Which term describes a rock texture with two crystal sizes, larger crystals embedded in a finer-grained matrix?

Which term describes a rock texture with two crystal sizes, larger crystals embedded in a finer-grained matrix?

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

Answer: (A)

Two-stage cooling of magma creates a texture with two crystal sizes: large crystals embedded in a finer-grained matrix. The big crystals, called phenocrysts, form during an initial slow cooling, while the remaining melt cools quickly to form the fine groundmass. This combination is the porphyritic texture. Phaneritic describes a uniform, coarse-grained rock where crystals are all large and similar in size, indicating slow, steady cooling. Aphanitic refers to a fine-grained rock where crystals are too small to see, showing rapid cooling. Glassy means the lava cooled so rapidly that no crystals formed, leaving a glassy structure.

Two-stage cooling of magma creates a texture with two crystal sizes: large crystals embedded in a finer-grained matrix. The big crystals, called phenocrysts, form during an initial slow cooling, while the remaining melt cools quickly to form the fine groundmass. This combination is the porphyritic texture.

Phaneritic describes a uniform, coarse-grained rock where crystals are all large and similar in size, indicating slow, steady cooling. Aphanitic refers to a fine-grained rock where crystals are too small to see, showing rapid cooling. Glassy means the lava cooled so rapidly that no crystals formed, leaving a glassy structure.