Model Answer
0 min readIntroduction
Metamorphism, derived from the Greek words ‘meta’ (change) and ‘morph’ (form), refers to the transformation of existing rocks (protoliths) into new rocks through changes in physical or chemical conditions. These changes occur in the solid state, without melting, and are driven by variations in temperature, pressure, fluid activity, and stress. Metamorphism is a crucial process in the rock cycle, responsible for a significant portion of the Earth’s crust. Understanding the types of metamorphism and their controlling factors is fundamental to deciphering the geological history of a region. This answer will explore the different types of metamorphism, their controlling factors, and the characteristic mineral assemblages observed during regional metamorphism of pelitic rocks.
Types of Metamorphism
Metamorphism is broadly classified into several types based on the geological setting and dominant controlling factors:
- Regional Metamorphism: This is the most widespread type, occurring over large areas and associated with mountain building (orogenic) events. It’s characterized by directed pressure and high temperatures.
- Contact Metamorphism: Occurs locally around igneous intrusions. The primary controlling factor is temperature, as heat from the magma alters the surrounding rocks. Pressure is generally low and uniform.
- Hydrothermal Metamorphism: Driven by the interaction of rocks with hot, chemically active fluids. Common in mid-ocean ridges and volcanic areas.
- Burial Metamorphism: Results from the increasing temperature and pressure due to the weight of overlying sediments. It’s a low-grade metamorphic process.
- Fault Zone Metamorphism: Occurs along fault lines due to frictional heating and deformation.
- Impact Metamorphism: Caused by the extreme pressure and temperature generated during meteorite impacts.
- Recrystallization: A type of solid-state alteration of rocks where minerals change in size and shape without changing in composition.
Controlling Factors of Metamorphism
Several factors control the type and degree of metamorphism:
- Temperature: Increases with depth and proximity to igneous intrusions. Provides the energy for metamorphic reactions.
- Pressure: Increases with depth. Can be lithostatic (uniform in all directions) or directed (stress).
- Fluid Activity: Fluids (primarily water with dissolved ions) act as catalysts, accelerating metamorphic reactions and transporting ions.
- Chemical Composition: The composition of the protolith influences the resulting metamorphic minerals.
- Time: Metamorphic reactions take time to reach equilibrium.
Regional Metamorphism of Pelitic Rocks and Characteristic Mineral Assemblages
Pelitic rocks, derived from clay-rich sediments (shale, mudstone), are commonly subjected to regional metamorphism. The resulting mineral assemblages are indicative of the pressure-temperature (P-T) conditions experienced during metamorphism. Metamorphic facies are defined by specific P-T ranges and characteristic mineral assemblages. The following table illustrates the mineral assemblages observed in pelitic rocks across different metamorphic facies:
| Metamorphic Facies | Pressure (kbar) | Temperature (°C) | Characteristic Mineral Assemblages (Pelitic Rocks) |
|---|---|---|---|
| Zeolite Facies | 0-2 | 200-300 | Zeolites, Chlorite, Epidote |
| Greenschist Facies | 2-5 | 300-500 | Chlorite, Epidote, Albite, Muscovite, Actinolite |
| Amphibolite Facies | 5-10 | 500-700 | Hornblende, Plagioclase, Garnet, Biotite, Kyanite/Sillimanite (depending on pressure) |
| Granulite Facies | 7-12 | 700-900 | Pyroxene, Plagioclase, Garnet, Sillimanite |
| Eclogite Facies | >12 | >800 | Omphacite (Na-rich pyroxene), Garnet, Coesite/Diamond |
The presence of index minerals like kyanite, sillimanite, and andalusite in pelitic rocks is particularly useful in determining the P-T conditions of metamorphism. For example, kyanite is stable at high pressure, sillimanite at high temperature, and andalusite at lower pressure and temperature. The stability fields of these minerals are represented in phase diagrams.
Conclusion
In conclusion, metamorphism is a complex process driven by changes in temperature, pressure, fluid activity, and chemical composition. Different types of metamorphism occur in various geological settings, each with its unique controlling factors. Regional metamorphism of pelitic rocks provides a valuable tool for understanding the P-T history of orogenic belts, as evidenced by the characteristic mineral assemblages that develop within different metamorphic facies. The study of metamorphic rocks continues to refine our understanding of Earth’s dynamic processes and the evolution of the crust.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.