The state of matter of Earth's inner core is solid. This highly compact sphere is due to the extreme pressures generated within the inner core.
The layers of Earth are classified based on chemical constitution and physical behavior of the rocks that comprise the planet. Based on physical behavior, the various layers of Earth include the lithosphere, asthenosphere, upper mesosphere, lower mesosphere, outer core and inner core. The outer core exhibits a molten, or liquid, physical behavior.
The inner core, discovered by Inge Lehmann in 1930, is a rigid metallic ball that is primarily composed of an iron-nickel alloy. The lithostatic pressure that increases with depth also increases the melting point of the alloy. The surrounding temperature is inadequate to liquefy the metal, hence, the inner core maintains its solidity.