Earth’s core is a blazing sphere of iron roughly the size of Mars, buried nearly 3,000 kilometers beneath your feet. No camera has ever captured it, and no drill has come close, but decades of seismic imaging and lab experiments have given scientists a remarkably detailed picture. The core has two main parts: a liquid outer shell of molten iron and a solid inner ball of crystalline metal, both glowing far hotter than the surface of the sun.
Two Layers, Two Very Different States
The core stretches from about 2,900 kilometers deep all the way to Earth’s center at 6,371 kilometers. The outer core is a churning ocean of liquid iron roughly 2,200 kilometers thick. Below it sits the inner core, a solid metal sphere about 1,220 kilometers in radius, a little smaller than the Moon. The boundary between the two is sharp. There’s a density jump of about 4.5% at this border, too large to be explained by the simple difference between liquid and solid iron. The inner core is denser because it has squeezed out most of its lighter elements as it slowly crystallized over billions of years.
What the Core Is Made Of
Both layers are primarily iron alloyed with roughly 5% nickel. Mixed in are lighter elements like silicon, oxygen, sulfur, and carbon. The inner core contains only about 2 to 3% of these light elements, while the outer core holds 5 to 10%. Recent studies suggest the inner core is specifically an iron-nickel-silicon alloy with 1 to 2% silicon. Oxygen plays a special role: it strongly resists being incorporated into solid iron, so as the inner core grows by crystallization, oxygen gets expelled into the surrounding liquid. This helps explain why the outer core is noticeably less dense than the inner core despite being made of the same basic ingredients.
Temperature and Pressure at the Center
If you could somehow look at the core, the first thing you’d notice is that it glows. At around 6,150 kelvin (roughly 5,880°C or 10,600°F), the center of the Earth is hotter than the surface of the sun. The inner core is nearly the same temperature throughout, dropping only about 20 degrees from its center to its outer edge. The outer core is cooler at its top, around 4,000 kelvin, and hotter at its base, near 6,130 kelvin. That temperature gradient is what keeps the liquid iron moving.
Pressure is equally extreme. At the boundary where the core meets the rocky mantle above, pressure sits around 135 gigapascals. At the center, it climbs to roughly 330 to 365 gigapascals. For perspective, one gigapascal is about 10,000 times atmospheric pressure at sea level, so the center of the Earth experiences pressure more than 3 million times what you feel standing outdoors.
A Solid Crystal Ball
The inner core isn’t just a featureless lump of metal. It’s crystalline. Iron at these extreme pressures and temperatures takes on a hexagonal close-packed crystal structure, the same arrangement you’d find in stacked oranges if each layer nestled into the gaps of the layer below. Lab experiments have confirmed this structure remains stable up to 377 gigapascals and 5,700 kelvin, conditions that match the inner core.
These iron crystals aren’t randomly oriented. They tend to align along a preferred direction, which is why seismic waves travel faster through the inner core along the north-south axis than along the equator. This anisotropy, a kind of grain to the metal like the grain in wood, is one of the inner core’s most distinctive features.
A Hidden Core Within the Core
In 2023, researchers at the Australian National University confirmed something that had been hypothesized for about 20 years: there’s a distinct innermost inner core, essentially a fifth layer of the Earth. By tracking seismic waves that bounced back and forth up to five times along Earth’s diameter (previous studies had only captured a single bounce), the team found that the crystalline structure near the very center appears to be different from the outer portion of the inner core. The iron crystals in this deepest region may be oriented differently, suggesting the innermost inner core formed under conditions or processes that changed at some point in Earth’s history.
The Churning Liquid Outer Core
The outer core is where things get dynamic. Picture an ocean of molten iron alloy 2,200 kilometers deep, constantly in turbulent motion. Two forces drive this movement. Thermal convection happens as the outer core slowly cools: denser, cooler liquid sinks toward the inner core while hotter material rises. Compositional convection happens at the inner core boundary, where light elements rejected by the crystallizing inner core are too buoyant to stay put and instead rise through the fluid, stirring it from the bottom up.
This constant churning of electrically conductive liquid iron is what generates Earth’s magnetic field through a process called the geodynamo. The vigor of the flow matters enormously. Simulations show that if the liquid moves too calmly, the magnetic field never reverses polarity. If it’s too turbulent, the field breaks apart into unstable patches with multiple poles rather than behaving like a bar magnet. Earth sits at a sweet spot where the field acts like a stable two-pole magnet most of the time but occasionally flips during brief periods of multipolar behavior. Compositional convection, not thermal convection, appears to be the primary driver that matches what we see in the long-term magnetic record preserved in rocks.
The viscosity of the outer core remains surprisingly hard to pin down. Estimates span 14 orders of magnitude, from roughly the consistency of water to something far thicker. Some recent work places the viscosity of the lowermost outer core around a billion pascal-seconds, which would make it enormously more viscous than everyday liquids, but the debate is far from settled.
Where the Core Meets the Mantle
The boundary between the core and the rocky mantle above it isn’t a clean line. Just above the molten outer core sits a region called the D” (D double-prime) layer, roughly 250 kilometers thick, with a seismic signature unlike anything else in the planet. This zone contains a mineral called silicate post-perovskite, which can absorb large amounts of iron, dramatically changing its properties.
Scattered across this boundary are ultralow-velocity zones: thin patches just 5 to 40 kilometers thick where seismic waves slow down dramatically, with shear wave speeds dropping 10 to 30% compared to surrounding rock. These patches may represent partially molten material, regions where the searing heat of the core has begun to alter the base of the mantle. The iron-enriched post-perovskite in this layer helps explain these strange slow zones and also produces the splitting of seismic shear waves that scientists observe when probing this depth.
How Scientists Actually “See” the Core
No one has ever directly observed the core, so everything we know comes from seismic waves generated by large earthquakes. When these waves travel through Earth, they bend, reflect, and change speed depending on the material they pass through. By measuring the arrival times of waves at seismograph stations worldwide, researchers build tomographic images of the core’s structure, similar in concept to a medical CT scan.
The most recent imaging techniques use a method called Hamiltonian Monte Carlo inversion, applied to thousands of seismic wave pairs that sample the outermost 50 to 100 kilometers of the inner core. These tomograms reveal that the top of the inner core isn’t a simple two-hemisphere pattern, as older models suggested, but has a more complex patchwork of velocity and density variations. Scientists can even estimate surface temperature maps of the inner core from attenuation data, showing how seismic energy is absorbed differently across the globe. The picture that emerges is a lumpy, textured surface rather than a smooth metallic sphere.
If you could somehow shield yourself from the heat and pressure and peer at the core directly, you’d see a blinding glow from thermal radiation at nearly 6,000°C, a roiling sea of liquid metal above, and a crystalline iron ball below with a subtle grain running roughly pole to pole. It would look less like a polished steel ball and more like a living, layered furnace at the heart of the planet.