Low-grade metamorphism

Definition

Low-grade metamorphism refers to the process where rocks undergo changes due to relatively low temperatures and pressures, typically ranging from about 200°C to 400°C and 1 to 4 kilobars. This type of metamorphism leads to the development of fine-grained textures and structures, such as slaty cleavage or schistosity, which are indicative of the initial stages of metamorphic transformation.

5 Must Know Facts For Your Next Test

1. Low-grade metamorphism typically results in the formation of minerals like chlorite, muscovite, and biotite, which are stable at lower temperatures and pressures.
2. Rocks affected by low-grade metamorphism often display a foliation texture, which is characterized by the parallel alignment of platy minerals.
3. Common examples of low-grade metamorphic rocks include slate, which forms from shale, and phyllite, which is a slightly more altered version of slate.
4. The process usually occurs in areas where tectonic forces are present but are not extreme enough to cause high-grade metamorphism.
5. Low-grade metamorphism is often associated with the burial of sediments in sedimentary basins or along convergent plate boundaries.

Review Questions

• How does low-grade metamorphism affect the texture and mineral composition of rocks?
  • Low-grade metamorphism significantly alters the texture and mineral composition of rocks through processes like recrystallization and foliation. This occurs at lower temperatures and pressures, leading to the formation of minerals such as chlorite and muscovite. The resulting textures, like slaty cleavage seen in slate, reflect these changes and are crucial for identifying the metamorphic history of the rock.
• Compare low-grade metamorphism to regional metamorphism in terms of conditions and outcomes.
  • Low-grade metamorphism occurs under relatively lower temperatures and pressures compared to regional metamorphism, which involves higher levels of heat and pressure often associated with tectonic plate interactions. While low-grade conditions lead to the formation of minerals like chlorite and fine-grained textures such as slate, regional metamorphism can produce coarser-grained rocks with more complex mineral assemblages like schist or gneiss. The scale and intensity of these processes greatly influence the resulting rock types.
• Evaluate the role of tectonic settings in influencing low-grade metamorphism and its resultant rock types.
  • Tectonic settings play a crucial role in determining the conditions for low-grade metamorphism, particularly in sedimentary basins or along convergent boundaries where tectonic forces apply moderate pressure. As sediments are buried deeper, they encounter increased temperature while remaining under relatively low pressure compared to high-grade environments. This leads to the development of specific rock types like slate from shale or phyllite with characteristic textures. Understanding these tectonic influences helps geologists interpret past geological events and reconstruct Earth's history.

Related terms

Metamorphic Rock: A type of rock that has been transformed by heat, pressure, and chemically active fluids from pre-existing rocks.

Regional Metamorphism: A type of metamorphism that occurs over large areas under high pressures and temperatures, often associated with mountain-building processes.

Foliation: The alignment of mineral grains within a rock, resulting in a layered or banded appearance that often develops during metamorphism.