Greenschist facies

Definition

Greenschist facies is a set of metamorphic conditions characterized by moderate temperature and pressure, where minerals such as chlorite, albite, and actinolite are typically formed. This facies indicates a specific range of metamorphic conditions usually found in subduction zones or areas of regional metamorphism, reflecting the geological processes that transform pre-existing rocks into metamorphic rocks.

5 Must Know Facts For Your Next Test

1. Greenschist facies typically forms at temperatures ranging from about 300°C to 500°C and pressures of 3 to 10 kilobars.
2. The presence of chlorite in greenschist facies rocks often indicates lower-grade metamorphism, while more stable minerals suggest higher grades.
3. Common rock types associated with greenschist facies include greenschist, schist, and amphibolite.
4. The characteristic green color of greenschist facies rocks is primarily due to the abundance of chlorite and other green minerals.
5. Understanding greenschist facies helps geologists interpret the tectonic history and metamorphic evolution of specific regions.

Review Questions

• How do temperature and pressure conditions define the characteristics of greenschist facies compared to other metamorphic facies?
  • Greenschist facies is defined by moderate temperatures between 300°C to 500°C and pressures ranging from 3 to 10 kilobars. These conditions lead to the formation of specific minerals such as chlorite and actinolite, which help identify the grade of metamorphism. In contrast, higher-grade facies like amphibolite have different mineral compositions due to increased temperature and pressure, demonstrating the varying conditions under which different metamorphic rocks form.
• Discuss the significance of identifying greenschist facies in understanding tectonic processes like subduction.
  • Identifying greenschist facies provides insight into tectonic processes such as subduction, where one plate moves under another, generating conditions suitable for this level of metamorphism. The formation of greenschist facies rocks can indicate active tectonic environments, allowing geologists to reconstruct past geological events and understand how these processes influence rock formation and mineral stability over time. This knowledge is crucial for interpreting regional geological histories.
• Evaluate how the study of greenschist facies contributes to our overall understanding of metamorphic geology and tectonics.
  • The study of greenschist facies is essential in understanding both metamorphic geology and tectonics because it links specific mineral assemblages to distinct pressure-temperature conditions. By analyzing these conditions, geologists can infer the tectonic setting where the metamorphism occurred, such as continental collision or subduction. Moreover, it helps in assessing the thermal gradients in different regions, ultimately enhancing our comprehension of Earth's dynamic processes during geological history.

Related terms

Metamorphism:

The process by which rocks are altered in mineralogy, texture, and chemical composition due to changes in temperature, pressure, and fluid composition.

Subduction Zone:

A geological feature where one tectonic plate moves under another, leading to increased pressure and temperature conditions that can result in metamorphism.

Metamorphic Index Minerals: Minerals that form under specific pressure-temperature conditions and are used to estimate the metamorphic conditions a rock has undergone.