Why is the earth divided into layers

The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.

answer; in the united states the plains include the great plains of the west and the tail grass prairie to the south of the great lakes extending east to the appalachian plateau