As you can tell from their name, metamorphic rocks are rocks that form from the transformation of pre-existing rocks of all types. Yes, the name is linked to the process that makes the rock, metamorphism. Keep on reading to find out exactly how they change!
THE FORMATIONMetamorphic rocks form under the surface of the Earth, in the crust. They can also form in the mantle, but those rocks are too dense to rise to the surface. Therefore, they are not observed. Uplift and erosion of the Earth's crust brings metamorphic rocks to the surfaceIt takes millions of years to form metamorphic rocks.
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Low grade metamorphic rocks form at low temperatures and pressures while high grade metamorphic rocks form at high temperatures and pressures. |
TWO MAJOR TYPES OF ROCK METAMORPHISM
REGIONAL METAMORPHISMRegional metamorphism also known as dynamic metamorphism is when changes in minerals and texture occur in large amounts of rock spanning thousands of kilometres. 5 km - 35 km below the crust rocks are subjected to high temperatures and great pressure from the immense amount of rocks above. This itself can be enough to change rocks, but sometimes tectonic forces are involved. Collisions between tectonic plates create orogenic belts, and cause increases in temperature and pressure. Plus collision can further squeeze rocks. All along these belts metamorphic rocks form. Mountain belts also known as metamorphic belts and cratonic areas were regionally metamorphosed rocks commonly occur. Ocean floor sediment that is subducted, with one tectonic plate under another, also undergoes regional metamorphism. These rocks are commonly foliated.
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CONTACT METAMORPHISMContact metamorphism occurs typically around intrusive igneous rocks. It's when changes occur when magma is injected into the surrounding rock. The rock's minerals and texture changes due to the extreme heat and contact with the magma. Metamorphic rocks form all around the boundary of the intrusion of magma. Important ores can form through metamorphism at or near the contact zone. The term hornfels is used by geologists to signify the fine-grained, compact, non-foliated products of contact metamorphism, while coarsely crystalline rocks are termed hornstones. In contact metamorphism, there is a tendency of metasomatism between the igneous magma and sedimentary country rock. This is when the chemicals from the rocks are exchanged or introduced into the other.
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CHARACTERISTICS OF METAMORPHIC ROCKS
There are two types of metamorphic rocks, the ones that are foliated and the ones that aren’t. Metamorphic rocks formed through regional metamorphism are commonly foliated, but in contact metamorphism, if the protolith was foliated this characteristic may not be destroyed. This means the rocks has ribbonlike layers of streaks of different colours and textures from different minerals. The layers can be curved and bent. Contortion or crumbling of the foliation is not uncommon. Foliation occurs when a rock within the Earth's interior is under extremely high pressure that is unequal. The pressure is directed, meaning that it is greater in one direction than in the others. This causes the rock to be shortened along one axis resulting in curved ribbonlike layers. Confining pressure results in non-foliated rocks, as the pressure is equal in all directions.
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Metamorphic rocks usually have no fossils as most would not be able to survive the temperatures as they would melt. However, sometimes they do survive the process, but the pressure and foliation of the rock alters the shape of the fossil, distorting and squashing it. Metamorphic rocks generally do not have any fossils in them.
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Most metamorphic rocks, foliated or not, have crystals. They are formed by minerals growing slowly over time. The crystals can be both large and small. Foliated rocks commonly have shiny crystals. New metamorphic minerals can form too.
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Examples of foliated metamorphic rock: Gneiss, Schist and Phyllite.
Examples of non-foliated metamorphic rock: Marble, Quartzite and Amphibolite
For more examples of metamorphic rocks see:
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ECONOMIC IMPORTANCE
Metamorphic rocks are used in sculptures. They are also used for construction, building, paving and in jewellery. Metamorphic rocks can be composed of important minerals and ores. Metamorphic rocks can be studied, and a lot can be found out, as all minerals are stable within certain limits. Therefore, new minerals can be formed by the reaction and metamorphosis of other minerals when they cannot stand the conditions any longer. Geologists can use this knowledge to find out the approximate temperature and pressure that formed the metamorphic rock.