Any rock derived from pre-existing rocks by mineralogical, chemical, structural, or textural changes in solid-state, in response to marked changes in temperature, pressure, and chemical environment at depth in earth’s crust i.e. below the zone of weathering and cementation is called sedimentary rock.
The rocks derived from igneous rocks are called ortho-metamorphic rocks e.g. Gneiss. The rocks derived from sedimentary rocks are called Para metamorphic rocks e.g. Slate.
Agents of Metamorphism
Metamorphism is the process of formation of metamorphic rocks from sedimentary and igneous rocks. During this process, different agents are involved. These agents are called agents of metamorphism or metamorphic agents. Following are the different agents of metamorphism:
Heat
It is the most important metamorphic agent because it provides the energy to drive chemical reactions that result in the recrystallization of existing materials and/or the formation of new materials. Here’s how heat causes metamorphism:
- Thermal energy results recrystallization and dehydration.
- Thermal energy breaks chemical bonds.
- They increase rate of reaction.
- Heat leads to changes in mineralogical composition and crystals and, that leads to make coarse texture.
Earth’s internal heat comes mainly from radioactive decay within the Earth’s interior and from converging plates.
Pressure
There are two types of pressure acting during the formation of metamorphic rocks: uniform pressure and directed pressure. They are also known as equidimensional pressure and directed pressure.
Uniform pressure leads to a change of volume of rock by pore space reduction due to closer atomic packing while directed pressure leads to changes in shape, texture, and structure.
Chemically Active Fluids
The change in rocks due to chemically active fluids take place through a partial or complete solution of the minerals. The most chemically reactive fluids derived from magma and other source are water, carbon dioxide, etc. They lead to chemical changes in rocks.
Types of Metamorphism
Metamorphism is broadly classified into three major categories on the basis of the dominant agent involved in the formation of rocks.
Contact metamorphism (Thermal metamorphism):
It results from the emplacement of a hot igneous intrusion into cooler country rocks at relatively low pressure. Here are some of the features of contact metamorphism.
- Heat is dominating factor.
- The metamorphism operation area around the magma intrusion called Contact Aureole.
- Changes in mineral composition and texture.
- Commonly formed in carbonate –rich country rocks.
- Contact metamorphic rocks are usually massive.
Example, Limestone to Marble
2. Regional metamorphism:
It occurs over wide areas (hundreds of kilometers) and within large Orogenic (mountain building process) belts. Temperature and directed pressure are dominating factors.
Regional metamorphism is also classified into the following two types.
- Regional Dynamothermal metamorphism: It results from the thermal gradient in a broad region. The rocks are characterized by the parallelism of platy and elongated minerals. It Changes both mineral composition and texture.
- Regional burial metamorphism(high grade diagenesis): It results from the thick piles of hydrous sediments or fragmental volcanic materials due to several hundred degrees temperature with high water pressure. It is due to lack of significant parallelism of metamorphic mineral grain. It is difficult to identify but only possible in a polarizing microscope.
For example, Zeolite, Glaucophane, etc are burial metamorphic minerals.
3. Dynamic metamorphism: (Cataclastic metamorphism)
Here are the features of dynamic metamorphism:
- Occurs in zone of intense deformation as fractures, faults etc at shallow depths.
- Change in texture i.e. decrease in grain size.
- Mechanical deformation produces rocks ranging from Breccias to Mylonites.
- Directed pressure is dominating factor.
Texture of Metamorphic Rocks
The texture is used to describe the size, shape, and arrangement of grains within a rock.
Foliated Texture:
Most igneous and sedimentary rocks consist of mineral grains that have a random orientation. By contrast, deformed metamorphic rocks that contain platy minerals (micas) and/or elongated minerals (amphiboles) typically display some kind of preferred orientation in which the mineral grains exhibit a parallel or specific alignment. This preferred orientation of a rock’s minerals is called a foliated texture.
Gneissic Texture: During high-grade metamorphism, ion migrations can result in the segregation of minerals. Although foliated, gneisses will not usually split as easily as slates and schists. Gneisses that do cleave tend to break parallel to their foliation and expose mica-rich surfaces that resemble schist.
Non-foliated Texture
Not all metamorphic rocks exhibit a foliated texture. Those that do not are referred to as non-foliated. Nonfoliated textures usually form in environments where parent rocks are composed of minerals that exhibit equidimensional crystals, such as quartz or calcite.
Another texture common to metamorphic rocks consists of particularly large grains, called porphyroblasts, that are surrounded by a fine-grained matrix of other minerals. Porphyroblastic textures develop in a wide range of environments and result in very large specimens of certain minerals, such as garnets.
Structure of Metamorphic Rocks
The large-scale feature developed in metamorphism is metamorphic structure. They have been measured in hand specimens.
- Cataclastic structure: It is the metamorphic structure formed under directed pressure i.e. stress resulted from deformation like folding
- Maculose structure: It is resulted from thermal metamorphism in argillaceous rocks, darked colored minerals giving a spotted appearance. E,g, slate
- Schistose structure: It is formed under stress by platy or flaky minerals in parallel orientation. In micas and inequidimensional minerals e.g. Biotite Schist
- Granulose Structure: It is resulted from heat & uniform pressure in equi-dimensional minerals by the process of recrystallisation. It is also known as sacchroidal structure. E.g. Quartzite, Marbles.
- Gneissose structure: It is banded structure due to alternation of dark colored and light colored minerals by highest grade of metamorphism through regional metamorphism, e.g. Gneiss
Also Read:
