The Earth's core is made up primarily of the metals nickel and iron. The inner core is mostly iron, while the outer core is composed of both nickel and iron.... The Earth's core is made up primarily of the metals nickel and iron. The inner core is mostly iron, while the outer core is composed of both nickel and iron. The inner core is very hot at 9,000 to 13,000 degrees Fahrenheit, however, the iron is not liquid because of the pressure of the Earth pressing in on it.