Which description best matches extrusive igneous rocks?

• A. Extrusive rocks form when magma erupts and cools at or near the surface
• B. Rocks formed when magma cools slowly underground
• C. Rocks formed by metamorphism of existing rocks
• D. Sedimentary rocks formed by lithification

Answer: (A)

Extrusive igneous rocks form when magma erupts onto the surface and cools rapidly. This quick cooling produces very small crystals or a glassy texture because crystals don’t have time to grow, which is the hallmark of rocks formed from lava at or near the surface. That’s why a description stating magma erupts and cools at or near the surface best matches extrusive rocks. In contrast, rocks that form when magma cools slowly underground are intrusive igneous rocks with larger crystals. Metamorphic rocks arise from existing rocks altered by heat and pressure, not from molten rock cooling, and sedimentary rocks come from sediments that become rock through lithification.