<human>:  Why is the crust of the Earth so thick?
<bot>: The crust of the Earth is about 30 km thick. The crust is the outermost layer of the Earth. It is made up of a variety of rocks, including granite, basalt, and sandstone. The crust is the result of the Earth’s cooling process. As the Earth cools, the outer layer of the Earth, the crust, forms. The crust is also the result of plate tectonics. The plates of the Earth’s crust move around, and the crust is constantly being broken up and reformed.