METAMORPHIC ROCKS

Metamorphism And Its Types

Metamorphism is a geological process that changes the mineralogical and chemical composition, as well as the structure of rocks. Metamorphism is typically associated with elevated temperature, pressure and shearing stress at considerable depth thus it affects rocks within the earth’s crust and mantle. The Older rocks may be either sedimentary, Igneous or other Metamorphic rocks. The process is driven by changing physical and/or chemical conditions in response to large-scale geological dynamics. During metamorphism recrystallization takes place essentially in the solid state and new minerals and new textures are produced.

Agents Of Metamorphism

Metamorphic changes in the rocks are primarily the result of three main factors that are also sometimes called agents of metamorphism

  1. Temperature 
  2. Pressure 
  3. Chemical Environment

 

 1. Temperature

Rocks are made up of minerals that are normally stable at temperature below 200º C. However when the temperature around these rocks changes due to one or the other reason, the mineral composition of the rocks undergoes some changes in order to adjust to the new temperature conditions. Two common source of heat for such metamorphism to take place are the internal heat (increase in temperature with depth) and magmatic heat ( rise in temperature around magmatic intrusions).The internal heat becomes operative when rocks formed at surface are pushed downwards due to some geological process where they have to withstand much higher temperature. Similarity, when a magmatic intrusion like a sill or a dyke invade the country rock from below the host rock around the margins of intrusion suffer sudden and enormous change in their temperature. They are also metamorphosed in order to be stable under the new conditions.

2. Pressure

Many metamorphic changes are induced solely due to the pressure factor whereas is great majority of case pressure is the dominant factor and is assisted considerably by the heat factor. Any given rock at some depth below the surface is subjected to pressure from two sources: first, load of the overlying burden and second, crustal movements during the convergence of the tectonic plates. The first type of pressure acts generally in a vertical direction and the process of change in the structure of the rock is often referred as load metamorphism. The pressure from orogenic activity is generally lateral ar horizontal and is commonly termed as directed pressure. Rocks situated near the plate boundaries or within the geosynclinal belts are especially prone to directed pressure and often show severest degree of metamorphic changes.

3.Chemical Environment

Chemically active fluids and gases when pass through the pores of rocks, they bring about changes in their original composition. The source of these chemical agents is generally the intrusive igneous body within country rocks.

 

Processes Of Metamorphism

The processes which operate together in the affected rock to bring about metamorphism are

  • Granulation: Pressure shatters rocks and the fraction is so great that the rocks are partially melted. This process where crushing of rocks takes place without loss of coherence, is called the “granulation”.
  • Plastic  Deformation:  When a solid is subjected to stresses, its shape changes. On the removal of stress if the solid dose not regain its original shape, it is said to be plastically deformed.
  • Recrystallization: “Recrystallization” means either the formation of new minerals or formation of new crystals of the pre-existing minerals. The pore fluid of rocks is thought to facilitate this process. Recrystallization causes mineralogical and textural changes in the rock during metamorphism.
  • Metasomatism: Metasomatism is the process in which the original composition of rocks are changed primarily by the addition or removal of material. This change caused by the movement of hydrothermal fluids through rocks usually under high temperature and pressures.

All above said process usually operate in combination to produce metamorphic rocks. During granulation individual crystals are plastically deformed. This deformation initiates recrystallization. During recrystallization the structures of original minerals are changed. They are elongated or flattened in the direction of minimum stress. Thus new texture are produced in the rock. New minerals are formed as a result of exchange of elements and compounds.

Types Of Metamorphism

The main types of metamorphism are:

  • Contact Metamorphism

Contact metamorphism is also called the “Thermal Metamorphism”. This metamorphism is caused due to local heating of rock by the intrusion of hot igneous bodies nearby. The zone of contact metamorphic rocks which occurs surrounding the intrusion is called “aureole”.  As temperature decreases away from the intrusive, the outer rocks in the aureole are less intensely metamorphosed than that of the innermost rocks. Thus depending upon the degree of alteration, the rocks in the aureole can be divided into concentric zones which may differ greatly in mineral assemblages.

