Factors that Control Metamorphism
Metamorphism occurs because some minerals are stable only under certain conditions of pressure and temperature. When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions. But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism.
Temperature
Temperature increases with depth in the Earth along the Geothermal Gradient. Thus higher temperature can occur by burial of rock.
Temperature can also increase due to igneous intrusion.
Pressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth. Pressure is defined as a force acting equally from all directions. It is a type of stress, called hydrostatic stress, or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress. If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock.
rounded grains can become flattened in the direction of maximum stress.
minerals that crystallize or grow in the differential stress field can have a preferred orientation. This is especially true of the sheet silicate minerals (the micas: biotite and muscovite, chlorite, talc, and serpentine).
These sheet silicates will grow with their sheets orientated perpendicular to the direction of maximum stress. Preferred orientation of sheet silicates causes rocks to be easily broken along approximately parallel sheets. Such a structure is called a foliation.
Fluid Phase - Any existing open space between mineral grains in a rocks can potentially contain a fluid. This fluid is mostly H2O, but contains dissolved mineral matter. The fluid phase is important because chemical reactions that involve one solid mineral changing into another solid mineral can be greatly speeded up by having dissolved ions transported by the fluid. Within increasing pressure of metamorphism, the pore spaces in which the fluid resides is reduced, and thus the fluid is driven off. Thus, no fluid will be present when pressure and temperature decrease and, as discussed earlier, retrograde metamorphism will be inhibited.
Time - The chemical reactions involved in metamorphism, along with recrystallization, and growth of new minerals are extremely slow processes. Laboratory experiments suggest that the longer the time available for metamorphism, the larger are the sizes of the mineral grains produced. Thus, coarse grained metamorphic rocks involve long times of metamorphism. Experiments suggest that the time involved is millions of years.
Types of Metamorphism
Cataclastic 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. Cataclastic metamorphism is not very common and is restricted to a narrow zone along which the sliding occurred.
Burial Metamorphism - When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than 300oC 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.
Contact 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.
Tuesday, May 19, 2009
The word "Metamorphism" comes from the Greek: Meta = change, Morph = form, so metamorphism means to change form. In geology this refers to the changes in mineral assemblage and texture that result from subjecting a rock to pressures and temperatures different from those under which the rock originally formed.
Note that Diagenesis is also a change in form that occurs in sedimentary rocks. In geology, however, we restrict diagenetic processes to those which occur at temperatures below 200oC and pressures below about 300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3,000 atmospheres of pressure.
Metamorphism, therefore occurs at temperatures and pressures higher than 200oC and 300 MPa. Rocks can be subjected to these higher temperatures and pressures as they become buried deeper in the Earth. Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction.
The upper limit of metamorphism occurs at the pressure and temperature of wet partial melting of the rock in question. Once melting begins, the process changes to an igneous process rather than a metamorphic process.
Grade of Metamorphism
As the temperature and/or pressure increases on a body of rock we say that the rock undergoes prograde metamorphism or that the grade of metamorphism increases. Metamorphic grade is a general term for describing the relatLow-grade metamorphism takes place at temperatures between about 200 to 320oC, and relatively low pressure. Low grade metamorphic rocks are characterized by an abundance of hydrous minerals (minerals that contain water, H2O, 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 320oC and relatively high pressure. As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O 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.
ive temperature and pressure conditions under which metamorphic rocks form.
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. Such a process is referred to as retrograde metamorphism. If retrograde metamorphism were common, we would not commonly see metamorphic rocks at the surface of the Earth. Since we do see metamorphic rocks exposed at the Earth's surface retrograde metamorphism does not appear to be common. The reasons for this include:
chemical reactions take place more slowly as temperature is decreased
during prograde metamorphism, fluids such as H2O and CO2 are driven off, and these fluids are necessary to form the hydrous minerals that are stable at the Earth's surface.
chemical reactions take place more rapidly in the presence of fluids, but if the fluids are driven off during prograde metamorphism, they will not be available to speed up reactions during retrograde metamorphism.
Note that Diagenesis is also a change in form that occurs in sedimentary rocks. In geology, however, we restrict diagenetic processes to those which occur at temperatures below 200oC and pressures below about 300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3,000 atmospheres of pressure.
Metamorphism, therefore occurs at temperatures and pressures higher than 200oC and 300 MPa. Rocks can be subjected to these higher temperatures and pressures as they become buried deeper in the Earth. Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction.
The upper limit of metamorphism occurs at the pressure and temperature of wet partial melting of the rock in question. Once melting begins, the process changes to an igneous process rather than a metamorphic process.
Grade of Metamorphism
As the temperature and/or pressure increases on a body of rock we say that the rock undergoes prograde metamorphism or that the grade of metamorphism increases. Metamorphic grade is a general term for describing the relatLow-grade metamorphism takes place at temperatures between about 200 to 320oC, and relatively low pressure. Low grade metamorphic rocks are characterized by an abundance of hydrous minerals (minerals that contain water, H2O, 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 320oC and relatively high pressure. As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O 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.
ive temperature and pressure conditions under which metamorphic rocks form.
