What are metamorphic rocks?

Metamorphic rocks are rocks whose structure and appearance have been affected or changed by greater heat, pressure and chemical processes, usually while buried deep below the Earth’s surface or where tectonic plates meet. Exposure to these extreme conditions has altered the mineralogy, texture, and chemical composition of these rocks. Examples of metamorphic rocks whose normal structure and appearance have been affected by great heat and pressure are

• Clay changed to slate
• Limestone changed to marble
• Granite to gneiss
• Shale to schist,
• Coal to graphite and lastly
• Sandstone changed to quartzite.

All types of rocks can be subjected to changes under temperature and pressure; they can be heated or deformed such that their appearance is completely changed. However, metamorphic rocks are derived from igneous or sedimentary rocks that have altered their form (recrystallized) as a result of changes in their physicalenvironment. The new rocks are formed from the old by the process of metamorphism. The term meta means change, while morph means form. However, sedimentary and volcanic processes take place outside our range of visibility.

Metamorphism

Metamorphism takes place in response to changes in temperature and pressure, as soon as sedimentary processes (lithification) stops, metamorphism begins at about 150⁰C but this depends on the pressure involved. The upper limit of metamorphism occurs when rock begins to melt. This however, depends on the composition of the rock involved. The presence of water and the pressure, the PT space between these two limits defines the conditions under which metamorphism normally takes place.

Metamorphic processes usually takes place in the solid state, the process does not melt the rocks, but instead changes them into denser, more compact rocks. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Pressure or temperature can even change previous metamorphosed rocks into new types. Metamorphic rocks are often squished, smeared out and folded. Despite these uncomfortable conditions, metamorphic rocks do not get hot enough to melt; else, they would become igneous rocks. If temperatures are so high as to melt a rock, the results on cooling are igneous rocks. Melting marks the upper limit of metamorphism. Metamorphic rocks form a vital part of the rock cycle. Metamorphic rocks are usually different from the original rocks from which they were formed for example a clay- rich sedimentary rock (a shale) can be metamorphosed to a coarse grained banded rock containing shiny micas, quartz and grains of garnet. Black, vesicular, fine-grained basaltic lava can be metamorphosed to become a heavy green and red rock consisting of red garnet and green pyroxene (an eclogite).

Causes and Effects of Metamorphism

Heat, compression and chemically active fluids (water) are the main agents that work singly or collectively to transform rocks. These metamorphic media increase the internal density of the rocks and the dimension of the crystals and create clear foliation. Due to the application of temperature and pressure, the minerals of the rock become compacted like the leaves of trees and are arranged in parallel layers. This characteristic of layered structure of rocks is called foliation.

Heat: pure limestone consists of calcite (CaCO₃), when limestone is heated, calcite grain boundaries migrate and the grain size increases. Any fossil that was present become indistinct and finally all trace of them disappears, and then the limestone has crystallized in the solid state. The rock melted, but has not changed its chemical composition. A textural change has taken place and the rock has become a marble- the product of formed from the metamorphism of limestone. Most rocks however form new minerals during metamorphism. Clay minerals for instance, are not stable at high temperatures, the reason may be that they contain so much water in their structure and therefore they breakdown to form mica. Newly formed large mineral grains in metamorphic rocks are called porphyroblasts; these are the metamorphic equivalents of phenocrysts in igneous rocks.

Pressure: pressure increases with depth below the surface of the earth simply because of the weight of the overlying rocks. This process is called lithostatic pressure. The density of the rocks involved plays a big role, but on average in continental crust. The effect of pressure can produce some metamorphic reactions, both pressure and temperature increase with depths.

Water: hot water that passes through rocks at depth below the surface (hydrothermal solutions) dissolves some materials and deposits others. The composition of the hydrothermal solution depends on the type of rocks involved. In limestone and marble for example, they will be carbonate-rich in granites they silica-rich. Hydrothermal solutions that pass through relatively cold rocks that can fracture commonly deposit minerals in joints to form veins, these veins are commonly filled with quartz (or calcite in carbonate-rich rocks). Quartz is such a common vein-filling mineral since SiO₂ is a product of many metamorphic reactions. This water contributes to hydrothermal solutions.

Differential stress

Material subjected to different pressures in different directions becomes deformed. There are two types of differential stress: normal stress and shear stress. Normal stress operates perpendicular to a surface, the most usual situation involves compression (where rocks are squashed) but tension can be involved (where rocks are stretched). Shear stress moves one part of a material sideways relative to another. Rocks near the surface of the earth reacts in a brittle way to deformation, they break to form faults or fractures. Rocks at the depth undergo metamorphism, however, are not brittle, they are ductile and react plastically. Deformation under elevated pressure and temperature conditions commonly results in the development of a preferred mineral orientation of foliation. Metamorphic foliation forms by several processes. Grains can deform plastically and become squashed, while part of a grain may dissolve where the pressure is greatest and be re-deposited where the pressure is less by a process known as pressure solution. However, grains may rotate and new metamorphic minerals can form; these can all contribute to the development of a metamorphic foliation.

