Iron is the main element in Earth's core.
Earth's core is primarily composed of iron, with nickel and lighter elements making up smaller portions. The core is divided into a liquid outer core and a solid inner core because pressure and temperature vary with depth.
The movement of electrically conductive liquid metal in the outer core generates Earth's magnetic field through the geodynamo process. The solid inner core is not simply a giant block of ordinary iron; it exists under extreme pressure and temperature and has distinct physical behavior.
Nickel is commonly mentioned alongside iron because it is an important core component, but it is not the primary element. Earth's mantle and crust have very different compositions, dominated by silicate minerals rather than metallic iron.