Porphyroblasts represent minerals that crystallise at a faster rate than the matrix minerals. Garnet is a common porphyroblast mineral. Introduction Features Igneous rocks Sedimentary rocks Metamorphic rocks.
Foliation planes in gneiss. Introduction Features Igneous rocks Sedimentary rocks Metamorphic rocks Metamorphic rocks Metamorphism is the alteration of pre-existing rocks in the solid state due to changes in temperature and pressure.
Metamorphic textures The two distinctive metamorphic textures are: Foliation planes in gneiss Foliation - This represents a distinct plane of weakness in the rock. Guide to the classification of metamorphic rocks by texture.
Multimedia Gallery. Park Passes. Technical Announcements. Employees in the News. Emergency Management. Survey Manual. Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous , sedimentary , or earlier metamorphic form.
Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet. Process of Metamorphism: The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks.
New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Pressure or temperature can even change previously metamorphosed rocks into new types. Metamorphic rocks are often squished, smeared out, and folded.
Despite these uncomfortable conditions, metamorphic rocks do not get hot enough to melt, or they would become igneous rocks! Common Metamorphic Rocks: Common metamorphic rocks include phyllite, schist, gneiss, quartzite and marble. Foliated Metamorphic Rocks: Some kinds of metamorphic rocks -- granite gneiss and biotite schist are two examples -- are strongly banded or foliated.
Foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance. Foliation forms when pressure squeezes the flat or elongate minerals within a rock so they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.
Non-Foliated Metamorphic Rocks: Non-foliated metamorphic rocks do not have a platy or sheet-like structure. There are several ways that non-foliated rocks can be produced. Some rocks, such as limestone are made of minerals that are not flat or elongate. 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. The folding and deformation of the rock while it is ductile may greatly distort the original shapes and orientations of the rock, producing folded layers and mineral veins that have highly deformed or even convoluted shapes.
The diagram below shows folds forming during an early stage of regional metamorphism, along with development of foliation, in response to normal stress. The photograph below shows high-grade metamorphic rock that has undergone several stages of foliation development and folding during regional metamorphism, and may even have reached such a high temperature that it began to melt.
Contact metamorphism occurs to solid rock next to an igneous intrusion and is caused by the heat from the nearby body of magma. Because contact metamorphism is not caused by changes in pressure or by differential stress, contact metamorphic rocks do not become foliated. Where intrusions of magma occur at shallow levels of the crust, the zone of contact metamorphism around the intrusion is relatively narrow, sometimes only a few m a few feet thick, ranging up to contact metamorphic zones over m over feet across around larger intrusions that released more heat into the adjacent crust.
The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole. The rocks closest to the contact with the intrusion are heated to the highest temperatures, so the metamorphic grade is highest there and diminishes with increasing distance away from the contact.
Because contact metamorphism occurs at shallow to moderate depths in the crust and subjects the rocks to temperatures up to the verge of igneous conditions, it is sometimes referred to as high-temperature, low-pressure metamorphism. Hornfels, which is a hard metamorphic rock formed from fine-grained clastic sedimentary rocks, is a common product of contact metamorphism. Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids.
The difference in composition between the existing rock and the invading fluid drives the chemical reactions. The hydrothermal fluid may originate from a magma that intruded nearby and caused fluid to circulate in the nearby crust, from circulating hot groundwater, or from ocean water. If the fluid introduces substantal amounts of ions into the rock and removes substantial amounts of ions from it, the fluid has metasomatized the rock—changed its chemical composition. Ocean water that penetrates hot, cracked oceanic crust and circulates as hydrothermal fluid in ocean floor basalts produces extensive hydrothermal metamorphism adjacent to mid-ocean spreading ridges and other ocean-floor volcanic zones.
Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. Greenschist contains a set of minerals, some of them green, which may include chlorite, epidote, talc, Na-plagioclase, or actinolite. The fluids eventually escape through vents in the ocean floor known as black smokers, producing thick deposits of minerals on the ocean floor around the vents. Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form.
This large boulder has bedding still visible as dark and light bands sloping steeply down to the right. The rock also has a strong slaty foliation, which is horizontal in this view, and has developed because the rock was being squeezed during metamorphism.
The rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation. Squeezing and heating alone as shown in Figure 7. This effect is especially strong if the new minerals are platy like mica or elongated like amphibole. Slate, for example, is characterized by aligned flakes of mica that are too small to see.
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. As already noted, slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress. Slate tends to break into flat sheets.
Phyllite is similar to slate, but has typically been heated to a higher temperature; the micas have grown larger and are visible as a sheen on the surface. Where slate is typically planar, phyllite can form in wavy layers. In the formation of schist, the temperature has been hot enough so that individual mica crystals are visible, and other mineral crystals, such as quartz, feldspar, or garnet may also be visible. In gneiss, the minerals may have separated into bands of different colours.
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.
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