The inner core is a solid ball of metal at the very center of the Earth, roughly 1,220 kilometers (about 760 miles) in radius. It sits beneath every other layer of the planet, surrounded by the liquid outer core, and begins at a depth of about 5,150 kilometers below the surface. Despite temperatures rivaling the surface of the sun, the inner core remains solid because the pressure at Earth’s center is so extreme that it forces atoms into a tightly packed crystalline structure.
What the Inner Core Is Made Of
The inner core is primarily iron, alloyed with roughly 10 percent of lighter elements. Scientists estimate that combination includes nickel, oxygen, sulfur, or some mix of the three. No one has ever sampled the inner core directly, so these estimates come from comparing how seismic waves travel through it with laboratory experiments that squeeze iron and iron alloys to enormous pressures.
The iron in the inner core is thought to be arranged in a hexagonal crystal structure, similar to how carbon atoms stack in a diamond but on a planetary scale. There’s also evidence for a distinct zone at the very center, sometimes called the “innermost inner core,” with a radius of about 300 kilometers. Within this zone, the crystal alignment appears to behave differently, suggesting the inner core may have formed in at least two phases or under shifting conditions over time.
Temperature and Pressure at Earth’s Center
Temperatures at the inner core reach approximately 6,150 kelvin (about 5,880°C or 10,600°F), based on experiments that melted iron under extreme compression in diamond-anvil cells. The temperature barely changes across the entire inner core. At the boundary with the outer core, it drops to only about 6,130 kelvin, a difference of just 20 degrees across more than a thousand kilometers.
The pressure at Earth’s center is roughly 364 gigapascals, which is about 3.6 million times the atmospheric pressure you feel at sea level. This is what keeps the inner core solid. Iron normally melts well below 6,000°C at the pressures we experience on the surface, but the melting point rises with pressure. At the center of the Earth, the pressure is high enough that iron’s melting point sits above the actual temperature, locking the metal into a solid state.
How Scientists Discovered It
Danish seismologist Inge Lehmann proposed the existence of the inner core in 1936, working with seismic recordings from earthquakes on the far side of the globe. At the time, scientists already knew the Earth had a core because certain seismic waves (pressure waves, or P-waves) bent sharply when they hit it, creating a “shadow zone” where no direct waves arrived. But Lehmann noticed faint P-waves showing up inside that shadow zone, waves that shouldn’t have been there if the core were entirely liquid.
She hypothesized that these unexpected arrivals were waves that had passed through a solid inner region where they traveled faster, bending back toward the surface at angles the liquid-only model couldn’t produce. Her idea was initially difficult to prove with the limited data available, but subsequent decades of seismic observations confirmed it. Today, scientists use specific wave phases with names like PKIKP (waves that travel through the mantle, outer core, inner core, and back out) to map the inner core’s size, density, and internal variations in detail.
Why It Matters for Earth’s Magnetic Field
The inner core plays a central role in generating the magnetic field that shields Earth from solar radiation. The field itself is produced by churning currents of liquid iron in the outer core, a process called the geodynamo. But the inner core is what keeps this system energized.
As the inner core slowly grows by crystallizing iron from the liquid outer core, it releases heat and expels lighter elements like oxygen and sulfur back into the surrounding liquid. Both effects drive convection, the rising and sinking of material that keeps the outer core in constant motion. Without this energy source, the geodynamo would gradually weaken. Simulations show that before the inner core existed, Earth’s magnetic field was likely weaker and more chaotic, with frequent polarity reversals. As the inner core has grown, the field has stabilized into the strong, predominantly two-pole configuration we have today.
How Old Is the Inner Core
The inner core is surprisingly young compared to the Earth itself. While the planet formed about 4.5 billion years ago, the inner core likely began solidifying between 1 and 1.4 billion years ago. The reason for the delay is that iron doesn’t simply freeze the moment temperatures drop to its melting point at these pressures. Crystallization requires “undercooling,” where the liquid drops several hundred degrees below its melting point before the first solid crystals can form.
Research published in the Proceedings of the National Academy of Sciences suggests the inner core may have nucleated in two steps: first forming crystals with one type of atomic arrangement, then transitioning into the hexagonal structure that dominates today. The required undercooling for this two-step process is about 470 kelvin for the initial phase, which at a cooling rate of roughly 100 degrees per billion years, lines up with the estimated age range. The inner core is still growing today, though extremely slowly, adding new iron to its surface as the outer core continues to cool.
The Inner Core’s Rotation
One of the more striking discoveries of recent decades is that the inner core appears to rotate at a slightly different speed than the rest of the planet. Because it floats within liquid iron, it isn’t mechanically locked to the mantle and crust the way a wheel is fixed to an axle. Seismologists detected this by comparing recordings of nearly identical earthquakes years apart and finding that the waves passing through the inner core had shifted slightly, as if the inner core had rotated to a new position.
A 2024 study in Nature provided the most detailed picture yet. By tracking how seismic waveforms changed and then reverted to earlier patterns over time, researchers concluded the inner core gradually “super-rotated” (spun slightly faster than the surface) from about 2003 to 2008, then slowly reversed direction and “sub-rotated” (spun slightly slower) from 2008 through 2023. The backward rotation was two to three times slower than the forward phase. This back-and-forth motion appears to follow a smoothly reversing path rather than a steady spin in one direction. The total movement is tiny, on the order of a fraction of a degree per year, but it provides the most definitive evidence so far that the inner core moves independently relative to the rest of the Earth.
Some scientists have linked this oscillation to a subtle six-year cycle observed in the length of Earth’s day, which varies by milliseconds. The idea is that gravitational coupling between density variations in the mantle and topography on the inner core boundary could transfer angular momentum back and forth. However, the exact mechanism remains an active area of investigation, and the amount of inner core motion needed to explain the day-length changes may be smaller than initially thought once the drag of surrounding liquid iron is factored in.