Igneous Rocks Rocks formed by the hardening and crystallization of molten material that originates deep within the earth. This material, magma , is a mixture of liquid rock, gases, and mineral crystals. Extrusive rocks form when magma flows onto the surface of the earth or ocean floor. Once the magma reaches the surface it is known as lava. As the lava cools, it either forms rocks of microscopic crystals, smooth volcanic glass or porose volcanic glass.
Intrusive rocks form when magma solidifies beneath the earth's surface.
As the magma solidifies, it forms rocks with large crystals.