Schistosity forms due to : A. Dynamothermal metamorphism Choose the correct answer from the options given below :
B. Contact metamorphism
C. Plutonic metamorphism
D. Thermal metamorphism
Schistosity Formation Explained
Schistosity is a type of metamorphic fabric characterized by the parallel alignment of platy or elongated mineral grains, such as micas. This texture results from the development of a pervasive planar fabric within the rock.
Key Metamorphic Processes
- Dynamothermal Metamorphism: This process involves both high temperatures and significant directed pressure (stress). The directed pressure causes minerals to recrystallize and align perpendicular to the maximum stress direction, leading to foliation like schistosity. It is commonly associated with regional mountain-building events.
- Contact Metamorphism: This type occurs when rocks are heated by proximity to an igneous intrusion. Heat is the dominant factor, and pressure is usually uniform (confining), not directed. Therefore, contact metamorphism typically produces non-foliated rocks (like hornfels), not schistosity.
- Thermal Metamorphism: This is essentially synonymous with contact metamorphism, emphasizing the role of heat. It does not typically involve the directed pressure needed for schistosity.
- Plutonic Metamorphism: While this term refers to metamorphism deep within the crust, often related to plutons, it doesn't specifically imply the directed pressure needed for schistosity. Its effects can vary depending on the specific pressure-temperature-stress conditions.
Conclusion on Schistosity
Based on the requirements for developing a foliated texture like schistosity, dynamothermal metamorphism (A) is the primary cause. Contact metamorphism (B) and thermal metamorphism (D) lack the necessary directed pressure. While plutonic settings can sometimes have directed pressure, dynamothermal metamorphism specifically describes the conditions (heat + directed pressure) that create schistosity.
Therefore, schistosity primarily forms due to dynamothermal metamorphism.
The correct option is A only.
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