Metamorphism

The mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions which have generally been imposed at depth below the surface zones of weathering and cementation, and which differ from the conditions under which the rocks in question originated.

Metamorphism means to “change form,”. In other words, metamorphism is a process that leads to changes in the mineral content, texture, and sometimes the chemical composition of parent rocks.

Process of Metamorphism

The process of metamorphism does not melt the rocks, but instead transforms 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 previously metamorphosed rocks into new types.

What are types of Metamorphism?

As already mentioned, metamorphism of rocks occurs due to the combined effect of temperature, pressure and chemically active fluids. But sometimes one or two of these metamorphic agents play a more active role in bringing about metamorphism. Based on this factor, different types of metamorphism are recognized. Some important types along with relevant details are as follows:

Types of metamorphism recognized based on agents of metamorphism– Thermal metamorphism, Dynamic metamorphism and Dynamothermal metamorphism.

Thermal Metamorphism

Metamorphism in which heat (temperature) plays a predominant role are given the common name “thermal metamorphism”. The surrounding region of magma chamber or magma intrusion in which the heat effect is appreciable is called the metamorphic aureole. Rise in temperature becomes an important factor at great depths and also in the vicinity of magma. In the vicinity of the magma, the country rocks are sometimes affected by emanating fluids. This results in mineral transformation.

Dynamic Metamorphism

Dynamic metamorphism is associated with zones of high strain such as fault zones. In these settings, mechanical deformation is more important than chemical reactions in transforming the rock. Dynamic metamorphism occurs mainly due to directed pressure, or stress. With little heat, when directed pressure acts, rocks are forced to move past one another resulting in their crushing and granulation. This kind of effect is called cataclasis.

The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the deformation mechanisms is governed by dominance of the temperature and confining pressure. At the shallowest depths, a fault zone will be filled with various kinds of unconsolidated cataclastic rock, such as fault gouge or fault breccia. At greater depths, consolidated cataclastic rock, such as crush breccia are found.

Dynamothermal Metamorphism

In this type of metamorphism directed pressure and heat play a dominant role. This is the most common type of metamorphism. The most common metamorphic rocks like gneisses and schists are produced by dynamothermal metamorphism. This type of metamorphism produces mainly recrystallization and consequent textural changes. It is effective at certain depths. The conditions that prevail during this metamorphism lower the melting points of minerals locally and facilitate diffusion and recrystallization of material.

Types of metamorphism recognized based on geological setting- Contact metamorphism, regional metamorphism, ocean floor metamorphism, Impact metamorphism and fault-zone metamorphism.

Contact Metamorphism

One of the principal local processes of thermal metamorphism, genetically related to the intrusion of magmas and taking place in rocks at or near their contact with a body of igneous rock. Metamorphic changes are affected by the heat and materials emanating from the magma and by some deformation connected with the emplacement of the igneous mass (Holmes, 1920).

The effect of metamorphism is restricted to environs of intruding magmas; and the zone is called contact aureoles Limestones are changed to marbles due to contact metamorphism. Unlike the regional metamorphism, contact metamorphism are confided to smaller areas near the heat source (magma, lava, or hydrothermal fluids).

Regional Metamorphism

Regional metamorphism is a general term for metamorphism affecting an extensive region, as opposed to local metamorphism that is effective only in a relatively restricted area. As introduced in the nineteenth century, the term covered only those changes due to deep burial metamorphism. But today, it is used almost synonymously with dynamothermal metamorphism (Holmes, 1920). This type of metamorphism is caused by mountain building or orogenic processes and is characteristic of orogenic belts where recrystallisation is accompanied by deformation. All of the foliated rocks fall into this metamorphic category and some non-foliated rocks as well.

Ocean-floor Metamorphism

The term ocean-floor metamorphism is introduced by Miyashiro (1973) for transformations occurring in the vicinity of the mid-oceanic ridges. The ocean-floor metamorphism is produced by convection of large amounts of heated sea water resulting in extensive veining and metasomatism. Water within the earth crust is forced to rise in the area close to the source of volcanic heat. This further requires drawing in more water from crust. This ultimately creates a convective system where cold seawater is drawn into the crust, heated to 200 °C to 300 °C as it passes through the crust, and then released again onto the seafloor near the ridge. This convective circulation leads to chemical interaction between rocks and sea water causing ocean-floor metamorphism.

Impact Metamorphism (Shock Metamorphism)

Impact metamorphism occurs and physical, chemical, mineralogic changes in rocks are observed resulting from the passage of shock waves produced by hypervelocity velocity impact of a meteorite or bolide. On its impact the energy represented by the meteorite’s mass and velocity is transformed into heat and shock waves pass through the impacted country rock. It lasts for a duration of a few microseconds which results in melting and vaporization of the rocks under impact. The mineralogical characteristics are defined by presence of shocked quartz and newly formed coesite and stishovite as well as minute diamonds.

Fault Zone Metamorphism

Metamorphism along fault-zone occurs due to mechanical deformation. Rocks along fault-zone are deformed due to pressures associated with shearing, compression, or extensional stresses, with minor changes due to heat generated by friction. The process involves pure mechanical forces triggering crushing and granulation of the rock fabric in the locale of faults and overthrusts. Rocks formed as a result of metamorphism are non-foliated. Shallow faults may grind the nearby rocks into smaller, angular fragments called fault breccia or fault gouge that is formed by the chemical alteration of fault breccia. Pseudotachylites likely to form at brittle ductile transition where shear heating generates sufficient heat to melt the rocks.

Metasomatic Metamorphism

The type of metamorphism in which significant compositional changes occur in the parent rock due to the predominant role played by chemically active fluids is called metasomatic metamorphism of simply metasomatism. This metamorphism alters the composition of the rock significantly. Due to this, similar rocks may be formed out of different parent rocks or different rocks may be formed out the same parent rock. Directly magma itself or hydrothermal solutions or volatiles of magma may play a leading role in bringing about these changes. When these solutions permeate through, the country rocks produce new minerals.