Metamorphic rocks are created when another type of rock is exposed to extreme levels of heat or pressure. They are formed deep within the earth, although they can come up to the surface eventually. There are many different kinds of metamorphic rocks. They are all unique in where they can be found, how they look, and what they can be used for.
What Are Metamorphic Rocks?
Metamorphic rocks were not always metamorphic, and that is how they get their classification. Metamorphic rocks are formed by another type of rock, either an igneous rock or sedimentary rock.
The main difference between these rocks is that metamorphic rocks are formed from already existing rocks. Sedimentary and igneous rocks, on the other hand, are formed into rocks by another material.
Igneous rocks are formed when magma, or lava, has emerged from the earth’s center and came to the earth’s surface. The lava then cools at the earth’s surface and hardens into what we know as an igneous rock.
Igneous rocks are defined as a rock made of silicate materials like muscovite, orthoclase feldspar, or quartz.
Sedimentary rocks are formed from sediments that were eventually compacted under extreme pressure.
Sedimentary rocks are the most common rocks on the surface of the earth. These are the rocks that are exposed to the elements that we are most likely to see. Although they are the most common rocks at the earth’s surface, they only make up a small portion of the earth’s crust.
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How Are Metamorphic Rocks Made? Can They Withstand The Heat & Pressure?
What is the process of metamorphic rock formation? A metamorphic rock formation is created when the original rock is exposed to extremely high heat, pressure, or fluids rich with minerals. Usually, it is not just one of these elements that causes a rock to become a metamorphic rock. Rather, it is usually a combination of these elements.
For a rock to be transformed by heat, it must be exposed to extremely high temperatures. When rocks are transformed by heat, it is referred to as “thermal metamorphism”. This simply means that the higher temperatures were the main factor in transforming the rock.
Temperatures of 302-392 degrees Fahrenheit (150-200 degrees Celsius) are needed to change a rock into a metamorphic rock.
Because of this, metamorphic rocks transformed by heat must be transformed deep within the surface of the earth. Specifically, the rocks are located where magma is either created or expelled from the center of the earth. The metamorphic rocks are created very close to the magma, but not actually inside it, which is one reason why the rocks do not melt.
In the same way, if the rocks are to be transformed by pressure, they must deal with extreme levels of pressure. When a metamorphic rock is transformed by extreme pressure, it is referred to as “cataclastic metamorphism”.
At least 100 megapascals of pressure is needed to turn a normal rock into a metamorphic rock. This incredible amount of pressure is formed by shifting of tectonic plates. Tectonic plates are always moving, and when two or more tectonic plates come together, there is a great deal of friction and pressure being created.
When tectonic plates rub against one another, parts of the plates become crushed, or become broken off altogether. When pieces of the tectonic plate are broken off, it results in pieces of rock that are very tiny.
There is another process called, “mylonitic metamorphism”. This process is exactly the same as cataclastic metamorphism, but the result is different. Rather than rocks being broken off into small particles, mylonitic rocks are ground down into pieces the size of grains.
Sometimes, metamorphic rocks are not formed solely by extreme heat or pressure. Sometimes, the rock is formed from exposure to both extreme heat and pressure.
When this occurs, the rock’s physical and chemical composition begins to change. The texture and mineral contents of the rock may shift as well. What is left is a rock that looks completely different than it did before, and is composed of completely different elements.
Rocks do not become metamorphic rocks every day, though, because it can be hard to find the right conditions to make the transformation possible. For a rock to be exposed to these conditions, it will need to be at the convergence of a tectonic plate, or it will be buried deep within the earth.
When rocks are exposed to these elements, they are compacted into denser rocks. They are not melted, but instead, new minerals are created. This occurs because mineral components are rearranged or created when the rock is exposed to fluids.
Metamorphic VS Igneous Rocks
Metamorphic rocks and igneous rocks are formed in similar ways, but there is an important distinction between the two. Metamorphic rocks can be transformed when exposed to high heat, but they will never melt. Igneous rocks are also formed through exposure to high heat, but unlike metamorphic rocks, they will melt before reforming into rocks.
Igneous rocks are formed by melting, cooling, and crystallization into a new rock. Metamorphic rocks, on the other hand, will change their composition over time.
Metamorphic rocks are not commonly seen because their transformation process occurs well below the surface of the earth. You may be wondering how they are ever seen at the earth’s surface, then. The answer is that the rocks are lifted to the surface after a geological shift.
