Igneous rocks form from magma . If the magma reaches the surface, we call it lava .
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 .