Which rock type forms when magma cools slowly beneath Earth’s surface?

Magma that cools slowly beneath Earth’s surface forms intrusive igneous rock.

Because underground magma loses heat gradually, mineral crystals have time to grow. This commonly produces a coarse-grained texture in which individual crystals can be seen without a microscope. Granite is a familiar example, and gabbro is another important intrusive rock.

The word intrusive refers to magma entering or intruding into older rocks. Geologists also call these rocks plutonic, a term connected with Pluto, the Roman god of the underworld. Large underground bodies are called plutons, while sheet-like intrusions include dikes and sills.

Intrusive rocks contrast with extrusive igneous rocks. Lava that erupts at the surface cools much faster, producing fine-grained rocks such as basalt or volcanic glass such as obsidian. The same magma composition can therefore produce different textures depending on where cooling occurs.