Where do igneous rocks form?

• A. Only in sedimentary basins.
• B. In deserts.
• C. Deep within Earth's mantle.
• D. At or near volcanic activity, either on the surface as volcanic rocks or underground as intrusive rocks.

Answer: (D)

Igneous rocks form when molten rock cools and crystallizes. This happens as magma makes its way toward the surface or sits underground: if it erupts, the lava reaches the surface and cools quickly to make volcanic (extrusive) rocks with very small crystals or glassy textures; if it solidifies below the surface, it cools slowly to form intrusive rocks with larger crystals. So the key idea is that solidification of magma near volcanic activity—either at the surface or beneath the crust—produces igneous rocks. The other options don’t capture this cooling and crystallization process in the right setting: sedimentary basins form sedimentary rocks, deserts aren’t a defining setting for igneous formation, and while some melting and crystallization occur deep in the mantle, igneous rocks most commonly form in association with volcanic activity near or at the surface.