The first rocks on Earth were igneous rocks. Igneous rocks are formed by the cooling and hardening of molten material called magma. The word igneous comes from the Latin word ignis, meaning fire. There are two types of igneous rocks: intrusive and extrusive. Intrusive igneous rocks form within Earth's crust; the molten material rises, filling any available crevices, into the crust, and eventually hardens. These rocks are not visible until the earth above them has eroded away. Intrusive rocks are also called plutonic rocks, named after the Greek god Pluto, god of the underworld. A good example of intrusive igneous rock is granite. Extrusive igneous rocks form when the magma or molten rock pours out onto the earth's surface or erupts at the earth's surface from a volcano. Extrusive rocks are also called volcanic rocks. Basalt, formed from hardened lava, is the most common extrusive rock...