Rapid cooling at earth's surface results in tiny mineral crystals that can only be seen under a microscope.

Rapid cooling at earth's surface results in tiny mineral crystals that can only be seen under a microscope.

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

Answer: (A)

Cooling rate shapes crystal size in igneous rocks. When lava erupts onto the surface, it loses heat very quickly, so mineral grains don’t have time to grow large. That creates a fine-grained texture where the individual crystals are too small to see with the naked eye and require a microscope to observe. This texture is called aphanitic. For comparison, rocks that cooled slowly underground develop visible crystals—phaneritic texture. If cooling is so rapid that no crystals form at all, you get a glassy texture. If some crystals formed early and others crystallized later, you’d see larger crystals set in a finer matrix—porphyritic texture. The scenario described fits the aphanitic texture.

Cooling rate shapes crystal size in igneous rocks. When lava erupts onto the surface, it loses heat very quickly, so mineral grains don’t have time to grow large. That creates a fine-grained texture where the individual crystals are too small to see with the naked eye and require a microscope to observe. This texture is called aphanitic.

For comparison, rocks that cooled slowly underground develop visible crystals—phaneritic texture. If cooling is so rapid that no crystals form at all, you get a glassy texture. If some crystals formed early and others crystallized later, you’d see larger crystals set in a finer matrix—porphyritic texture. The scenario described fits the aphanitic texture.