Which type of rock forms from the cooling of molten rock at or below the surface?

• A. Sedimentary rock
• B. Metamorphic rock
• C. Erosion
• D. Igneous rock

Answer: (D)

Igneous rocks form when molten rock cools and solidifies. When rock melts, its minerals are in a liquid form, and as that melt loses heat, crystals begin to form and lock into place, creating solid rock. If cooling happens underground, crystals grow larger, producing coarse-grained intrusive rocks; if cooling happens quickly at the surface, crystals are small or may form a glassy texture. This is distinct from sedimentary rocks, which come from weathered and transported pieces of other rocks that are deposited and cemented together, and from metamorphic rocks, which arise when existing rocks are altered by heat and pressure without melting. Erosion is a process that wears away rock and isn’t a type of rock.