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Metamorphic Rocks

How Are Metamorphic Rocks Formed and What Do they Look Like?

Metamorphic rocks are formed from the alteration of pre-existing rock types (igneous, sedimentary or already formed metamorphic rocks) through metamorphism. The pre-existing rock, also known as protolith, is subjected to both heat and pressure, which causes chemical or physical changes. 

These types of rocks form a huge part of the Earth's crust and make up 12 percent of the Earth's surface.

They are grouped according to texture and how they undergo metamorphism:

  • Some metamorphic rocks are formed when rocks deep beneath the surface of the earth are subjected to high temperature and pressure of the layers of rocks above them.
  • Some are formed from tectonic processes which bring about friction, horizontal pressure and shape alteration.
  • Others are formed when a pre-existing rock is heated by magma.

More so, some examples of metamorphic rocks include: quartzite, slate, marble, gneiss and schist.

Foliation

Foliation is simply the forming of layers within metaphoric rocks. It happens when a rock is being reduced in size along a single axis as re-crystallization occurs. Foliated rocks occur as a result of increased strain that distorts the shape of the rock in a single plane. Slate, schist and gneiss are examples of foliated metamorphic rocks.

Non-foliated rocks occur when the temperatures are high but the pressure is low or equalized on both sides. These rocks do not have planar shapes of strain. Marble is a non-foliated rock; a characteristic that allows it to be used as a sculpting material and in architecture.

Where a rock has undergone differential strain, the kind of foliation that is formed is dependent on the metaphoric grade. For example, beginning with mudstone, the next process happens with increasing heat:

  • Slate is a metamorphic rock with very fine grains that has undergone foliation, this is the character of 'very low' grade metamorphism;
  • Phyllite has fine grains and is seen in sites of 'low grade' metamorphism;
  • Schist has coarse grains and is seen in sites of 'medium' grade metamorphism; and
  • Gneiss which has very coarse grains is seen in sites of 'high-grade' metamorphism.

Chemical reactions that are in-between minerals are yet another vital aspect of metamorphism. Atoms are exchanged between one mineral and another, leading to formation of new minerals. A lot of complex chemical reactions may occur in high-temperature.

Every mineral formed gives us an idea on the degree of temperature and pressure during metamorphosis.

Metamorphism is the significant alteration in the heavy chemical state of a rock that usually happens during metamorphism. It is as a result of introduction of new chemicals from the surrounding rock materials. Water is usually involved in transporting the chemicals over long distances. Due to the effect of water, in general, metamorphic rocks have a lot of elements that were not present in the original rock, and are deficient of some that existed in the original rock.

There are different types of metamorphism. The major ones are contact metamorphism and regional metamorphism.

Types of Metamorphism

1. Contact metamorphism

Contact metamorphism occurs when magma is induced into the surrounding rock structure. The changes that happen are drastic wherever the hot molten rock gets into contact with the solid rock. This is because the temperatures are greatest at this point and reduce with distance away from it.

Surrounding the igneous rock, is the 'contact metamorphism aureole'. A contact metamorphism aureole is a product of the cooling magma. Aureoles might display all levels of metamorphism from the contact zone to the unmetamorphosed solid rock a few distances away.

The production of essential mineral ores may happen through metasomatism at or just next to the contact region. Rocks that are changed by an igneous intrusion usually get more indurated and their crystals are increasingly coarse. A lot of rocks formed this way were in the past known as 'hornstones'.

'Hornfels' is a name used by geologists to describe the fine-grained, compacted and non-foliated rocks formed from contact metamorphism.

  • Shale may be altered to form dark argillaceous hornfels.
  • Impure limestone (marl) may become lime-silicate hornfels that are green, grey, or yellow in color. It may be changed also to siliceous marble which is splintery and firm with loads of garnet, augite, wollastonite and other minerals containing calcite.
  • Andesite (diabase) may be altered into andesite hornfels/diabase hornfels with formation of 'hornblende and biotite' and a semi-recrystallization of the pre-existing rock.
  • Chert/flint may be changed into quartz rock with fine crystals.
  • Sandstones may be transformed into quartzite.

If the pre-existing rock was foliated, such as a foliated calc-schist, this characteristic might not be distorted. Instead, these activities may occur:

  • Banded hornfelsmay form
  • Fossils may retain their shapes, although fully recrystallized
  • Vesicles can still be seen in lavas that have been changed by contact. Their components have normally gone into new reactions to create minerals that did not exist before.
  • The vesicles, however, may disappear entirely if the alteration by heat is profoundly great. As a result, small quartz grains in shale disappear or combine with the surrounding clay particles. The mass of lava (finely-grounded) is therefore completely reconstructed.

Recrystallization of this kind leads to production of peculiar rocks:

  • Shales may become cordierite rocks. They may also display huge crystals of garnet, andalusite, kyanite, staurolite and sillimanite, all gotten from the aluminous composition of the pre-existing shale.
  • Mica is usually simultaneously made and the resulting rock material is similar to many types of schist.
  • Pure limestones are normally transformed into marbles with coarse crystals. However, if there existed a mixture of clay/sand in the pre-existing rock, minerals like epidote, wollastonite, garnet and idocrase, will be seen.
  • When intensely heated, sandstones may be transformed into quartzites with coarse grains made of huge transparent quartz grains.

Metasomatism also happens between igneous rock and sedimentary solid rock, where chemicals in both are exchanged. Granites may take up pieces of shale or basalt. After this, hybrid rocks known as 'skarn' are formed, which do not have the properties of the original igneous magma or sedimentary rocks.

At times, granite magma invades and permeates the surrounding rocks, filling them with strings of feldspar and quartz.

