When a rock has a two-stage cooling history (slow then fast)or has minerals that crystallized at different temperatures and grew at different rates it is ________.
A)aphanitic
B)phaneritic
C)porphyritic
D)pyroclastic

Question

Quizplus · Accepted Answer

A porphyritic rock has two different crystal sizes due to two different cooling rates or mineral growth rates. This matches the description of a rock with a two-stage cooling history or minerals that grew at different temperatures and rates. Aphanitic rocks have fine-grained textures, phaneritic rocks have coarse-grained textures, and pyroclastic rocks are made up of fragments from explosive volcanic eruptions, none of which match the description given.

When a Rock Has a Two-Stage Cooling History (Slow Then