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One of three main rock types in the world, metamorphic rocks are made up of other rocks when some physical or chemical process changes them and is the hardest of the three rocks.
The process of one rock changing into another is termed metamorphosis and this is the reason why these rocks are called metamorphic rocks. The previous rocks after transformation become more compact and denser than the original rocks.
The three types of rocks that are known to us are sedimentary rocks, igneous rocks and metamorphic rocks. Among them, the igneous and sedimentary can be regarded as parent rock and metamorphic can be termed as child rock since it is formed from the alteration of the parent rocks. Metamorphic rocks can even form from other metamorphic rocks. When metamorphism takes place, the minerals in the pre-existing rocks change into new minerals having different shapes, orientations and sizes. It might also be observed that the chemical properties of the previous rock may change due to some elements being carried away and some new elements added by the liquids that flow through the rocks. There might be layered or granular metamorphic rocks. Extreme pressure and heat can lead to metamorphism in a number of rocks. There are various examples of metamorphic rocks based on the metamorphic grade that are available throughout the world. Fossils are mostly not found in metamorphic rocks, so it is not known how they would look.
Check out space rocks and three types of Metamorphic Rock as well for more great content.
Metamorphic rocks can be broadly divided into two types: foliated metamorphic rocks and non-foliated metamorphic rocks.
The first type is the foliated metamorphic rock, which is the most common group of metamorphic rocks. They are created when the rock gets direct exposure to pressure along with heat. The foliated metamorphic rocks normally have a layered or banded appearance. They have aligned textures that are might be one of four different varieties. The word ‘foliated’ is a special term that means that the rocks have a striped appearance due to the parallel arrangement of some of the mineral grains. This type of rock is formed when the minerals present inside the rock are squeezed or elongated so that they come into alignment. The sheet-like or platy structure of foliated metamorphic rocks reflects the direction in which pressure was applied.
Some common examples of foliated metamorphic rock formation are phyllite, schist, gneiss and slate.
Non-foliated metamorphic rocks, on the other hand, are created due to direct pressure or by tectonic movements under the surface of the planet. These rocks lack a layered or banded appearance like the foliated metamorphic rock type. Non-foliated metamorphic rocks like limestone, which is also the protolith of marble, are made up of minerals that are neither elongate nor flat. The minerals will not align even if you apply a fairly large amount of high pressure to them.
The most commonly known example of non-foliated metamorphic rock is a marble. Some of the few other examples include novaculite, quartzite and hornfels.
During metamorphism, the rocks change so much that they turn into a wholly new rock. This change might be due to chemical reactions or a physical process. The rock cycle is the reason why metamorphic rocks form.
Metamorphism often causes the layering of rocks. When this happens, it might seem that the layers will peel off from the upper surface if you try to break them. The new rocks might be folded, smeared out, or squished.
Metamorphic rocks are originally igneous rock, sedimentary rock or other metamorphic rock. Pressure or heat causes the rearrangement of mineral crystals in the rocks. The new minerals are more suitable and stable in the environment that has been newly created. Foliation of rocks occurs when extreme pressure causes rocks to get squeezed. This can happen only if the pressure gets exerted from one direction. But no foliation will happen if this pressure is coming from all directions.
The conditions which are favorable for the formation of metamorphic rocks can be found deep inside the Earth’s surface and in places where a meeting of tectonic plates occurs. But it must be remembered that the heat of the magma in the core is not high enough for the rocks to melt, otherwise, they would change into igneous rocks.
Magma intrusions and tectonic movements cause the rocks to shift and move. This movement is the driving factor behind the heat and extreme pressure to which other rocks are exposed. And consequently, changes occur in the chemical composition, mineralogy and texture of the rocks. High -temperatures between 302-1463 °F (150 °C - 795 °C) have the ability to produce the high energy that is required to break and realign the chemical nature of rocks.
Metamorphism can be classified into two broad types. These are regional metamorphism and contact metamorphism.
Contact metamorphism is a process that takes place when hot magma comes into contact with a rock transforming the rock by extreme heat. The parent rock does not become completely melted rock due to the heat. Only the mineral crystals get rearranged which can be noticed by us. Contact metamorphism occurs when a parent rock is intruded on by hot igneous rock. The former is almost baked due to the heat, changing its mineral structure.
Regional metamorphism is a process in which large rock masses undergo a change over a wide area when the rocks face pressure near the plate boundaries. This pressure is formed due to the presence of tectonic forces.
More than a hundred varieties of rocks formed from metamorphism are present on the earth, each having its own texture and physical compositions. Let us read about some of them.
One of the most well-known rocks in the world, marble, is a product of metamorphosis. It is a non-foliated metamorphic rock that is formed from limestone or dolostone. It can be polished and therefore marble is used all over the world as a building material and for making sculptures. The main component of marble is calcium carbonate. The metamorphosed limestone has crystals that are in a medium-grained interlocking position without having any alignment. There are several colors of marble because of some impurity that is present in the rock. The colors can be gray, green, black, red and white, among others.
Slate is both fine-grained metamorphic rock and low-grade metamorphic rock. It can be divided into thin pieces as it is a foliated rock. The low-grade metamorphism of shale creates the slate which consists mainly of realigned clay minerals. The crystals all lie in the same direction that is also known as the cleavage direction. The rock gets split very easily along the cleavage direction. Clay and silt get deposited in the shale and it goes deeper into Earth. The pressure and temperature under the ground change the rock into slate. The rock is dark or light brown in color. Red slate is found on the east coast of Newfoundland.
Quartzite that is a non-foliated metamorphic rock is formed from quartz sandstone. It consists of interlocking quartz crystals. The rock is granular and white in color. This rock is used in the crushed form in the building of railroad tracks.
Schist has a shiny appearance as it has the mica mineral in it. It has crystals in the medium-grained interlocked position. The rock has a wrinkled texture. It is formed from the metamorphosis of shale or claystone. The grains and the layers can be seen going in the same way in schist rocks. The presence of mica allows the rock to be split into finer layers. Some amount of chlorite may be present in these foliated rocks.
Clay rock exposed to heat by the process of contact metamorphosis creates fine-grained rock hornfels. The rocks get warm when it is present near a source of heat such as a magma chamber, dike or sill.
The metamorphic rock that is formed from igneous rock granite is gneiss. Heat and pressure create gneiss. A banded texture of dark and pale layers can be seen on these foliated rocks. The dark and light bands are distinct features of gneiss.
Here at Kidadl, we have carefully created lots of interesting family-friendly facts for everyone to enjoy! If you liked our suggestions for what do metamorphic rocks look like then why not take a look at a rock made of shells and corals cemented together, or facts about little rock Arkansas.
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