2. Regional Metamorphism/ Dynamic Metamorphism

Regional metamorphism refers to the alterations made to huge masses of rock over a large area. Rocks can undergo metamorphism simply by existing deep underneath the Earth's surface. These rocks are subjected to extreme temperatures and intense pressure brought about by the great weight of the overlying layers of rocks.

Most of the underlying earth's crust is metamorphic in nature, apart from the newly made igneous intrusions. Horizontal tectonic forces like the collision of continents produce orogenic belts. The tectonic forces also lead to increased temperatures, pressures and alteration of the rocks along the orogenic belts.

When the metamorphosed rock structures are eventually raised and exposed due to erosion, they might form in elongated belts or other huge areas in the earth's surface. Metamorphism might have distorted the pre-existing features that could have shown the history of the rock. Recrystallization destroys the fossils and texture that exist in sedimentary rockswhereasmetasomatism will alter the pre-existing composition.

Regional metamorphism has a tendency of making the rock increasingly indurated. It also gives it a foliated, gneissic orschistosic texture, made up of planar setting of the minerals.Prismatic minerals such as hornblende and mica have their lengthiest axes in parallel position with each other. As a result, most of these rocks break easily in a single direction along the mica- bearing regions.

For gneiss, minerals are usually formed into bands. There exists seams of both quartz and mica in a schist of mica; very slim, but is made up of one mineral.

Along the layers of minerals made up of soft minerals, the rock will break very easily. The newly broken specimens will seem like they're coated with the mineral; for instance, a specimen of mica schist may visually seem to be made up of shiny mica scales. White folia of quartz which is granular in nature will however appear on the edges of the specimens.

For gneiss, the alternating folia are at times thicker and less defined than that of the schists, but they are also less micaceous. Gneiss has more feldspar compared to schist, and is tougher and less soft.

The two major kinds of foliation (gneissic and schistosity banding) are produced when direct pressure and extreme temperatures are subjected to interstitial movement. This leads to the alignment of particles of minerals as they crystalize in that pressure field. 

Pre-existing sedimentary and igneous rocks may be transformed into schists and gneisses via metamorphism. If they had similar composition in their original form, they may be hard to differentiate if the metamorphism was intense. For instance, both a quartz-porphyry and fine-grained feldspathic sandstone may be transformed into mica schist that is gray or pink in color through metamorphism.

3. Hydrothermal Metamorphism

This type of metamorphism occurs when a rock interacts with a hot fluid that has a different composition. Metaphoric and metasomatic processes occur due to the interaction of the different components. The hydrothermal fluid can come from magma, rotating ground water, or oceanic water.

Convectional movements of hydrothermal fluids in basalt rocks that are found in ocean floors, causes extreme hydrothermal metamorphism. The ocean has vents called "black smokers" through which the hot fluids escape. Patterns formed by the hydrothermal changes are utilized as guides when looking for metal ores. 

4. Shock Metamorphism

Shock metamorphism is caused by the collision of an extraterrestrial component with the surface of the earth during a powerful volcanic eruption.

There are many metamorphic rocks in existence with varying compositions and textures. To learn about the different metaphoric rocks, it is beneficial to see them visually and handle them. Here are some examples of well-known metamorphic rocks:

Some Examples of Metamorphic Rocks

Amphibolite

It is a metamorphic rock that consists mainly of plagioclase and hornblende/amphibole, usually with small amounts of quartz. Amphibolite is a non-foliated rock that occurs under directed pressure levels and great viscosity via recrystallization.

Gneiss

It is a metamorphic rock that is foliated and consists of granular mineral particles. It has high amounts of feldspar minerals, quartz bands, and at times mica. Gneiss resembles granite.

Hornfels

Hornfels are non-foliated metamorphic rocks that are formed through contact metamorphism, where clay is subjected to extreme heat. Hornfels do not have a unique composition. They get heated when they're adjacent to a source of heat like a magma chamber.

Lapis Lazuli

This is one of the rarest form of metamorphic rocks, particularly due to its blue hue. As a result, Lapis lazuli is well-known for its blue gem-like nature. It is crushed into small round stones to form beads for decorative purposes.

Marble

This is a well-known non-foliated metamorphic rock that is formed after dolostones or limestones are metamorphosed. It is naturally white in color and is used for creating sculptures and in building projects. It is mainly made up of calcium carbonate mineral.

Novaculite

It is a fine-grained, hard and non-foliated metamorphic rock that is naturally dense. It occurs in marine areas from the deposits of sediments. Novaculites usually split with conchoidal fractures. 

Phylitte

Phylitte is a non-foliated metamorphic rock that is mainly composed of mica (fine-grained) and at times chlorite. It has a lustrous surface and may be wrinkled as well in some instances.

Quartzite

Quartzite is a non-foliated metamorphic rock that is formed when sandstone is metamorphosed. It is made up of quartz crystals that interlock each other. Quartzite is naturally a hard rock.

Schist

This is a foliated metamorphic rock. It is made up of large amounts of mica minerals, as a result, schists can easily break into smaller layers. In some cases schist may have high levels of chlorite.

Slate

Slate is a foliated metamorphic rock that occurs when shale is metamorphosed. It has fine grains and belongs to the low-grade category of metamorphic rocks.

Soapstone

Soapstone is a soft textured, dense and heat resistant metamorphic rock. Soapstone is mainly composed of talc. It has a soapy feel and also contains amphiboles, carbonates, chlorite, mica and pyroxenes in different amounts. Soapstone is mainly used for artistic work and construction projects.

Yeah, this shit is boring! - I know. It’s for the serious knowledge seeking bums out there in the first place. So, let me know if I missed anything out. And, yes, that’s probably all you gotta know.

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