Earth’s core has two distinct layers: a liquid outer core and a solid inner core. They share a similar iron-nickel composition, but they differ in physical state, size, temperature, pressure, and the roles they play in keeping our planet habitable. The boundary between them sits about 5,150 kilometers below the surface, and the contrast between molten metal on one side and solid metal on the other drives everything from Earth’s magnetic field to the slow growth of the inner core itself.
Size and Location
The entire core begins about 2,900 kilometers beneath the surface, where the rocky mantle ends and molten metal begins. The outer core extends from that depth down to roughly 5,150 kilometers, giving it a thickness of about 2,260 kilometers. The inner core sits at the very center of the planet with a radius of approximately 1,220 kilometers, making it slightly smaller than the Moon. Together, the two layers account for about 18% of Earth’s total volume, but because iron is so dense, they represent roughly a third of the planet’s mass.
Liquid vs. Solid
The most fundamental difference is physical state. The outer core is liquid, a churning ocean of molten iron alloy. The inner core is solid, a dense ball of crystallized iron. This might seem counterintuitive since the inner core is hotter, but the extreme pressure at the center of the planet forces atoms so tightly together that the metal solidifies despite the heat. Farther out, where pressure drops enough, the same iron-nickel mixture stays molten.
Scientists figured this out by studying how seismic waves from earthquakes travel through the planet. A type of wave called an S-wave (a shearing motion, like shaking a rope side to side) cannot pass through liquid. When seismologists noticed that S-waves disappeared after entering the core, they knew the outer core had to be fluid. But faint signals arriving at unexpected locations on the far side of the planet hinted that something solid existed deeper inside.
How the Inner Core Was Discovered
In 1936, Danish seismologist Inge Lehmann noticed that certain P-waves (compression waves that can travel through both solids and liquids) were showing up at seismic stations where existing models predicted a “shadow zone” with no arrivals. She proposed that a smaller, solid inner core existed inside the liquid outer core, and that waves were refracting off its surface and reaching those unexpected locations. She estimated the wave speed inside the inner core at about 8.6 kilometers per second, faster than in the outer core, which is exactly what you’d expect from a solid versus a liquid.
The boundary she identified is now called the Lehmann discontinuity. A few years later, in the 1940s, physicists Francis Birch and Keith Bullen independently proposed that the inner core was solid based on calculations of how iron behaves under extreme pressure. Decades of increasingly precise seismic data have confirmed them all.
Composition
Both layers are primarily iron with a smaller fraction of nickel, roughly 5% by weight. But neither layer is pure metal. The outer core contains lighter elements, most likely sulfur and silicon, possibly along with oxygen and hydrogen. These light elements are important because they lower the melting point of the alloy, helping explain why the outer core stays liquid. Laboratory experiments have tested iron alloys with about 12% sulfur or 15% silicon at pressures approaching 94 gigapascals (nearly a million times atmospheric pressure) and found densities that match what seismic data tell us about the outer core.
The inner core is thought to contain fewer of these lighter elements. As the inner core slowly crystallizes from the surrounding liquid, it preferentially locks in iron and nickel while rejecting lighter components back into the outer core. This process creates a density jump at the boundary between the two layers that seismologists can detect.
Temperature and Pressure
Temperatures in the outer core range from about 4,400°C near the mantle boundary to roughly 6,000°C near the inner core. The inner core itself likely reaches temperatures comparable to the surface of the Sun, somewhere around 5,000 to 7,000°C depending on the estimate. Pressure climbs steadily with depth, from around 135 gigapascals at the top of the outer core to over 360 gigapascals at Earth’s center. It’s that enormous pressure gradient that determines where iron freezes and where it stays molten.
The Outer Core Generates Earth’s Magnetic Field
The liquid outer core is the engine behind Earth’s magnetic field. Because it’s fluid, the molten iron alloy can flow in large convection currents, rising when heated from below and sinking as it cools. Two forces drive this circulation. Thermal buoyancy pushes hotter material upward, while chemical buoyancy arises at the inner core boundary, where lighter elements are released as iron crystallizes onto the growing inner core. These lighter, less dense fluids rise through the surrounding liquid.
This constant churning of electrically conductive liquid iron, combined with Earth’s rotation, creates what physicists call a geodynamo. The moving metal generates electric currents, and those currents produce the magnetic field that shields the planet from solar radiation and makes compass needles point north. Without a liquid outer core, Earth would have no global magnetic field.
The character of that field has changed over time. Early in Earth’s history, before the inner core existed, the geodynamo likely produced a weaker, more complex magnetic field with multiple poles. As the inner core formed and began to grow, the field gradually shifted into the stronger, predominantly two-pole (dipolar) pattern we have today.
The Inner Core Is Still Growing
The inner core formed roughly 1 to 1.5 billion years ago when temperatures deep inside the planet finally dropped enough for iron to begin crystallizing at the center. It has been growing ever since, solidifying from the liquid outer core at an estimated rate of about 1 millimeter per year. That’s slow by any human standard, but over geological time it has built up a sphere more than 2,400 kilometers across.
This gradual freezing releases heat and ejects lighter elements into the outer core, both of which help sustain the convection currents that power the magnetic field. In a sense, the inner core’s growth is what keeps the geodynamo running in its current strong configuration.
The Inner Core Spins Independently
Because the inner core is a solid ball floating within a sea of liquid metal, it isn’t mechanically locked to the rest of the planet. Seismic studies have found evidence that the inner core rotates in the same direction as the surface but slightly faster, gaining roughly one extra degree of longitude per year compared to the mantle and crust. At that rate, the inner core completes a full extra revolution relative to the surface about every 400 years.
That may sound modest, but it’s about 100,000 times faster than the speed at which tectonic plates drift. The driving force is thought to be electromagnetic coupling between the inner core and the convecting outer core. More recent studies have debated whether this super-rotation has slowed or even briefly reversed in recent decades, but the basic observation that the inner core can rotate independently remains one of the more striking consequences of having a liquid layer separating it from the rest of the planet.