The Earth’s interior is structured in layers, with the core residing at the center beneath the mantle. This central mass is composed primarily of an iron-nickel alloy, yet it is divided into two separate regions: a fluid outer core and a solid inner core. Although they share a similar metallic composition, the physical properties and dynamic behavior of these two layers are fundamentally different, leading to separate roles in the planet’s overall function.
Physical State and Boundaries
The most significant distinction between the two core regions is their physical state. The outer core is an approximately 2,260-kilometer-thick shell of molten metal, while the inner core is a dense, solid sphere with a radius of about 1,220 kilometers. This difference is confirmed by tracking the passage of seismic waves generated by earthquakes.
Seismic shear waves (S-waves) cannot transmit through liquids, providing definitive proof of the outer core’s fluid nature. Conversely, compressional waves (P-waves) travel through both liquid and solid material, but their speed increases sharply where the outer core meets the inner core. This sudden velocity increase signals the change from a liquid to a solid medium.
The transition zone separating the layers is the inner core boundary, occurring at a depth of roughly 5,150 kilometers. This boundary is also called the Lehmann discontinuity, named after Inge Lehmann, who first hypothesized the solid inner core in 1936. The sharp contrast in rigidity at this deep boundary causes the observed reflection and refraction of seismic energy.
Temperature and Pressure Gradients
The opposing physical states result from the complex relationship between temperature and pressure with depth. Both temperature and pressure increase toward the Earth’s center, but the rate of increase dictates the state of the iron-nickel alloy. The liquid outer core has temperatures estimated between 4,000 K and 8,000 K, which is enough heat to melt the metal at the pressures found there.
Pressure climbs dramatically across the inner core boundary, reaching an estimated 330 to 360 Gigapascals at the center. This immense pressure forces the metallic atoms into a rigid, crystalline structure, overwhelming the heat. The melting point of the alloy rises with pressure, and the conditions in the inner core push the alloy past this elevated melting curve, causing it to solidify.
The inner core’s temperature is estimated to be approximately 5,700 K at its surface, comparable to the temperature at the surface of the sun. The pressure difference is the deciding factor in maintaining the solid state. The outer core is liquid because heat dominates pressure, while the inner core is solid because extreme pressure dominates heat.
Unique Geophysical Functions
The physical differences between the layers lead to unique functional roles in the planet’s dynamics. The fluid outer core, composed of electrically conductive molten iron and nickel, is the site of the geodynamo mechanism. The movement of this liquid metal, driven by thermal and compositional convection, generates the Earth’s magnetic field.
As the planet cools, iron in the outer core slowly crystallizes onto the inner core surface, releasing lighter elements into the surrounding liquid. This compositional convection drives vigorous, buoyant flows, which are influenced by the Earth’s rotation to create self-sustaining electric currents. These turbulent motions produce the magnetic field, which extends into space and protects the surface from solar radiation.
The solid inner core contributes to the magnetic field’s stability and structure. It is believed to rotate slightly faster than the mantle, affecting the convection pattern in the fluid core above it. Its presence and slow growth sustain the compositional convection that powers the geodynamo.