Model Answer
0 min readIntroduction
Metamorphism, derived from the Greek words ‘meta’ (change) and ‘morph’ (form), refers to the transformation of existing rocks (igneous, sedimentary, or even other metamorphic rocks) into new forms due to changes in physical or chemical conditions. These changes occur in the solid state, meaning the rock doesn’t melt. The primary drivers of metamorphism are increasing temperature and pressure, often in conjunction with chemically active fluids. The resultant rock exhibits altered mineralogy, texture, and sometimes, chemical composition, reflecting the new equilibrium established under the changed conditions. Understanding how these factors influence rock transformation is crucial in deciphering Earth’s geological history.
Effects of Increasing Pressure
Pressure plays a significant role in metamorphism, manifesting in two primary forms: confining pressure and directed pressure.
- Confining Pressure: This is equal pressure applied in all directions, typically associated with burial under increasing layers of sediment. It leads to increased density and a reduction in pore space, resulting in a more compact rock. While it doesn’t drastically alter mineralogy, it promotes recrystallization of existing minerals into more stable forms. For example, shale under increasing confining pressure transforms into slate.
- Directed Pressure (Differential Stress): This occurs when pressure is unequal in different directions, often associated with tectonic forces during mountain building. It causes minerals to align perpendicular to the direction of maximum stress, leading to the development of foliation – a layered or banded texture.
Examples of Pressure-Induced Metamorphism:
- Slate from Shale: Low-grade metamorphism under directed pressure.
- Schist from Shale/Igneous Rocks: Medium-grade metamorphism with prominent foliation.
- Gneiss from Granite/Sedimentary Rocks: High-grade metamorphism with distinct banding.
Effects of Increasing Temperature
Increasing temperature provides the energy needed for chemical reactions that drive metamorphic changes. Higher temperatures increase the rate of diffusion, allowing atoms to migrate and rearrange themselves into more stable mineral assemblages.
- Recrystallization: Existing minerals grow larger and more well-formed.
- Phase Change: Minerals transform into different polymorphs (same chemical composition, different crystal structure). For example, andalusite, sillimanite, and kyanite are all polymorphs of Al2SiO5, forming under different temperature and pressure conditions.
- Neomorphism: One mineral transforms into another mineral with a different chemical composition, often involving the introduction of fluids.
Examples of Temperature-Induced Metamorphism:
- Hornfels: Formed by contact metamorphism around igneous intrusions, characterized by fine-grained, non-foliated texture.
- Marble from Limestone: High temperature causes recrystallization of calcite.
Combined Effects of Pressure and Temperature
The most significant metamorphic changes occur when pressure and temperature increase simultaneously. This leads to the development of metamorphic facies – a set of metamorphic mineral assemblages that are stable under specific P-T conditions.
| Metamorphic Facies | Temperature (°C) | Pressure (kbar) | Typical Rocks |
|---|---|---|---|
| Zeolite Facies | 200-300 | 1-2 | Zeolite-bearing rocks |
| Greenschist Facies | 300-500 | 2-5 | Greenschist, amphibolite |
| Amphibolite Facies | 500-700 | 5-10 | Amphibolite, granulite |
| Granulite Facies | 700-900 | 7-12 | Granulite |
The P-T path a rock follows determines the final metamorphic product. For instance, rocks subjected to high temperature and low pressure (e.g., near a magma chamber) will exhibit different characteristics than those subjected to high pressure and low temperature (e.g., in a subduction zone).
Conclusion
In conclusion, both increasing pressure and temperature, acting individually or in combination, are potent agents of metamorphic change. Pressure dictates the texture, particularly the development of foliation, while temperature drives mineralogical transformations through recrystallization, phase changes, and neomorphism. The interplay of these factors, defined by metamorphic facies, results in a diverse range of metamorphic rocks, each reflecting the unique P-T conditions under which it formed. Understanding these processes is fundamental to interpreting the geological evolution of the Earth’s 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.