Magma Differentiation and Metamorphism in Rocks
Fractional Crystallization: As magma cools, its components begin to crystallize, forming minerals. Crystals form over time but successively. As magma cools and pressure changes, different minerals crystallize at different temperatures. This process, fractional crystallization, leads to the separation of solid phases from the liquid magma. The solid phases consist of minerals that have already crystallized, while the liquid phase is formed from remaining minerals, water, and gases.
Magma Differentiation: This refers to processes that alter the composition of magma. If crystals that form do not remain with the magma, the final rock composition will differ from the original magma. Separation of solid crystals or migration of fluids through fissures can leave elements behind, enriching the residual magma. Distinct magma compositions can result from processes like assimilation of original rocks.
Assimilation: Hot magma can incorporate surrounding rock fragments into the magma chamber. These assimilated components change the magma’s composition.
Magma Mixing: When two magma chambers with different compositions communicate, their magmas mix, creating a new magma with a different composition from the originals. Differentiation also occurs through fluid migration, assimilation of surrounding rock fragments, and the separation of solid phases.
Metamorphism: This involves changes in rocks due to heat, pressure, and chemical reactions.
- Changes in Texture: Texture relates to the size, shape, and arrangement of crystals within a rock. Increased pressure and the orientation of minerals can cause changes in texture.
- Changes in Structure: Structure refers to the distribution of crystals within a rock. In metamorphic rocks, pressure can cause minerals to align, creating a layered or banded appearance known as foliation.
- Foliation Grades:
- Schistosity: Characterized by wavy, larger crystals.
- Gneissic: More intense metamorphism, where crystals are arranged in alternating bands of light and dark colors.
- Slaty: Crystals define a planar structure, resulting from low-intensity metamorphism.
- Foliation Grades:
- Changes in Mineralogy: Minerals in rocks subjected to metamorphism can become unstable. This can lead to the formation of new minerals with different chemical compositions or crystal structures (polymorphs). Recrystallization can increase crystal size (blastesis). New minerals can also form through chemical changes caused by fluids, which can introduce or remove elements, leading to new mineral assemblages.
Types of Metamorphism:
- Contact Metamorphism: Occurs due to increases in temperature, typically when magma intrudes into surrounding rocks. The heat from the magma alters the nearby rocks, forming metamorphic aureoles where alteration is most intense closest to the magma.
- Regional Metamorphism: Caused by the simultaneous effect of increased pressure and temperature over large areas and long periods. It affects extensive regions of the Earth’s crust, such as subduction zones or collision orogens. The intensity varies depending on location.
- Dynamic Metamorphism: Primarily caused by pressure, occurring in fault zones with significant movement between rock blocks. These processes form mylonite rocks.