Igneous rock is the first kind of rock. It is formed by the hardening and crystallization of molten material that comes from deep within the earth. This material, magma, is a mixture of liquid rock, gases, and mineral crystals.

Scientists separate igneous rock into two groups- extrusive and intrusive. Extrusive rocks are formed when magma flows onto the surface of the earth or the floor of the ocean and then cools and hardens. Intrusive rocks form when magma becomes a solid beneath the earth's surface.

Extrusive rocks are formed from magma that reaches the earth's surface along fissures (deep cracks) and at volcanic vents. This magma is called lava. It may form broad flat sheets or even build up into a volcano shape.

Intrusive rocks are found in mines and tunnels, or at the surface where they have been exposed by geological uplifting (the earth's plates moving toward each other) and by erosion. Intrusive rock formations vary from thin sheets to huge, uneven masses.

Some examples of igneous rocks are basalt, feldspar, granite, pyroxene, and quartz.