Layers of Earth The first layer is the crust. It is the thinnest layer. It is made of igneous, metamorphic, and sedimentary rocks. The second layer is the mantle. It is the thickest layer. It is made up of magma and rock. The third layer is the outer core. It is thick and mostly composed of mostly iron and nickel. The fourth and last layer is the inner core. It is made of iron and nickel.