Igneous rocks are produced when magma exits and cools above the Earth's surface. What term best describes rocks formed in this way?

• A. Extrusive Igneous
• B. Intrusive Igneous
• C. Clastic Sedimentary
• D. Metamorphic

Answer: (A)

When magma erupts to the surface and cools rapidly, it forms extrusive igneous rocks. The quick cooling at the surface produces very small crystals or a glassy texture, since there isn’t time for large crystals to form. That surface-forming process defines rocks created from lava that erupts and solidifies outside the crust. In contrast, rocks formed from magma cooling underground become intrusive igneous rocks with larger crystals, while clastic sedimentary rocks come from cemented fragments, and metamorphic rocks come from existing rocks altered by heat and pressure.