The Earth’s outer core is a layer situated beneath the mantle and surrounding the solid inner core. This dynamic shell begins roughly 2,889 kilometers below the surface and extends to a depth of about 5,150 kilometers, resulting in a thickness of approximately 2,260 kilometers (1,400 miles). It is a low-viscosity fluid layer that plays a unique role in the planet’s structure and function.
Composition and Physical State
The outer core is primarily composed of a molten iron-nickel alloy. Its measured density is slightly lower than that of pure iron-nickel, suggesting the presence of lighter elements. These elements likely include sulfur, oxygen, and silicon, which lower the alloy’s melting point.
It is the only liquid layer within the Earth’s interior, a state determined by the balance between temperature and pressure. Although the temperature is high enough to melt iron, the surrounding pressure is not sufficient to force the material to crystallize into a solid. This contrasts with the inner core, where greater pressure overrides the thermal effect. Seismology confirms the fluidity of the outer core, as seismic shear waves (S-waves) cannot travel through it.
Extreme Conditions within the Outer Core
Temperatures within the outer core range from about 4,000°C (7,200°F) near the mantle boundary to 6,000°C (10,800°F) near the inner core. Pressure is immense, increasing from approximately 135 to 330 Gigapascals (GPa) with depth. These thermal and compositional gradients drive vigorous convection currents in the liquid metal.
As heat transfers outward and lighter elements are rejected from the crystallizing inner core, the fluid outer core experiences thermal and compositional buoyancy. Hotter, less dense fluid rises, while cooler, denser fluid sinks, creating turbulent flows. This churning motion of the electrically conductive liquid metal sets the stage for the planet’s most significant geophysical process.
Earth’s Magnetic Shield
The dynamic movement within the outer core is the engine for the Geodynamo, the mechanism that generates Earth’s magnetic field. The molten iron-nickel alloy is an excellent electrical conductor, and its convection currents act like a massive, moving wire. As this conductive fluid flows through an existing magnetic field, it generates new electrical currents through electromagnetic induction.
These electrical currents sustain and amplify the magnetic field in a positive feedback loop, maintaining the field over geological timescales. The Earth’s rotation organizes these turbulent flows into structured, spiraling columns, ensuring the continuous generation of the magnetic field.
The magnetosphere is a shield that deflects the constant stream of high-energy charged particles, known as solar wind, and cosmic radiation. Without this magnetic protection, the solar wind would gradually strip away the Earth’s atmosphere, making the surface uninhabitable. The churning liquid metal of the outer core is responsible for maintaining the conditions that allow life to thrive on our planet.
The Transition to the Inner Core
The boundary where the outer core meets the solid inner core is a structural transition known as the Lehmann discontinuity. At this depth, the composition of the core material is largely the same, but the physical state changes abruptly from liquid to solid. The reason for this phase change is the extreme pressure.
The pressure at the inner core boundary is significantly higher than in the outer core, raising the melting point of the iron-nickel alloy above its actual temperature. This squeezing force overrides the thermal energy, causing the metal to crystallize and remain solid. This freezing process releases latent heat and forces lighter elements back into the liquid layer, driving some of the outer core’s convection.