This shift occurs over time as the rocks on the surface are eroded by wind and rain that eventually tears down the rock. Because of this, metamorphic rocks will come to the surface of the earth slowly over time rather than becoming exposed all at once.
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Types Of Metamorphic Rocks
Metamorphic rocks can be formed either from extreme heat or pressure. However, these rocks are not classified by the way they’re formed. Instead, they are classified by how the minerals fit in with the newly metamorphosed rock.
Therefore, there are two different kinds of metamorphic rock: foliated rocks, and non-foliated rocks.
Foliated rocks are characterized by minerals who have become aligned with one another. This is generally caused by exposure to extreme pressure combined with extreme heat. Together, the pressure and heat force the minerals to form into a unified pattern.
When metamorphic rocks are foliated, their appearance is more uniform. You will notice the rock is created in layers. This is most commonly found in rocks such as gneiss, schist, or slate.
Non-foliated rocks are characterized by minerals that have not been uniformly aligned with one another. In foliated rocks, the minerals match up so that the rock appears to have somewhat uniform layers. Non-foliated rocks, on the other hand, produce minerals that are irregular. The minerals do not elongate, so they don’t form layers like the foliated rocks do.
Non-foliated rocks include rocks like quartzite, hornfels, marble, or soapstone.
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Metamorphic Rocks Examples
Below are a few of the most common metamorphic rock examples:
A metamorphosed quartz that has a strong crystalline structure that is dense and hard to the touch. These rocks are formed from regional metamorphism from sandstone or chert.
Grains of quartz sand will melt together when exposed to heat and pressure. The sand will then recrystallize to form the quartzite.
There is a second way in which quartzite can be formed, though. Instead of melting, the sandstone will be exposed to much lower pressures and temperatures. Instead, the spaces in the stone will be filled with silica cement.
However, this kind of quartzite is considered a sedimentary rock rather than a metamorphic rock. This is because the original minerals found within the rock are still present.
The stone will usually be white or gray, but it may also be a light shade of pink if the sand contains iron oxide.
A metamorphic rock derived from limestone or dolomite which are sedimentary rocks that have been carbonated.
Limestone is composed mostly of calcium, while dolomite is composed mostly of magnesium. Marble is extremely strong and beautifully patterned, so it is highly sought out for building materials.
These rocks are created by regional metamorphism where the microscopic grains of the rock combine to form larger crystals.
There is a large variety in the color of these rocks, with some being a pure white, and others being a dark black. Most marbles are a combination of a lighter and darker color.
Formed when mineral talc is transformed into a hard, solid rock. Mineral talc is rich in magnesium. It is derived from the alteration of peridotite and ultramafic rocks.
These rocks are best suited for creating carvings, especially countertops because they are resistant to cracks and stains.
This type of metamorphic rock can fall into two different categories: orthogneiss or paragneiss. Orthogneiss is derived from igneous rock, while paragneiss is derived from sedimentary rock.
These rocks are noticeably foliated because you will see distinct bands across the surface of the rocks. Often, these rocks are a white color with prominent bands of dark gray. There also might be some hints of copper.
The dark gray bands that you see on these rocks are formed by minerals such as hornblende which cannot be found within regular sedimentary rocks.
Although you can clearly see these bands, they only make up a fraction of the rock’s composition. Less than 50% of the minerals found within the rock are formed into the layers. However, this makes the layers stand out more clearly against the lighter parts of the rock.
Unlike schist, the layers formed in gneiss are not very thick or uniform. Instead, they are thinner and spread out like vines or roots across the rock.
Gneiss is produced by regional metamorphism where the rock is formed from a sedimentary or igneous rock deep within the earth’s crust. This rock would have been subjected to very high temperature and pressures so that the rock’s original structures have been changed.
If given enough time under these conditions, gneiss can even continue to transform. It will go from gneiss to migmatite, and can eventually turn into granite.
Gneiss are believed to be the oldest kinds of rock on earth. Some gneiss have been found in northern Canada that are about 4 billion years old. This is also the kind of rock that makes up the largest portion of the earth’s lower crust.
A type of shale that is metamorphosed through regional metamorphism.
It is formed when its clay minerals are put under extreme pressure and lower temperatures. The caly then begins to transform back into the mica minerals that it consisted of originally.
If slate is allowed to metamorphose for long enough, it will eventually transform into phyllite, which will further metamorphose into schist or gneiss.
