At Earth’s center, temperatures are comparable to those at the visible surface of the Sun. Yet the innermost part of the planet is not a molten sphere. It is a solid body about 1,220 kilometers in radius, enclosed by a much larger layer of liquid metal. The contrast seems contradictory only if temperature is considered by itself.
Whether a substance is solid or liquid depends on pressure and composition as well as heat. In the inner core, the weight of the entire planet creates pressures so extreme that iron-rich metal remains crystallized despite temperatures of several thousand degrees. The solid center and liquid outer core are therefore two parts of one thermal and chemical system, divided where the conditions cross the alloy’s melting boundary.
Pressure Changes the Meaning of Hot
Atoms in a hot material vibrate vigorously, and enough thermal energy can disrupt an ordered crystal lattice. Pressure works in the other direction. When matter is compressed, the stable arrangement of its atoms can change, and the temperature required to melt it generally rises. Near the inner-core boundary, pressure is about 330 gigapascals—more than three million times average atmospheric pressure at sea level. It increases further toward Earth’s center.
Iron at ordinary pressure melts at roughly 1,538 degrees Celsius. That familiar value does not apply deep inside the planet. Laboratory experiments, shock-compression measurements, and atomic-scale calculations indicate that iron alloys at core pressures have melting temperatures of several thousand degrees. Estimates vary because neither the exact temperature nor the exact mixture of elements in the core is known perfectly.
The key comparison is not between the core and a surface furnace. Scientists compare the estimated internal temperature, called the geotherm, with the melting curve of core material at each pressure. Inside the central sphere, the temperature lies on the solid side of that curve. Farther out, pressure falls enough for the same broad family of iron-rich material to be liquid.
One Boundary Separates Two Physical States
The inner-core boundary lies about 5,150 kilometers below the surface. Across it, a solid iron-rich alloy meets the fluid outer core. This does not mean the outer core is hotter everywhere. Temperature generally rises with depth, so the solid center can be hotter than liquid metal above it. The difference is that compression raises the melting threshold more steeply toward the center.
Composition also matters. The core is dominated by iron, with nickel and smaller quantities of lighter elements likely including some combination of silicon, sulfur, oxygen, carbon, and hydrogen. These additions can lower melting temperatures. When solid metal crystallizes at the inner-core boundary, some light elements preferentially remain in the liquid. The outer and inner cores consequently differ slightly in density and chemistry, further influencing where freezing occurs.
The boundary is unlikely to behave as a perfectly smooth, motionless surface. Variations in heat flow through the overlying mantle can favor local crystallization in one region and melting in another. Even so, the long-term balance has allowed the inner core to grow as Earth has cooled.
Earthquakes Reveal the Solid Center
No instrument has approached the core directly. Its physical state is inferred mainly from seismic waves generated by earthquakes. Compressional waves can cross both liquids and solids, but their speeds and paths change at material boundaries. Waves that traverse the center arrive at stations at times consistent with a distinct inner sphere whose elastic properties differ from those of the surrounding liquid.
The strongest basic evidence for a liquid outer core is the disappearance of direct shear waves, which require material with rigidity. Establishing the inner core’s solidity is more demanding because signals sampling it are fewer and weaker. Compressional-wave refraction, reflection from the inner-core boundary, normal modes produced by very large earthquakes, and evidence for shear behavior within the center together constrain a rigid body rather than another liquid layer.
Seismic waves do not travel through the inner core identically in every direction. Many paths near the rotational axis move faster than comparable paths closer to the equatorial plane, a property called anisotropy. This behavior is usually linked to the alignment, deformation, or growth pattern of iron crystals. It shows that “solid” does not mean simple or uniform.
A Solid That Continues to Evolve
Earth loses internal heat over geological time. As the core cools, iron-rich liquid freezes onto the inner core, releasing latent heat. Crystallization also rejects buoyant light elements into the outer core. Both effects help drive fluid motion in the electrically conducting metal, supporting the geodynamo that maintains Earth’s magnetic field.
Scientists still debate the inner core’s age, detailed composition, crystal structure, texture, and patterns of rotation and deformation. Those uncertainties do not overturn the central phase relationship. Immense heat favors melting, but pressure raises the temperature at which melting can begin. Under the conditions at Earth’s center, the iron-rich alloy remains predominantly solid while liquid metal circulates above it.