Igneous rocks form from magma. Magma is the liquid rock (and dissolved volatile gasses) originating deep within Earth (most often the mantle). If the magma reaches the surface, we call it lava.
Extrusive igneous rocks form from lava cooled at Earth's surface; intrusive rocks form within Earth, directly from the cooling of magma. Most magma actually cools beneath Earth's surface.
Igneous rock classification is based on the rock texture and composition.
The texture of a rock is defined by the size and distribution of the mineral crystals that make up the rock.
The composition is measured by the types and quantities of mineral in the rock.
You can view samples of igneous rocks in the display case on the southeast corner of the second floor of Macelwane Hall.