Large crystals found in a fine-grained groundmass can indicate:
A) origin from a magma with an early intrusive phase followed by an extrusive phase.
B) origin from a magma with an early extrusive phase followed by an intrusive phase.
C) a rock of mixed volcanic-plutonic origin.
D) origin from a magma with an early intrusive phase followed by an extrusive phase AND a rock of mixed volcanic-plutonic origin.
E) origin from a magma with an early extrusive phase followed by an intrusive phase AND a rock of mixed volcanic-plutonic origin.

Question

Quizplus · Accepted Answer

Large crystals in a fine-grained groundmass suggest that the rock has undergone two phases of cooling: a slow cooling phase that allows large crystals to form (intrusive), followed by a rapid cooling phase that results in a fine-grained texture (extrusive). This indicates a rock of mixed volcanic-plutonic origin, where the early intrusive phase allowed large crystals to grow, followed by an extrusive phase that caused the surrounding material to cool quickly.

Large Crystals Found in a Fine-Grained Groundmass Can Indicate