical factors
Metamorphism is generally isochemical: minerals that appear are formed from the same composition from those of the original rock (it ignores fluid loss). The rocks formed in this way are called ectinites.
If métasomatose (replacement of elements by others), this is most often water and CO2 involved.
Triggering factors
Metamorphism is not uniform in a rock, some areas may not suffer (they allow besides to serve as controls). Indeed the minerals remain in metastable equilibrium throughout the metamorphism and only those areas where there has been destabilization were transformed. For low metamorphism, low temperature, deformation enough to destabilize, to a high temperature metamorphic rocks are retained in their original state if there is no fluids.
Metamorphic rocks
Structure of metamorphic rocks
Metamorphic rocks often undergo deformations. These constraints lead to the onset of particular structures in the rock. We can distinguish 3 types one after another with the intensity of metamorphism:
Stratification that is derived sedimentation phenomena. It is perpendicular to the forces at play (lithostatic pressure). It relates the flow rate of the rock.
Schistosity where the rock is cut up into sheets of the same mineralogical composition. This provision appears from 5 km deep. It can occur during diagenesis (lithostatic pressure) but is often linked to the tectonic stress. Most often the cleavage is perpendicular or oblique to the forces in play.
Foliation where some rock minerals are transformed. The new minerals that appear flatten and are moving in the direction of the cleavage. They can be grouped as bed The foliation front is located around 10 km deep. (Mica schist, gneiss).
During metamorphism, the same rock undergoes mineralogical changes. Some minerals appear, others disappear. Gold minerals appear only under certain conditions of temperature and pressure, this is called their stability range. To avoid errors of interpretation by studying only a single mineral, it has defined assemblages. In fact, not a mineral is observed, but a mineral association or paragenesis.
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