Describing an igneous rock composed of both large and small crystals.

Describing an igneous rock composed of both large and small crystals.

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

Answer: (A)

Porphyritic texture describes an igneous rock that has two distinct crystal sizes: large crystals embedded in a finer-grained matrix. This happens when magma begins cooling slowly enough to allow some crystals to grow into noticeable size, then moves to shallower levels or erupts, where cooling is rapid and the remaining melt solidifies into many small crystals. The result is big crystals (phenocrysts) set in a background of tiny crystals. This is different from rocks with uniform large crystals (phaneritic), where cooling was consistently slow; rocks with crystals too small to see (aphanitic), where cooling was rapid; and rocks with no crystals at all (glassy), where cooling was extremely instantaneous. The described texture fits porphyritic.

Porphyritic texture describes an igneous rock that has two distinct crystal sizes: large crystals embedded in a finer-grained matrix. This happens when magma begins cooling slowly enough to allow some crystals to grow into noticeable size, then moves to shallower levels or erupts, where cooling is rapid and the remaining melt solidifies into many small crystals. The result is big crystals (phenocrysts) set in a background of tiny crystals. This is different from rocks with uniform large crystals (phaneritic), where cooling was consistently slow; rocks with crystals too small to see (aphanitic), where cooling was rapid; and rocks with no crystals at all (glassy), where cooling was extremely instantaneous. The described texture fits porphyritic.