Model Answer
0 min readIntroduction
Metamorphism, derived from the Greek words ‘meta’ (change) and ‘morph’ (form), refers to the alteration of pre-existing rocks (igneous, sedimentary, or metamorphic) by heat, pressure, or chemically active fluids. Thermal metamorphism, a specific type, occurs due to an increase in temperature, typically associated with igneous intrusions or geothermal gradients. This process doesn’t involve significant changes in pressure or fluid activity. The resulting rocks exhibit distinct characteristics reflecting the intensity and duration of heating, providing valuable insights into the Earth’s geological history. Understanding these features is crucial for deciphering past geological events and resource exploration.
Causes of Thermal Metamorphism
Thermal metamorphism primarily occurs due to:
- Igneous Intrusion: Magma intrusions into cooler country rocks provide the heat source. The zone around the intrusion experiences metamorphism, with the intensity decreasing with distance.
- Geothermal Gradient: Increase in temperature with depth within the Earth. This is a slower process but can lead to regional metamorphism at depth.
- Contact Metamorphism: Occurs locally around igneous intrusions, creating a metamorphic aureole.
Salient Features Exhibited by Rocks due to Thermal Metamorphism
1. Mineralogical Changes
Thermal metamorphism leads to the formation of new minerals stable at higher temperatures. These changes are often indicative of the protolith (parent rock) and the temperature reached.
- Index Minerals: Certain minerals, known as index minerals (e.g., sillimanite, kyanite, andalusite), are indicative of specific temperature and pressure conditions. Their presence helps determine the metamorphic grade.
- Recrystallization: Existing minerals recrystallize into larger, more stable forms. For example, clay minerals in shale can recrystallize into micas like muscovite and biotite.
- Neomorphism: A change in the mineral composition without a change in the overall chemical composition. For instance, kaolinite transforms into pyrophyllite.
- Formation of New Minerals: New minerals form through chemical reactions. For example, limestone (CaCO3) can transform into marble, with recrystallization of calcite.
2. Textural Changes
The texture of rocks undergoes significant changes during thermal metamorphism.
- Granoblastic Texture: Characterized by an equigranular, interlocking texture of equidimensional grains. This is common in rocks like quartzite and marble.
- Porphyroblastic Texture: Larger crystals (porphyroblasts) grow within a finer-grained matrix. Garnet porphyroblasts in schist are a classic example.
- Hornfelsic Texture: Fine-grained, dense, and often dark-colored texture resulting from high-temperature, low-pressure metamorphism.
- Development of Foliation (limited): While thermal metamorphism doesn’t typically produce strong foliation like regional metamorphism, some degree of preferred mineral orientation can occur, especially near the contact zone.
3. Structural Changes
Structural features are less prominent in purely thermal metamorphism compared to regional metamorphism, but they can still be observed.
- Contact Aureole: A zone of altered rock surrounding an igneous intrusion. The width and intensity of the aureole depend on the size and temperature of the intrusion, and the thermal properties of the country rock.
- Jointing and Fracturing: Thermal expansion and contraction can cause jointing and fracturing in the surrounding rocks.
- Minor Folding and Faulting: In some cases, localized folding or faulting may occur due to stress induced by the intrusion.
Examples of Rocks and their Features
| Protolith | Metamorphic Rock | Salient Features |
|---|---|---|
| Shale | Hornfels | Fine-grained, dense, dark-colored; recrystallization of clay minerals into micas and other minerals. |
| Limestone | Marble | Recrystallization of calcite, loss of bedding planes, increased hardness and durability. |
| Sandstone | Quartzite | Recrystallization of quartz grains, increased hardness and resistance to weathering, granoblastic texture. |
| Granite | Hornfels (altered granite) | Recrystallization of feldspars and quartz, formation of new minerals like hornblende. |
Conclusion
Thermal metamorphism, driven by heat, results in significant alterations in the mineralogy, texture, and structure of rocks. The features exhibited – including the formation of index minerals, recrystallization, and the development of contact aureoles – provide crucial information about the thermal history of a region. Understanding these changes is essential for geological mapping, resource exploration, and reconstructing past tectonic events. Further research into the kinetics of metamorphic reactions and the role of fluid activity will continue to refine our understanding of this important geological process.
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.