Its texture is smooth, and the coloration is typically a uniform, dark gray. These rocks are fairly easy to split, so they are often used for building materials.
Schist is created through regional metamorphism. It is characterized by coarse mineral grains which are split up into thin layers. It is formed when it is put under high pressure and extreme heat so that the minerals align with the other components within the rock, like mica or hornblende.
At least 50% of these minerals are arranged within the rock so that it forms very thin layers
Blueschist is transformed when it is exposed to high pressures, but significantly lower temperatures. This usually occurs at areas of subduction, where parts of the earth’s crust are dragged down into the earth’s core. This most commonly occurs in marine territories where the marine crust is dragged down beneath a continental plate.
Then, the crust is worked by the grinding of the tectonic plates and is subjected to sodium-rich fluids.
It is characterized as a schist because the original structure of the rock has been completely altered, as well as the mineral composition.
Not all blueschists are actually blue, however. The rocks that appear the most blue have high concentrations of sodium. These are most often rocks like gabbro or basalt.
Like blueschist, greenschist is formed by regional metamorphism in conditions of very high pressure, but significantly low temperatures. The conditions needed to create greenschist are not as extreme as those needed to create blueschist.
As the name suggests, Amphibolite is a rock composed primarily of amphibole minerals. Most often, it is a hornblende schist because hornblende is the most common kind of amphibole.
These rocks are formed from basaltic rock that is exposed to temperatures of 1,022-1,382 degrees Fahrenheit (550-750 degrees Celsius), as well as pressure that is slightly higher than that which produces greenschist, another type of metamorphic rock.
Argillite is a type of metamorphic rock that looks very similar to slate. It is a type of claystone that was exposed to a more moderate temperature and pressure.
These rocks are very easy to carve and were readily used by Native Americans to create pipes, ceremonial objects, and other decorative pieces.
This metamorphic rock is composed of very fine grains that were created by the grinding of rocks. The rocks are grinded so much that it results in very fine particles called cataclasis.
Eclogite is a metamorphic rock created when basalt is put under extremely high temperatures and levels of pressure.
Greenstone is a type of basaltic rock that has been altered. These rocks come from solida lava that was created deep within the sea. Basalt is made up of olivine and peridotite. A rock is transformed into greenstone when the basalt is subjected to high pressures and warm temperatures.
These rocks are generally formed within subduction zones, and they rarely make it to the surface of the earth without their composition being changed.
You will notice that these rocks are mostly a dark gray, but that they have a white, marble-like pattern throughout them. The white portion is made up of aragonite which is a crystal-like form of calcium carbonate.
These metamorphic rocks are definitively foliated with its tough, fine-grained rocks. They are typically a darker gray color with even and distinct layers in a lighter gray.
Hornfels are created through contact metamorphism which refers to magma baking and recrystallizing around the rocks.
Migmatite rocks are made out of the same materials as gneiss, but it is brought closer to its melting point than gneiss is. It is formed through regional metamorphism and is exposed to temperatures so hot that it forms layers that look like chaotic waves.
These rocks are usually made up of a combination of rocks, the darker parts created from biotite mica and hornblende, while the lighter parts are made of quartz and feldspar.
Mylonite is created when the rock is stretched and crushed under extreme heat and pressure. The rocks are molded in a way that resembles plastic. They are typically formed along fault surfaces that are buried deep within the earth.
Phyllite is one of the more picturesque kinds of metamorphic rock because it has a definite shine to it. The rocks are usually a medium gray, but may have some green hues to it. You will notice white veins running throughout the rock.
The texture and look of phyllite looks very similar to that of slate. They are easy to tell apart, though, as long as you have the sun on your side. When in the sun, phyllite very clearly has a shine to it, while slate will remain quite dull.
As I mentioned, you will notice fine, white veins running throughout the rock. If the rock is held under pressure and heat for long enough, it will continue to metamorphose. The more it metamorphosizes, the more veins will sprout up on the surface of the rock.
Slate is generally an easier material to work with when building things as well. Slate will typically break off into flat, even sheets. Phyllite, on the other hand, breaks off with grooves and dips.
Serpentinite is also formed by regional metamorphism. They are created from rocks from the ocean mantle found within the deep sea.
The rocks are made up of serpentine minerals, which make the rock look like it is made from snakeskin. The rock has a waxy appearance with mottled coloring.