Texture developed when crystals are unable to form; Rapid rate of cooling prevents crystals from forming.

Texture developed when crystals are unable to form; Rapid rate of cooling prevents crystals from forming.

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

Answer: (A)

Cooling rate controls whether crystals can form. When magma or lava cools so rapidly that atoms can’t arrange into a repeating lattice, no crystals develop and the melt becomes volcanic glass. This results in a glassy texture—an amorphous solid like obsidian that typically fractures conchoidally. Other textures reflect some crystal formation: aphanitic rocks are fine-grained but crystalline (tiny crystals), porphyritic rocks have crystals of noticeable size paired with a finer groundmass due to two-stage cooling, and phaneritic rocks are coarse-grained from slow cooling. The rapid cooling that prevents crystal growth specifically yields glassy texture.

Cooling rate controls whether crystals can form. When magma or lava cools so rapidly that atoms can’t arrange into a repeating lattice, no crystals develop and the melt becomes volcanic glass. This results in a glassy texture—an amorphous solid like obsidian that typically fractures conchoidally. Other textures reflect some crystal formation: aphanitic rocks are fine-grained but crystalline (tiny crystals), porphyritic rocks have crystals of noticeable size paired with a finer groundmass due to two-stage cooling, and phaneritic rocks are coarse-grained from slow cooling. The rapid cooling that prevents crystal growth specifically yields glassy texture.