In the contact metamorphism heat plays dominate role and its general effect is to promote recrystallization. In this process,minerals grow haphazardly in all directions and the metamorphic rock acquires a granular fabric which is called the “hornfels texture”. Contact metamorphic rock do not show schistosity.

During contact metamorphism transfer of magmatic vapours and gases from an igneous body into the country rocks often takes place. These emanations react with the country rocks ans form new minerals. Such a process is called the “pneumatolytic metamorphism”. A localized burning of baking effect may be produced at the contact of an igneous body and the country rocks. This effect is described as the ” Pyrometamorphism”.

contact metamorphic aureole

  • Plutonic Metamorphism

Plutonic Metamorphism is a process of metamorphism that takes place due to equally important role of imposed loads and very high temperatures that become natural at those great depth. Such changes take place in rocks that are pushed down during crustal movements to positions where high temperature and high pressure become almost a permanent feature.

In all types of thermal metamorphism the change is generally in the direction of mineralogical reconstitution. These process may induce change varying from simple baking effect to complete or nearly complete recrystallisation of almost all of the original minerals.

  • Dynamic Metamorphism

A metamorphism which is associated with high pressure with little increase in temperature, is called the “dynamic metamorphism”. In this case a new rock is formed partly by the mechanical effect  of flow and partly by the growth of new minerals that develop in the direction of flow. Slates which possess flow cleavage, are perhaps the best example of a dynamically metamorphosed rock.

  • Regional Metamorphism

When directed pressure and heat act together in the presence of migrating hydro thermal fluids, the rock are metamorphosed over wider areas. This kind of metamorphism is called the “regional” or “dynamo thermal metamorphism”. Regional metamorphism take place at great depths, such as in root grgions of fold mountains, where temperature and stresses are high.

Heat promotes recrystallization and the stresses cause shearing and flow movements which produce new structure in rocks. The new minerals that grow under directed pressure are usually flat, elongated, bladed or flaky in nature. Example of such minerals are muscovite biotite, chlorite, talc and amphiboles. These minerals arrange themselves in parallel layers and produce a banded or laminated structure, called ” foliation”. The most common foliated metamorphic rock are slate, phyllites, schists and gneisses. Foliated rocks split easily into flaky sheets.

When shale are subjected to regional metamorphism, the characteristic minerals that develop in succession with the rise in temperature and stress are chlorite, biotite, garnet, staurolite, kyanite and sillimanite. Thus shale changes to “slate” in early stage to “schist” in middle stage and finally to “gneiss” at the highest temperature of regional metamorphism.

  • Retrogressive Metamorphism

When high temperature metamorphic minerals assemblage are changed to a low temperature mineral assemblage, the process is called the Retrogressive metamorphism. Such changes take place when an intensely metamorphosed rock is subjected either to strong differential movement or to hydrothermal activity. In zones of displacement an amphibolite may be converted to a green- schist and due to hydrothermal alteration a sepentite rock may change to talc- magnesite-schist.

  • Metasomatism

It may be broadly defined as a”metamorphic process involving essentially formation of new minerals by the mechanism of chemical replacement of pre-existing minerals under the influence of chemically active fluids”.

The metasomatic replacement of minerals takes place at atomic level and in solid state. The chemically active fluids may be provided:

  1. From within the rock, such as pore fluid, in which case the end result of metasomatic change would be a mere replacement of the atoms, the total chemical composition of the rock remaining the same. This is sometimes referred as mineral metasomatism.
  2. Form outside the rock, such as from magmatic  emanations or ground water sources in which case many new minerals may be formed in the rock by the interaction of the atoms of the invading fluids with those of the rock. The net result would be a definite change in the bulk chemical composition of the rock as whole. This is, therefore ,sometimes referred as rock metasomatism.

The process of metasomatism is sometimes further distinguished into:

Hydrothermal- When the fluids are in the form of solution.

Pneumatolytic- When the fluids are in the form of solutions.

Additive- When the net result of the process is addition of a new constituent

Expulsive- When some component get removed from the original composition of the rock.

A common fact observed in the case of metasomatism is that total volume of the rock remains by and large unchanged after the process is completed.

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