This changing of rock types is called the "Rock Cycle". Solid rock can be changed into a new rock by stresses that cause an increase in heat and pressure. There are 3 main agents that cause metamorphism. Factors that cause an increase in Temperature, Pressure, and Chemical changes are the three agents that we are going to study. Temperature increases can be caused by layers of sediments being buried deeper and deeper under the surface of the Earth.
As we descend into the earth the temperature increases about 25 degrees Celsius for every kilometer that we descend. The deeper the layers are buried the hotter the temperatures become. The great weight of these layers also causes an increase in pressure, which in turn, causes an increase in temperature. The descending of rock layers at subduction zones causes metamorphism in two ways; the shearing effect of the plates sliding past each other causes the rocks coming in contact with the descending rocks to change.
Some of the descending rock will melt because of this friction. When rock melts it is then considered igneous not metamorphic, but the rock next to the melted rock can be changed by the heat and become a metamorphic rock. There are 3 factors that cause an increase in pressure which also causes the formation of metamorphic rocks. These factors are;. The huge weight of overlying layers of sediments.
Stresses caused by plates colliding in the process of mountain building. Stresses caused by plates sliding past each other, such as the shearing stresses at the San Andreas fault zone in California. Factors that cause chemical changes in rocks also contribute to the formation of metamorphic rocks.
Very hot fluids and vapors can, because of extreme pressures, fill the pores of existing rocks. These fluids and vapors can cause chemical reactions to take place, that over time, can change the chemical makeup of the parent rock.
Metamorphism can be instantaneous as in the shearing of rocks at plate boundaries or can take millions of years as in the slow cooling of magma buried deep under the surface of the Earth. There are three ways that metamorphic rocks can form. The three types of metamorphism are Contact, Regional, and Dynamic metamorphism. Contact Metamorphism occurs when magma comes in contact with an already existing body of rock. When this happens the existing rocks temperature rises and also becomes infiltrated with fluid from the magma.
The area affected by the contact of magma is usually small, from 1 to 10 kilometers. Contact metamorphism produces non-foliated rocks without any cleavage rocks such as marble, quartzite, and hornfels.
In the diagram above magma has pushed its way into layers of limestone, quartz sandstone and shale. The heat generated by the magma chamber has changed these sedimentary rocks into the metamorphic rocks marble, quartzite, an hornfels. 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. Because rocks undergoing burial metamorphism encounter the uniform stress of lithostatic pressure, not differential pressure, they do not develop foliation. Burial metamorphism is the lowest grade of metamorphism. The main type of mineral that usually grows during burial metamorphism is zeolite, a group of low-density silicate minerals.
It usually requires a strong microscope to see the small grains of zeolite minerals that form during burial metamorphism. During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle. In most subduction zones the subducting plate is relatively cold compared with the high temperature it had when first formed at a mid-ocean spreading ridge. Subduction takes the rocks to great depth in the Earth relatively quickly.
This produces a characteristic type of metamorphism, sometimes called high-pressure, low-temperature high-P, low-T metamorphism, which only occurs deep in a subduction zone.
In oceanic basalts that are part of a subducting plate, the high-P, low-T conditions create a distinctive set of metamorphic minerals including a type of amphibole, called glaucophane, that has a blue color.
Blueschist is the name given to this type of metamorphic rock. Blueschist is generally interpreted as having been produced within a subduction zone, even if the plate boundaries have subsequently shifted and that location is no longer at a subduction zone.
The pressure and temperature conditions under which specific types of metamorphic rocks form has been determined by a combination labratory experiments, physics-based theoretical calculations, along with evidence in the textures of the rocks and their field relations as recorded on geologic maps. The knowledge of temperatures and pressures at which particular types of metamorphic rocks form led to the concept of metamorphic facies.
Each metamorphic facies is represented by a specific type of metamorphic rock that forms under a specific pressure and temperature conditions. Even though the name of the each metamorphic facies is taken from a type of rock that forms under those conditions, that is not the only type of rock that will form in those conditions.
For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for. However, if the protolith is shale, a muscovite-biotite schist, which is not green, will form instead. The diagram below shows metamorphic facies in terms of pressure and temperature condiditons inside the Earth.
Rocks are much denser than air and MPa is the unit most commonly uses to express pressures inside the Earth. One MPa equals nearly 10 atmospheres.
A pressure of MPa corresponds to a depth of about 35 km inside the Earth. Although pressure inside the Earth is determined by the depth, temperature depends on more than depth.
Temperature depends on the heat flow, which varies from location to location. The way temperature changes with depth inside the Earth is called the geothermal gradient, geotherm for short. In the diagram below, three different geotherms are marked with dashed lines. The three geotherms represent different geological settings in the Earth.
High-pressure, low-temperature geotherms occurs in subduction zones. As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions. High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a volcanically active area. Rocks that have their pressure and temperature conditions increased along such a geotherm will metamorphose in the hornfels facies and, if it gets hot enough, in the granulite facies.
Blueschist facies and hornfels facies are associated with unusual geothermal gradients. The most common conditions in the Earth are found along geotherms between those two extremes. Most regional metamorphic rocks are formed in conditions within this range of geothermal gradients, passing through the greenschist facies to the amphibolites facies.
At the maximum pressures and temperatures the rocks may encounter within the Earth in this range of geotherms, they will enter either the granulite or eclogite facies. Regionally metamorphosed rocks that contain hydrous fluids will begin to melt before they pass beyond the amphibolite facies. Metamorphic rock fall into two categories, foliated and unfoliated. Most foliated metamorphic rocks originate from regional metamorphism. Select personalised ads. Apply market research to generate audience insights.
Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Andrew Alden. Geology Expert. Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U. Geological Survey. Updated September 18, Featured Video. Cite this Article Format. Alden, Andrew. Properties of Metamorphic Rocks.
How to Identify the 3 Major Types of Rocks. Get to Know 24 Types of Sedimentary Rock. Slate Rock Definition, Composition, and Uses. Your Privacy Rights. To change or withdraw your consent choices for ThoughtCo. This will result in the formation of an igneous rock , not a metamorphic rock. Consider how granite changes form. Granite is an igneous rock that forms when magma cools relatively slowly underground.
It is usually composed primarily of the minerals quartz, feldspar, and mica. 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. Tyson Brown, National Geographic Society.
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