Lesson 1 Earth’s Layers: We learned that the mantle is about 2900 km thick. It is made up of oxygen, silicon, magnesium, and iron at a temperature of more than 1000°C. Although it is solid it is able to flow like a fluid or plastic material because of its high temperature. Then at the center of the Earth is the core which is made up of a liquid outer core and a solid inner core. The outer core is predominantly iron and nickel and the inner core is almost pure iron-nickel alloy. We watched a video about the layers of the Earth and the Theory of Plate Tectonics.