Question

In which type of rock generally, the rock cleavage can be noticed?

• A. fine grained rock
• B. coarse grained rock
• C. both fine and coarse-grained rocks
• D. acidic rocks

Hint:

Finer grains = more cohesive mineral alignment = better cleavage development.

Answer 1

The Correct Option is A

To determine in which type of rock cleavage is generally observed, let's analyze the question systematically:

1. Understanding Rock Cleavage:
Rock cleavage refers to the tendency of rocks to split along closely spaced, parallel planes of weakness. This is different from mineral cleavage and occurs due to:

• Alignment of platy minerals (like micas)
• Recrystallization under stress during metamorphism
• Preferred orientation of mineral grains

2. Grain Size Considerations:
Cleavage development depends on rock texture:

• Fine-grained rocks (e.g., slate, phyllite) show excellent cleavage because their small mineral grains can align more uniformly under stress
• Coarse-grained rocks typically don't develop good cleavage as their large mineral grains resist uniform alignment

3. Rock Type Analysis:
 

• Metamorphic rocks most commonly exhibit cleavage (especially low-grade metamorphic rocks like slate)
• Among sedimentary rocks, only shales (fine-grained) show cleavage-like fracture
• Igneous rocks (including acidic rocks) generally lack cleavage

4. Evaluating the Options:
- "Fine grained rock" is correct as cleavage develops best in fine-grained metamorphic rocks
- "Coarse grained rock" is incorrect as these rarely show cleavage
- "Both" is incorrect as coarse-grained rocks generally don't exhibit cleavage
- "Acidic rocks" is irrelevant as composition doesn't determine cleavage

Final Answer:
Rock cleavage is generally noticed in fine grained rock.