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. The lines are known as isograds. Regional metamorphism occurs where large areas of rock are subjected to large amounts of differential stress for long intervals of time, conditions typically associated with mountain building.
Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other. Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are formed during regional metamorphism. As the rocks become heated at depth in the Earth during regional metamorphism they become ductile, which means they are relatively soft even though they are still solid.
In the example shown in Figure 7. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable. Unlike slate and phyllite, which typically only form from mudrock, schist, and especially gneiss, can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks. Schist and gneiss can be named on the basis of important minerals that are present.
For example a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist. One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist. Similarly, a gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or, more accurately, an amphibolite. If a rock is buried to a great depth and encounters temperatures that are close to its melting point, it will partially melt.
The resulting rock, which includes both metamorphosed and igneous material, is known as a migmatite Figure 7. JPG] As already noted, the nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions.
The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 7. Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees. Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated.
In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust.
This is contact metamorphism. What type of rocks can contain fossils on them? Do metamorphic rocks contain lectures crystals? What countries contain metamorphic rocks? What types of stones are most likely to contain fossils? Why are metamorphic rocks called metamorphic rocks?
What types of rocks contain crystals? Does a rock contain fossils? Where is the gemstone garnet found? Can metamorphic rocks form from other metamorphic rocks? Do sedimentary rocks contain fossil?
Does igneous sedimentary and metamorphic rocks contain fossils? What is metamorphic and how is it formed? Two types of metamorphic rocks? What is metamorphic rocks most likely form? Why are you likely to find fossils in evaporite sedimentary rocks but not in detrital sedimentaryor metamorphic rocks? Study Guides. Trending Questions. Still have questions? A rock that shows a banded texture without a distinct foliation is termed a gneiss.
All of these could be porphyroblastic i. A rock that shows no foliation is called a hornfels if the grain size is small, and a granulite , if the grain size is large and individual minerals can be easily distinguished with a hand lens.
Protolith Protolith refers to the original rock, prior to metamorphism. In low grade metamorphic rocks, original textures are often preserved allowing one to determine the likely protolith.
As the grade of metamorphism increases, original textures are replaced with metamorphic textures and other clues, such as bulk chemical composition of the rock, are used to determine the protolith. Bulk Chemical Composition The mineral assemblage that develops in a metamorphic rock is dependent on The pressure and temperature reached during metamorphism The composition of any fluid phase present during metamorphism, and The bulk chemical composition of the rock.
Just like in igneous rocks, minerals can only form if the necessary chemical constituents are present in the rock i. Based on the mineral assemblage present in the rock one can often estimate the approximate bulk chemical composition of the rock.
Some terms that describe this general bulk chemical composition are as follows:. These are as follows:. In general, metamorphic rocks do not drastically change chemical composition during metamorphism, except in the special case where metasomatism is involved such as in the production of skarns, as discussed above. The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism.
Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to. This pressure and temperature environment is referred to as Metamorphic Facies. This is similar to the concept of sedimentary facies, in that a sedimentary facies is also a set of environmental conditions present during deposition. The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism.
A high geothermal gradient such as the one labeled "A" , might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies. Under a normal to high geothermal gradient, such as "B", rocks would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism or depth of burial increased.
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.
This relationship between geothermal gradient and metamorphism will be the central theme of our discussion of metamorphism. Examples of questions on this material that could be asked on an exam. Types of Metamorphism. Metamorphism is defined as follows: The mineralogical and structural adjustment of solid rocks to physical and chemical conditions that have been imposed at depths below the near surface zones of weathering and diagenesis and which differ from conditions under which the rocks in question originated.
Note that Diagenesis is also a change in form that occurs in sedimentary rocks. Metamorphism, therefore occurs at temperatures and pressures higher than o C and MPa. Rocks can be subjected to these higher temperatures and pressures as they are 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 where melting of the rock in question begins. Once melting begins, the process changes to an igneous process rather than a metamorphic process.
Low-grade metamorphism takes place at temperatures between about to o C, and relatively low pressure. Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals. 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. Types of Metamorphism Contact Metamorphism Contact metamorphism occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion.
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