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. Such a process is referred to as retrograde metamorphism. If retrograde metamorphism were common, we would not commonly see metamorphic rocks at the surface of the Earth. Since we do see metamorphic rocks exposed at the Earth's surface retrograde metamorphism does not appear to be common. The reasons for this include:
chemical reactions take place more slowly as temperature is decreased
during prograde metamorphism, fluids such as H2O and CO2 are driven off, and these fluids are necessary to form the hydrous minerals that are stable at the Earth's surface.
chemical reactions take place more rapidly in the presence of fluids, but if the fluids are driven off during prograde metamorphism, they will not be available to speed up reactions during retrograde metamorphism.
Monday, May 4, 2009
About sedimentary rocks
Sedimentary rocks can be categorized into three groups based on sediment type.Most sedimentary rocks are formed by the lithification of weathered rock debris that has been physically transported and deposited. During the transport process, the particles that make up these rocks often become rounded due to abrasion or can become highly sorted. Examples of this type of sedimentary rock include conglomerate and sandstone. Scientists sometimes call this general group of sedimentary rocks clastic. The remaining types of sedimentary rocks are created either from chemical precipitation and crystallization, or by the lithification of once living organic matter. We identify these sedimentary rocks as being non-clastic.
All sedimentary rocks are lithified into some collective mass. Lithification is any process that turns raw rock sediment into consolidated sedimentary rock. The process of lithification usually produces identifiable layering in these type of rocks . Lithification can occur by way of:
* Drying and compaction.
* Oxidation of iron and aluminum.
* Precipitation of calcium and silica.
The classification of clastic sedimentary rocks is based on the particle types found in the rock. Some types of clastic sedimentary rocks are composed of weathered rock material like gravel, sand, silt, and clay. Others can be constructed from the break up and deposition of shells, coral and other marine organisms by wave-action and ocean currents.
Earlier it was suggested that there were two types of non-clastic sedimentary rocks. One group forms through the chemical precipitation and crystallization of elements and compounds from solution. Elements such as calcium, sodium, potassium, and magnesium are commonly released into the environment through a variety of chemical weathering processes. These elements can then become dissolved into aqueous solutions that are often transported via runoff, stream flow, or groundwater flow. If this solution enters a basin environment where evaporation exceeds precipitation and in-flow, sedimentary evaporites can form because of the loss of water from the solution.
The oceans are almost saturated with dissolved calcium carbonate. This compound originates from the shells of a variety of marine organisms that use it for the construction of shells and other hard body parts. Because these organisms are surrounded in a solution, some of the calcium carbonate dissolves into the ocean waters. Under the right circumstances the dissolved calcium carbonate can precipitate out forming chemically created limestone deposits. The formation of dolomite involves the chemical modification of limestone deposits by a magnesium rich solution.
Several types of sedimentary rocks are formed from the lithification of once living organisms. Limestone deposits can be formed by the direct lithification of coral reefs, marine organism shells, or marine organism skeletons. Chalk is a particular variety of limestone that is composed of the skeletons of marine microorganisms like forminifera. Coal and lignite are the lithified remains of plants.
All sedimentary rocks are lithified into some collective mass. Lithification is any process that turns raw rock sediment into consolidated sedimentary rock. The process of lithification usually produces identifiable layering in these type of rocks . Lithification can occur by way of:
* Drying and compaction.
* Oxidation of iron and aluminum.
* Precipitation of calcium and silica.
The classification of clastic sedimentary rocks is based on the particle types found in the rock. Some types of clastic sedimentary rocks are composed of weathered rock material like gravel, sand, silt, and clay. Others can be constructed from the break up and deposition of shells, coral and other marine organisms by wave-action and ocean currents.
Earlier it was suggested that there were two types of non-clastic sedimentary rocks. One group forms through the chemical precipitation and crystallization of elements and compounds from solution. Elements such as calcium, sodium, potassium, and magnesium are commonly released into the environment through a variety of chemical weathering processes. These elements can then become dissolved into aqueous solutions that are often transported via runoff, stream flow, or groundwater flow. If this solution enters a basin environment where evaporation exceeds precipitation and in-flow, sedimentary evaporites can form because of the loss of water from the solution.
The oceans are almost saturated with dissolved calcium carbonate. This compound originates from the shells of a variety of marine organisms that use it for the construction of shells and other hard body parts. Because these organisms are surrounded in a solution, some of the calcium carbonate dissolves into the ocean waters. Under the right circumstances the dissolved calcium carbonate can precipitate out forming chemically created limestone deposits. The formation of dolomite involves the chemical modification of limestone deposits by a magnesium rich solution.
Several types of sedimentary rocks are formed from the lithification of once living organisms. Limestone deposits can be formed by the direct lithification of coral reefs, marine organism shells, or marine organism skeletons. Chalk is a particular variety of limestone that is composed of the skeletons of marine microorganisms like forminifera. Coal and lignite are the lithified remains of plants.
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