Volcanic rocks tend to have small crystals because they cool rapidly at the surface.

Volcanic rocks tend to have small crystals because they cool rapidly at the surface.

• A. True
• B. False
• C. No crystals
• D. Large crystals

Answer: (A)

Crystal size in igneous rocks is controlled by cooling rate. When magma erupts at the surface, it meets air or water and cools very quickly, leaving little time for crystals to grow. This rapid cooling yields a fine-grained texture, with minerals too small to see with the naked eye. If cooling happens even faster, glass can form instead of crystals, as seen in obsidian. In contrast, rocks that crystallize underground cool slowly, allowing larger crystals to develop. So volcanic rocks commonly showing small crystals because they cool rapidly at the surface is true.

Crystal size in igneous rocks is controlled by cooling rate. When magma erupts at the surface, it meets air or water and cools very quickly, leaving little time for crystals to grow. This rapid cooling yields a fine-grained texture, with minerals too small to see with the naked eye. If cooling happens even faster, glass can form instead of crystals, as seen in obsidian. In contrast, rocks that crystallize underground cool slowly, allowing larger crystals to develop. So volcanic rocks commonly showing small crystals because they cool rapidly at the surface is true.