Which description correctly identifies extrusive igneous rock?

• A. Forms from molten rock cooling underground.
• B. Forms from cementation of sediments.
• C. Forms from high pressure altering existing rocks.
• D. Forms from lava erupting onto the surface and cooling rapidly.

Answer: (D)

Extrusive igneous rocks form when lava erupts onto the surface and cools rapidly in air or water. That quick cooling leads to very small crystals or a glassy texture, which is exactly what this description describes. So, rocks produced in this way are classified as extrusive. In contrast, rocks that form underground as magma cools slowly become coarse-grained intrusive igneous rocks; sedimentary rocks come from cementing sediments, and metamorphic rocks come from existing rocks transformed by pressure and heat.