The Earth is composed of three main layers - the crust, mantle, and core. The crust is the outer layer and exists in two types, oceanic and continental. Below the crust is the mantle, a very thick middle layer. The core is at the center and is made mostly of iron, accounting for 1/3 of the Earth's mass. The Earth's layers are similar to an egg, with the crust as the shell, mantle as the egg white, and core as the yolk.