Like an onion, the earth has several layers. The outer most layer of the earth is a earth’s crust. It was an average depth of about 40 km. As compared with the diameter of the earth, which is around 12700 km , the earth crust is very small. It is primarily made up of solid rocks. Below the crust is the mantle, reaching to a depth of the about 2900 km. Due to the heat inside the earth the mantle continues of solid and molten rocks. Below the mantle is the core, made up of iron and nickle.