Question

Which texture is characteristic of rapidly cooled volcanic rocks?

• A. Pegmatitic
• B. Phaneritic
• C. Porphyritic
• D. Aphanitic

Hint:

{Aphanitic texture = Rapid cooling = Volcanic rocks}. Always associate fine-grained texture with surface cooling.

Answer 1

The Correct Option is D

Step 1: Understanding volcanic rock formation.
Volcanic rocks are formed when magma erupts onto the Earth's surface as lava. At the surface, lava is exposed to atmospheric conditions and cools very rapidly compared to magma that cools below the surface.
Step 2: Effect of rapid cooling on crystal size.
Rapid cooling does not allow sufficient time for mineral crystals to grow large. As a result, the crystals formed are extremely fine-grained and often cannot be seen with the naked eye. This type of texture reflects quick solidification.
Step 3: Analyzing the given options.
(A) Pegmatitic: Characterized by very large crystals, formed by very slow cooling at great depths. Hence, incorrect.
(B) Phaneritic: Consists of coarse-grained crystals formed by slow cooling of magma beneath the Earth's surface. Hence, incorrect.
(C) Porphyritic: Indicates two stages of cooling, not purely rapid cooling. Hence, incorrect.
(D) Aphanitic: Characterized by very fine-grained crystals formed due to rapid cooling of lava at or near the Earth's surface. This correctly matches volcanic conditions.
Step 4: Conclusion.
Since rapidly cooled volcanic rocks develop very fine-grained textures, the correct answer is (D) Aphanitic.