Question

Which of the following statement is TRUE with regards to textural feature development in igneous rocks ?

• A. During slow cooling of magma, nucleation rate is slow and growth rate of crystal is high
• B. During slow cooling of magma, nucleation rate is high and growth rate of crystal is low
• C. During fast cooling of magma, nucleation and growth rate of crystal both are slow
• D. Fast cooling of magma results in coarse-grained texture

Answer 1

Answer:

A. During slow cooling of magma, nucleation rate is slow and growth rate of crystal is high

• During slow cooling of magma, nucleation rate is slow and growth rate of crystal is high: When magma cools slowly deep underground, it remains at high temperatures near its melting point for a long time. Under low degrees of undercooling, few crystal nuclei manage to form (low nucleation rate). However, because the molecules have plenty of time to travel and attach to those few existing nuclei, the existing crystals can grow significantly larger (high growth rate). This creates a coarse-grained (phaneritic) texture.

• Slow Cooling $\rightarrow$ Low Nucleation + High Growth $\rightarrow$ Coarse-grained (e.g., Granite)

• Fast Cooling $\rightarrow$ High Nucleation + Low Growth $\rightarrow$ Fine-grained (e.g., Basalt)