Types of Metamorphic Rocks

1. Foliated metamorphic rocks
2. Non-foliated metamorphic rocks

There are two main types of metamorphic rocks; there are those that foliated and those that are non-foliated metamorphic rocks. Most non-foliated rocks have been subjected to the effects of temperature (under constant pressure) whereas foliated rocks have been deformed during heating.

Foliated metamorphic rocks

The progressive metamorphism and deformation of clay-rich sedimentary rocks are called pelites and the metamorphic products are metapelites. One important constituent of pelites in terms of their chemical composition is that they are aluminium rich. During the metamorphism of politic rocks, a series of mineral with layered structures are formed (phyllosilicates), when these phyllosilicate becomes oriented in a parallel fashion style, they impart a foliated structure to the rock.

The first foliated metamorphic rock to form during metamorphism of pelite rock is slate. Some of the clay minerals become unstable under the elevated temperature and break down to form new phyllosilicates. These develop with their layered-structure perpendicular to the direction of pressure. Slate used to be widely utilized as a roofing material, it is sometimes possible to see the trace of the original bedding plane in a slate where sand rich layer are preserved. Slaty cleavage is formed at the same time as folds during the deformation and metamorphism of pelittic rocks. With higher temperature, the white mica muscovite starts to form from the breakdown of clay materials. The rocks becomes slightly coarser-grained and the surface develops a shiny appearance (like a siliky lustre. once slate has become a phyllite, the surface of the phyllite is commonly crenulated. As the temperature increases the grain size increases as well, the muscovite flakes become larger and dark mica may form ( biotite). The phyllite then becomes a mica schist. Additionally, mica schists mostly contain other metamorphic minerals. As the conditions of the metamorphism increase, the rock becomes even coarser-grained and a banding (layering0 is formed as well. The banding commonly consists of light and dark layers in which the light layers are dominated by quartz and felspar, while the dark layers contain minerals such as boitite. Hornblende, garnet, pyroxene etc. the shcist then becomes a gneiss.

Slate: Slate is produced from shale. This type of rock contains very fine crystals which are mainly formed by small plates created from mica. Slate is used to produce writing slates, blackboards etc.

Non-foliated metamorphic rocks

Non-foliated metamorphic rocks are formed when metamorphism takes place under constant pressure, when this happens, an intrusion of granite magma will heat its so-called country rock envelope. The temperature of the country rocks depends on the distance from the intrusion. The size of the intrusion and the composition of the magma are also very important. For example, granite, (rhyolitic) magma at 700⁰C will not heat the country rocks as gabbroic (basaltic) magma at 1200⁰C. The country rocks further away from the intrusion will simply crystallize where new metamorphic minerals will form as the heat source is approached. non-foliated metamorphic rocks that have been baked because of their nearness to an intrusion are called hornfels. Hornfels is a relatively fine-grained, massive rock. Any kind of protolith can be subjected to contact metamorphism to produce, for example, metamorphic (clay-rich sedimentary rock prior to metamorphism). Two important non- foliated rock types include marble (metamorphosed limestone) and quartzite 9 metamorphosed sandstone) both of these rocks may occur as foliated varieties if they are deformed during metamorphism.

Marble: It is a coarse-grained crystalized rock which has been formed from limestone or dolomite.

Quartzite: It is a very hard metamorphic rock which is mainly created from sandstone.

Picture showing the formation of foliation

Examples of Metamorphic Rocks

1. Gneiss
2. Slate
3. Phyllite
4. Schist
5. Hornfels
6. Amphibolite
7. Marble
8. Quartzite
9. Novaculite
10. Lapis Lazuli
11. Soapstone

Gneiss- gneiss is a foliated metamorphic rock that is made up of granular mineral grains. Gneiss is coarser than schist and it has distinct banding. This banding has alternating layers that are composed of different minerals. The minerals that compose gneiss are the same as granite. Feldspar is the most important mineral that makes up gneiss along with mica and quartz. Gneiss can be formed from a sedimentary rock such as sandstone or shale, or it can be formed from the metamorphism of the igneous rock granite. Gneiss is commonly used as paving and building stone.

Gneiss is an example of foliated metamorphic rock

Slate- slate is a foliated metamorphic rock that is produced from the metamorphism ofshale. This type of rock contains very fine crystals which are mainly formed by small plates created from mica. Slate is used to produce writing slates, blackboards, for making headstones or grave materials. etc. slate is not very hard and can be engraved easily, hence it is a low-grade metamorphic rock that splits into thin pieces.

Slate is a foliated metamorphic rock

Phyllite- thisis fine foliated metamorphic rock that is found in areas of low-grade metamorphism, it is made of mainly of very fine-grained mica. The surface of phyllite is typically lustrous and sometimes wrinkled. It is intermediate in grade betweenslateandschist.

