Preexisting rock that is subsequently altered to form a metamorphic rock is termed a


Metamorphism

Igneous rocks are rocks that are solidified from magma or lava. These rocks can be altered by heat and pressure to form metamorphic rocks. The temperatures and pressures required to form metamorphic rocks are usually higher than those found at the Earth’s surface. Most metamorphic rocks form when preexisting rocks are subjected to heat and pressure deep below the Earth’s surface.

Definition

Metamorphism is the change of minerals or objetcs into new ones with different chemical and physical properties due to changes in temperature, pressure, and/or the addition of fluids. The preexisting rock that is subsequently altered to form a metamorphic rock is termed a protolith. A metamorphic facies is a set of metamorphic mineral assemblages formed under similar pressures and temperatures.

Types

There are two principal types of metamorphism: thermal (contact and regional) and barometrical (dynamic, burial). -Thermal metamorphism occurs when rocks are heated by bodies of magma intruding into the Earth’s crust or by the heat released during exothermic volcanic eruptions. -Barometrical metamorphism, also called dynamic metamorphism, results from the differential pressures to which rocks are subjected as they descend into the Earth’s interior.

Causes of Metamorphism

There are three main types of metamorphism: thermal, burial, and contact metamorphism. Thermal metamorphism occurs when heat from the Earth’s interior alters the minerals in preexisting rocks. Burial metamorphism takes place when rocks are buried deep underground and experience high temperatures and pressures. Contact metamorphism occurs when rocks come into contact with a heat source, such as a magma body.

Temperature

While rocks naturally bake under the heat of the sun at the earth’s surface, and freeze in the frigid polar regions, most metamorphism takes place much deeper underground, where high temperatures are generated by the earth’s internal heat. This heat can come from three main sources: geothermal (earth) energy from radioactively decaying elements in the earth’s mantle and crust; frictional energy created as two continental plates collide; and impact energy generated when a bolide (a large meteorite or comet) strikes the earth. In most cases, heat is transferred by the flow of hot aqueous solutions (water + dissolved minerals), which carry the heat deep into the rock mass.

Pressure

Most metamorphism occurs while rocks are buried deep beneath the Earth’s surface. The heat and pressure of burial reduces the porosity of the rock, which helps to drive chemical reactions. The weight of the overlying material also creates physical stress that can deform rocks. While rocks are being buried, they may be thrust faulted or sheared, which can also contribute to metamorphism.

Fluids


Fluids play an important role in metamorphism. They act as both a solvent and a heat transfer medium. In low-grade metamorphism, fluids promote the decomposition of minerals and the recrystallization of new minerals. In high-grade metamorphism, fluids help to transport heat and volatiles out of the reaction zone. Fluids also play an important role in the formation of many types of metamorphic rocks, such as marble, quartzite, and certain schists.

Fluids can be either aqueous (water-rich) or gaseous (gas-rich). Aqueous fluids are the most common type of fluid in metamorphism and are typically derived from either magmatic or meteoric sources. Gaseous fluids are less common, but can be found in some metamorphic rocks, such as those that form from coal oroil.

Effects of Metamorphism

Metamorphism is the process of change that takes place within a rock as a result of temperature, pressure, or both. These agents of change can be either natural or artificial, but most metamorphic rocks are formed underground, where temperatures and pressures are higher than at the Earth’s surface.

Textural


Textural effects refer to changes in the shape, size and arrangement of minerals within the rock. Common textural effects include:

  • Recrystallisation: this is when new crystals form from the existing ones. The new crystals are usually larger than the original ones.
  • Foliation: this is when minerals grow in approximately parallel sheets. It gives rocks a ‘layered’ appearance.
  • Lineation: this is when minerals grow in parallel streaks or bands. It gives rocks a ‘striped’ appearance.
    Structural
    Alteration of preexisting rock to form metamorphic rock takes place primarily due to the changing temperature and pressure within the Earth’s crust. As rocks are heated, they expand; as they are cooled, they contract. The net effect of these thermal changes is that rocks tend to deform in a disorderly fashion. The fact that rocks deform under the influence of heat and pressure is extremely important in understanding how metamorphism takes place.
    Importance of Metamorphism
    Metamorphism is an important process that affects the formation of rocks and the composition of the Earth’s crust. It is a process of change, which can occur at any stage in the rock cycle.
    Economic

    A metamorphic rock may be formed simply by increase or decrease in temperature, With no change in composition. This type of metamorphism takes place at relatively low temperatures and pressures and is called thermal metamorphism. Important economic minerals such as graphite, asbestos, and some types of clay are formed by this process.

Most metamorphic rocks, however, form when both temperature and pressure are changed. This process is called dynamic metamorphism, and the rocks produced are termed dynamic metamorphic rocks. Most commercial-grade metamorphic rocks (marble, slate, gneiss, amphibolite) form under these conditions.

Geological


The degree to which a rock is altered depends on the temperature and pressure conditions to which it is exposed during metamorphism. For example, a sedimentary rock such as limestone will recrystallize into a different type of sedimentary rock, such as dolostone, under conditions of moderate heat and pressure. If the temperature and pressure conditions are extreme, however, the original sedimentary rock may be completely altered into an entirely different kind of rock, such as an igneous or metamorphic rock.

The Importance of Metamorphism
Metamorphism is an important process in the formation of many types of rocks, including igneous, sedimentary, and metamorphic rocks. Metamorphism can also be used to modify existing rocks to create new types of rocks with desired properties. For example, metamorphism can be used to change the structure of existing rocks to make them more resistant to weathering or erosion.


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