Why Earth’s hottest iron does not melt
Temperature alone does not decide whether a material is solid or liquid. Pressure matters too: squeeze iron hard enough and its atoms need more heat to break out of their orderly arrangement. In other words, pressure pushes iron’s melting point upward.
At Earth’s center, temperatures are high enough to melt iron under ordinary conditions. But the weight of all the rock above the center creates immense pressure, raising iron’s melting point beyond the temperature there. The iron therefore remains solid. Farther out, pressure drops; at the boundary with the outer core, the same general iron-rich material can be liquid at the local temperature.
A boundary between freezing and flowing
Earth’s core is not pure iron: it contains other elements, and those affect its melting behavior. The inner core is a solid, iron-rich region surrounded by a liquid, iron-rich outer core. The change in state happens where pressure and temperature meet the conditions for melting—not because the inner core is cool by everyday standards.
The boundary also shifts over time. As Earth gradually loses heat, some liquid in the outer core freezes onto the inner core. This slow growth releases heat and helps move the liquid metal around it. That circulation, together with Earth’s rotation, contributes to the magnetic field that surrounds the planet.
So Earth’s center is not a motionless ball simply because it is solid: it is part of a changing system, with solid metal growing at the center and liquid metal circulating just beyond it.
