What texture is typical of rocks formed by rapid cooling at the surface?

What texture is typical of rocks formed by rapid cooling at the surface?

• A. Fine-grained or Glassy
• B. Coarse-grained
• C. Medium-grained
• D. Porphyritic

Answer: (A)

Rocks that crystallize quickly at the surface end up with a fine-grained or glassy texture. When lava erupts and cools in the air or in water, there isn’t time for large crystals to form, so the crystals that do develop are very small (fine-grained) or the rock solidifies into a glassy, non-crystalline state (like obsidian). In contrast, slow cooling deep underground yields coarse-grained textures with visible crystals, and porphyritic textures come from cooling in two stages, producing some large crystals within a finer matrix. So the texture that matches rapid surface cooling is fine-grained or glassy.

Rocks that crystallize quickly at the surface end up with a fine-grained or glassy texture. When lava erupts and cools in the air or in water, there isn’t time for large crystals to form, so the crystals that do develop are very small (fine-grained) or the rock solidifies into a glassy, non-crystalline state (like obsidian). In contrast, slow cooling deep underground yields coarse-grained textures with visible crystals, and porphyritic textures come from cooling in two stages, producing some large crystals within a finer matrix. So the texture that matches rapid surface cooling is fine-grained or glassy.