The main type of rock found in the Earth's crust is igneous rock. This type forms through the cooling and solidification of magma or lava. Igneous rocks can be classified into two categories: intrusive (or plutonic) rocks, which cool slowly beneath the Earth's surface, and extrusive (or volcanic) rocks, which cool quickly on the surface following a volcanic eruption. Common examples of igneous rocks include granite (intrusive) and basalt (extrusive). These rocks play a crucial role in the geological cycle and provide valuable information about the Earth's history and composition.