Metamorphic Rocks: Formation, Types, and Minerals
Metamorphic Rocks
Metamorphic rocks are formed from other rocks in a process called metamorphism. Metamorphism never involves the fusion of the parent rock and occurs equally in igneous rocks, sedimentary rocks, or other metamorphic rocks. This process happens when rocks are subjected to high pressures (about 1500 bars), high temperatures (between 150 and 200°C), or a working fluid (which causes changes in the composition of the rock, introducing new substances).
Metamorphic rocks are classified according to their physical-chemical properties. The factors that define metamorphic rocks are the minerals that form them and the textures they present. There are two main types of textures: foliated and non-foliated.
- Foliated texture: Some examples include:
- Slate (from which broken plates are obtained)
- Schist (breaks easily)
- Gneiss (consisting of alternating light and dark minerals)
- Non-foliated texture: Some examples include:
- Marble (crystalline appearance, formed by the metamorphism of limestones and dolomites)
- Quartzite (is white but can change color due to impurities)
- Serpentinite (which can transform into asbestos)
- Cancagua
Types of Metamorphism
The main types of metamorphism depend on the nature of the energy supplied for its implementation, which may be in the form of heat or pressure:
- Thermal Metamorphism: Occurs when the transformation of rocks is primarily due to the high temperatures they are exposed to. This type is also called contact metamorphism. It occurs in circumstances such as the intrusion of magma into existing rocks, forming plutons, dikes, or sills. Marble is an example of a rock formed by this process.
- Regional Metamorphism: This is the most common form of metamorphism. When both pressure and temperature are present simultaneously, it is called regional metamorphism. These processes occur more frequently at greater depths and in regions where large mountains are formed. Gneiss is an example of a rock formed by this process.
- Dynamic Metamorphism: It is produced by strong pressures, such as those generated in the vicinity of tectonic deformations and faults. Rocks derived from dynamic metamorphism are called cataclasites. Mylonites are an example of this type of rock.
- Retrograde Metamorphism: Occurs when rocks formed in areas subject to high pressure and temperature conditions transition to lower pressure and temperature conditions. Serpentinites are the main rocks associated with this process.
Metamorphic Minerals
Metamorphic minerals are those formed under high temperatures associated with metamorphic processes. Among the minerals formed by this process, we find kyanite, staurolite, sillimanite, andalusite, and garnet.
Other minerals, such as olivine, pyroxene, amphibole, quartz, feldspar, and mica, can be identified in metamorphic rocks but are not necessarily the result of metamorphism. These minerals can also form during the crystallization of igneous rocks. They have a very high melting point, making them stable at high temperatures and pressures. During metamorphic processes, rocks can experience an altered chemical composition. However, all minerals are stable at high temperatures up to certain limits. The presence and type of minerals in the rocks indicate the temperature and pressure conditions under which they were formed.