Question

The basement rocks are often:

• A. highly sedimentary and non-crystalline in nature
• B. highly sedimentary but crystalline in nature
• C. highly metamorphic and non-crystalline in nature
• D. highly metamorphic and crystalline in nature

Hint:

Basement rocks = ancient, hard, crystalline metamorphic rocks underlying sedimentary layers.

Answer 1

The Correct Option is D

To determine the characteristics of basement rocks, we analyze their geological nature and formation processes.

1. Understanding Basement Rocks:
Basement rocks form the ancient, stable foundation of continental crust, typically lying below younger sedimentary cover.

2. Evaluating Each Option:
- Highly sedimentary and non-crystalline: Incorrect - basement rocks predate sedimentary cover
- Highly sedimentary but crystalline: Incorrect - while some may be sedimentary in origin, they're typically metamorphosed
- Highly metamorphic and non-crystalline: Contradiction - metamorphic rocks are crystalline
- Highly metamorphic and crystalline: Correct answer

3. Characteristics of Basement Rocks:
- Primarily consist of metamorphic and igneous rocks
- Exhibit well-developed crystalline structures
- Typically Precambrian in age (>541 million years)
- Often intensely deformed and recrystallized
- Examples include gneiss, schist, granite

4. Formation Process:
1. Original igneous/sedimentary protoliths
2. Subjected to multiple orogenic cycles
3. High-grade metamorphism and deformation
4. Development of crystalline textures
5. Cratonization (stabilization of continental crust)

5. Why Other Options Are Incorrect:
- Sedimentary rocks are too young to form basement
- Non-crystalline materials (like glass) don't survive geologic time
- The crystalline nature results from prolonged thermal history

Conclusion:
Basement rocks are characteristically highly metamorphic and crystalline in nature.

Final Answer:
The correct option is: highly metamorphic and crystalline in nature.