Metamorphic rocks are almost always harder than sedimentary rocks. They are generally as hard and sometimes harder than igneous rocks. They form the roots of many mountain chains and are exposed to the surface after the softer outer layers of rocks are eroded away. Many metamorphic rocks are found in mountainous regions today and are a good indicator that ancient mountains were present in areas that are now low hill or even flat plains. Metamorphic rocks are divided into two categories- Foliates and Non-foliates.
Foliates are composed of large amounts of micas and chlorites. These minerals have very distinct cleavage.
Foliated metamorphic rocks will split along cleavage lines that are parallel to the minerals that make up the rock. Slate, as an example, will split into thin sheets. Foliate comes from the Latin word that means sheets, as in the sheets of paper in a book.
Silt and clay can become deposited and compressed into the sedimentary rock shale. The layers of shale can become buried deeper and deeper by the process of deposition. Deposition is the laying down of rock forming material by any natural agent wind, water, glaciers over time.
Because these layers are buried, temperatures and pressures become greater and greater until the shale is changed into slate. Slate is a fine-grained metamorphic rock with perfect cleavage that allows it to split into thin sheets. Slate usually has a light to dark brown streak.
Slate is produced by low grade metamorphism, which is caused by relatively low temperatures and pressures. Slate has been used by man in a variety of ways over the years. One use for slate was in the making of headstones or grave markers. Slate is not very hard and can be carved easily. The problem with slate though is its perfect cleavage. The slate headstones would crack and split along these cleavage planes as water would seep into the cracks and freeze which would lead to expansion.
This freeze-thaw, freeze-thaw over time would split the headstone. Today headstones are made of a variety of rocks, with granite and marble being two of the most widely used rocks. Slate was also used for chalk boards. The black color was good as a background and the rock cleaned easily with water. Today it is not very advantageous to use this rock because of its weight and the splitting and cracking over time. Schist is a medium grade metamorphic rock.
This means that it has been subjected to more heat and pressure than slate, which is a low grade metamorphic rock. As you can see in the photo above schist is a more coarse grained rock. The individual grains of minerals can be seen by the naked eye.
Many of the original minerals have been altered into flakes. Because it has been squeezed harder than slate it is often found folded and crumpled. Schists are usually named by the main minerals that they are formed from. Bitotite mica schist, hornblende schist, garnet mica schist, and talc schist are some examples of this.
Gneiss is a high grade metamorphic rock. This means that gneiss has been subjected to more heat and pressure than schist. When granite is subjected to intense heat and pressure, it changes into a metamorphic rock called gneiss. Slate is another common metamorphic rock that forms from shale. Limestone, a sedimentary rock , will change into the metamorphic rock marble if the right conditions are met. This happens due to geologic uplift and the erosion of the rock and soil above them.
At the surface, metamorphic rocks will be exposed to weathering processes and may break down into sediment. These sediments could then be compressed to form sedimentary rocks, which would start the entire cycle anew. Any rock type can become any other. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.
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Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. 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. During metamorphism the protolith undergoes changes in texture of the rock and the mineral make up of the rock. These changes take place mostly in the solid state and are caused by changes in physical or chemical conditions, which in turn can be caused by such things as burial, tectonic stress, heating by magma or interactions with fluids.
Factors that Control Metamorphism. Metamorphism occurs because rocks undergo changes in temperature and pressure and may be subjected to differential stress and hydrothermal fluids. 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. There are two kinds of differential stress. Normal stress causes objects to be compressed in the direction of maximum principal stress and extended in the direction of minimal stress.
If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock. Shear stress causes objects to be smeared out in the direction of applied stress.
Differential stress if acting on a rocks can have a profound affect on the appearance or texture of the rock. Grade of Metamorphism Metamorphic grade is a general term for describing the relative 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 H 2 O and CO 2 are driven off, and these fluids are necessary to form the hydrous minerals that are stable at the Earth's surface.
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.
They have a significant amount of sheet silicate platy minerals and are classified by composition, grain size, and foliation type. Non-foliated — These have no evident planar fabric or foliation, crystallized under conditions where there was no differential stress, and are comprised of equant minerals only. These are classified mainly by the minerals present or the chemical composition of the protolith.
Foliated Metamorphic Rocks. Example - metamorphism of a shale, made up initially of clay minerals and quartz all of clay or silt size.
Slate - Slates form at low metamorphic grade by the growth of fine grained chlorite and clay minerals. The preferred orientation of these sheet silicates causes the rock to easily break along the planes parallel to the sheet silicates, causing a slatey cleavage.
Note that in the case shown here, the maximum stress is applied at an angle to the original bedding planes, so that the slatey cleavage has developed at an angle to the original bedding. Because of the nearly perfect breakage along planes, slates are useful for blackboards and shingles. Phyllite - Fine mica-rich rock, formed by low — medium grade metamorphism. In a phyllite, the clay minerals have recrystallized into tiny micas biotite and muscovite which reflect a satiny luster.
Phyllite is between slate and schist. Schist - The size of the mineral grains tends to enlarge with increasing grade of metamorphism. Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates mainly biotite and muscovite. Quartz and Feldspar grains, however show no preferred orientation. The irregular planar foliation at this stage is called schistosity.
Schist often has other minerals besides micas. When these non-mica minerals occur with a grain size greater than the rest of the rock, they are called pophyroblasts.
Gneiss As metamorphic grade increases, the sheet silicates become unstable and dark colored minerals like hornblende and pyroxene start to grow. These dark colored minerals tend to become segregated in distinct bands through the rock, giving the rock a gneissic banding. Because the dark colored minerals tend to form elongated crystals, rather than sheet- like crystals, they still have a preferred orientation with their long directions perpendicular to the maximum differential stress.
Granulite - At the highest grades of metamorphism all of the hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation.
The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks. Migmatites — If the temperature reaches the solidus temperature first melting temperature , the rock may begin to melt and start to co-mingle with the solids. Usually these melts are felsic with the mafic material remaining metamorphic.
Non-foliated Metamorphic Rocks. Non-foliated rocks lack a planar fabric. Absence of foliation possible for several reasons:. Non-foliated rocks are given specific names based on their mineralogy and composition: Amphibolite - These rocks are dark colored rocks with amphibole usually hornblende as their major mineral. They are usually poorly foliated and form at intermediate to high grades of metamorphism of basaltic or gabbroic protoliths.
Hornfels - These are very fine grained rocks that usually form as a result of magma intruding into fined grained igneous rocks or shales. The magma causes a type of metamorphism called contact metamorphism to be discussed later.
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