How do intrusive igneous rocks form?

• A. They form on the surface.
• B. They form by weathering.
• C. They form when molten magma solidifies below the surface as intrusive rocks.
• D. They form from sediment compaction.

Answer: (C)

When magma rises into the crust and then cools and solidifies underground, it forms intrusive igneous rocks. The key here is the cooling environment: below the surface, cooling happens slowly, allowing large crystals to grow and produce a coarse-grained texture. This is distinct from rocks that form on the surface from erupted lava, which cools rapidly and becomes fine-grained or glassy. It’s also different from weathering or sediment compaction, which are processes that create sedimentary rocks, not intrusive igneous ones.

So the best description is that intrusive rocks form when molten magma solidifies below the surface, producing the characteristic large crystals.