Phylite, a foliated metamorphic rock

Schist- schistis a metamorphic rock with well-developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces. It is a rock of intermediate metamorphic grade between phyllite andgneiss.

Schist, a foliated metamorphic rock

Hornfels- this is a non-foliated metamorphic rock which is normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rock. In some cases, hornfels has visible crystals of minerals like biotite or andalusite. If the hornfels in this situation occur without directed pressure, then these minerals would be randomly orientated, not foliated as they would be if formed with directed pressure.

Hornfels- is a non-foliated metamorphic rock

Amphibolite- this is a non-foliated metamorphic rock that is formed through recrystallization under conditions of high viscosity and directed pressure. It is composed primarily ofhornblende(amphibole) andplagioclase, usually with very littlequartz. Amphibole should not be mistaken with the metamorphic facies of the same name. Amphibolites is a widespread rock type

Amphibolite is a non-foliated metamorphic rock

Marble– marble is a non-foliated metamorphic rock that is produced from the metamorphism oflimestoneordolostone. Marble has many different sizes of crystals and has many color variances due to the impurities present at its formation. Some of the different colours of marble are white, red, black, mottled and banded, gray, pink, and green. This stone has many uses such as for building, for making sink tops, bathtubs, and it is a carving stone for artists.

Marble, a non-foliated produced from the metamorphism of limestone or dolostone.

Quartzite- it is a non-foliated metamorphic rock composed of sandstone that has been metamorphosed. It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Quartzite is much harder than the parent rock, sandstone. It forms from sandstone that has been exposed to deeply buried magmas. Quartzite looks similar to its parent rock. The best way to differentiate quartzite from sandstone is to break the rocks apart. Sandstone will shatter into many individual grains of sand while quartzite will break across the grains.

Quartzite, a non-foliated metamorphic rock composed of sandstone

Novaculite- novaculiteis a dense, hard, fine-grained siliceous rock that breaks with a conchoidal fracture. It is formed from sediments deposited in marine environments where organisms such as diatoms (single-celled algae that secrete a hard shell composed of silicon dioxide) are in abundant in the water.

Novaculite, a dense, hard, fine-grained siliceous rock

Lapis Lazuli– this is one of the rarest metamorphic rocks, especially because of its blue color. Thus, Lapis Lazuli is famously known for its blue gem material and they are used for decoration and to make beads in the form of round small stones.

Lapiz Lazuli, is one of rarest metamorphic rocks used in decorations and making of beads

Soapstone- soapstoneis a soft dense heated resistant metamorphic rock with a high specific heat capacity consisting primarily oftalcwith varying amounts of other minerals such as micas, chlorite, pyroxenes, amphiboles and carbonates. These properties make it useful for a wide variety of architectural, practical, and artistic uses. The reason why it is named soapstone is because of its slippery feel when touched. Soapstone may be schistose (talc schist) or massive.

Soap Stone is a metamorphic rock consisting primarily of talc with varying amounts of other minerals

Metamorphic rocks characteristics

Below is a list of the characteristics of metamorphic rocks.

1. Metamorphic rocks are formed form pre-existing rocks through the process of metamorphism.
2. Metamorphic rocks rarely contain fossils.
3. Common examples of metamorphic rocks are gneiss, schist, marble, slateetc.
4. Metamorphic rocks can become banded or foliated (the arrangement of minerals that gives the rock a striped appearance).
5. Different colours and texture exist in metamorphic rocks.
6. These rocks mainly occur in layers.
7. Metamorphic rocks are crystalline rocks (they have visible crystals) and the amount of time they take to cool depends on the size of these crystals.
8. Metamorphic rocks may be composed of only one mineral, for example, marble and quartzite.
9. Metamorphic rocks are usually made of mineral crystals of different sizes.
10. Metamorphic rocks may be hard or soft.
11. They may have pores or openings.

Where are Metamorphic Rocks Found?

• Metamorphic rocks are found anywhere plates tectonic meet.
• They are anywhere that sedimentary or other igneous rocks have undergone transformation due to heat and/or pressure.
• Metamorphic rocks are found near the environment of igneous intrusive bodies.
• Metamorphic rocks are found around old mountains and where mountains have been eroded away. Metamorphic rocks often represent the roots of mountains. They also form where the rock has been deformed by folding.
• Some of the places you can find metamorphic rock are New EnglandStates, Southern New York. New York City is built on metamorphic rocks, the Adirondacks and the region called the Canadian Shield is all metamorphic rocks.
• Most metamorphic rocks found at the surface of the earth in the Precambrian shield areas (ancient continents) and on mountain belts, the Precambrian shields are dominated by gneissic rocks. Mountain belts are formed as a result of continental collision. The young mountain belt that stretches roughly east-west from the Alps to the Himalayas was formed, and still being formed as a result of the collision of the African and Indian continental plates with the European and Asian continental plates. Gneissic areas represent the root zones of ancient mountain belts.