Metamorphic Rocks: Types, Formation And Metamorphism

Formation of Metamorphic Rocks and Metamorphism

Earth is an active and dynamic planet. Rocks once buried at a great depth of the earth’s surface, may be deformed and their temperature may be changed as a consequence of burial or by the intrusion of hot magma. After such changes in the surrounding condition of rocks, the characteristics of the rocks commonly become modified by undergoing metamorphism and finally transformed into metamorphic rocks.

Surely, changes in temperature and pressures are the causes of rocks in metamorphism, but they are not the only ones. Changes may have occurred as a result of changes in chemical composition. In this case, changes are most commonly associated with the movement of fluids combination like carbon dioxide (CO2) and water (H2O).

Metamorphism often progresses slowly from slight (low-grade metamorphism) to substantial (high-grade metamorphism) changes. For example, under low-grade metamorphism, sedimentary rock like shale becomes the more compact metamorphic rock slate.

Agents of Metamorphism

There are some important, influential factors which help rocks to be metamorphosed. The degree of metamorphism and the role of factors differ greatly from one environment to another.

Heat: High temperature is one of the obvious causes of rock metamorphism. It accelerates the chemical reaction that leads to the recrystallization of existing minerals and the formation of new minerals. For example, limestone is recrystallized in a solid state, but the rock is not melted however, only the mineral or textural changes take place that results in marble (CaCO3).

In contrary to this, some rocks are not stable at higher temperatures because of containing so much water and break down to form new minerals, for instance, clay is transformed into mica. Pre-existing rocks experience an elevation in temperature when they are intruded by magma from below, called contact or thermal metamorphism. The temperature increases with the depth of the earth’s surface. So, the minerals of rock start becoming unstable, leading to recrystallization into other minerals.

Pressure: Metamorphism is not purely thermal as pressure is another important factor in the process of metamorphism. Greater pressure in rocks consolidates mineral grains and produces a more compact rock with greater density. Bounding pressure at a greater depth may cause minerals to recrystallize into new minerals displaying more compact crystalline forms. In the Mountain building process, the differential stress deforms rocks.

Chemical: These become active when they are in contact with water. If it is hot water passing through rocks that eats the greater depth of the surface, then it is called hydro-thermal solution, which dissolves some materials and deposits in some other places.

Such solutions act as accelerators to promote recrystallization by enhancing ion migration from one chemical to another. The chemical reaction is more advanced in a hot environment where the solutions or fluid supply OHˉ² (the hydroxyl ion) to create certain key minerals like chloride and actinolite. For example, after being in contact with water, asbestos forms serpentine.

Types of Metamorphic Rocks

Classification on the Basis of Place of Occurrence

Contact Metamorphism

Such metamorphism happens when the mineral composition of surrounding rocks is altered due to the high temperature of ascending magma. It is also known as thermal metamorphism. The best example is limestone which is altered to marble because of contact metamorphism. The area between the altered rocks and intruded magma is called aureoles.

Regional Metamorphism

When pressure is dominant to change the forms of rock in an extensive area, the process is known as regional metamorphism. This process is also known as dynamic metamorphism. Apart from pressure, the temperature is also active in changing the forms of rocks. Both pressure and heat change the original form of sedimentary rock leading to folding during mountain building. Consequently, rocks are crystallized and crystallized can be further recrystallized by greater pressure and heat.

Classification on the Basis of Foliation or Structure

Foliated Rocks

The term foliation comes from the Latin word ‘leaf’ (as the parallel leaves or pages of a book) thereby characterising it as the nearby flat arrangement of minerals in a rock. Foliation may develop in both igneous and sedimentary rocks. Granite among igneous rocks can be metamorphosed in a similar foliation process to become granitic gneiss.

Slate: It is composed of very fine-grained mica flakes which are too small to be visible. Under higher temperatures, some clay minerals become unstable and break down to form new phyllosilicates. The most important characteristic is that it has excellent rock cleavage that can be broken down into flat slabs. The low-grade metamorphism of shale produces slate.

Schist: Schists are platy that split into thin flakes or slabs very easily. This process is called schistosity or schistose structure. Coarse-grained minerals like Muscovite and biotite are the composition of mica schist.

Gneiss: This rock is composed of medium and coarse grain minerals in which there may be a limited or partial development of the schistose structure that often gives rise to a layered or lenticular structure. The most predominant minerals of gneiss are feldspar, quartzite, muscovite, biotite and hornblende.

Non-Foliated Rocks

Unlike foliated rocks, non-foliated rocks form under constant pressure but a minimum deformation takes place and the parent rocks are composed of mineral crystals like quartzite or calcite. For example, when the intrusion of magma heats a limestone composed of fine-grained calcite, the fine grains recrystallize to form larger interlocking crystals producing marble with large equidimensional grains which are actually randomly oriented.

Marble: It is characterized by a coarse-grained crystalline rock which originates from limestone. It is discussed earlier that marble is made of large interlocking calcite crystal that was in the form of smaller sizes in limestone. Marble is useful as a building stone due to its colour and relative softness. For example, the Taj Mahal in India and Lincoln Memorial in Washington D.C. are important exhibitions of white marble.

Quartzite: This rock is composed of more than 80 per cent of quartz because the parent rock is quartz sandstone. Another important characteristic of this rock is that this is a very hard metamorphic rock due to the high grade of metamorphism in which quartzite grains in the parent rock blend. The colour of pure quartzite is white, while iron oxide may exhibit reddish or pinkish marks and the grey colour is because of dark minerals.

Metamorphic Rocks: Types, Formation and Metamorphism

Formation of Metamorphic Rocks and Metamorphism

Agents of Metamorphism

Types of Metamorphic Rocks