Earth's Iron Core and Why It Stays Molten

What the core is actually made of

Seismology, meteorites, and high-pressure experiments converge on a core that is dominantly metallic iron. The density of the core inferred from seismic waves is a few percent lower than pure iron under the same conditions, which forces the presence of light alloying elements. The current best estimate is roughly:

• Iron (Fe): ~85 wt% — the overwhelming majority, matching the iron-nickel composition of metallic meteorites (the shattered cores of ancient planetesimals).
• Nickel (Ni): ~5 wt% — chemically similar to iron, alloys completely with it.
• Light elements: ~10 wt% — most likely a mix of oxygen (O), sulfur (S), silicon (Si), carbon (C), and possibly hydrogen (H).

Iron dominates the core for a cosmochemical reason: it is the most stable nucleus (peak binding energy near ⁵⁶Fe) and therefore the most abundant heavy element produced by stellar nucleosynthesis and supernovae. During Earth's accretion, molten iron — being siderophile ('iron-loving') and denser than silicate rock — sank through the mantle in a global-scale differentiation event, dragging gold, platinum, and other siderophile metals with it. This left the silicate mantle and crust depleted in iron and formed the core in perhaps the first ~30 million years of Earth's history.

Why molten at all: pressure vs melting point

The everyday intuition is that hotter means more likely to be liquid — but pressure is the decisive control at these depths. Increasing pressure raises the melting temperature of iron, because melting expands the material slightly and high pressure fights that expansion (a direct expression of the Clausius–Clapeyron relation, dT/dP = ΔV·T/ΔH_fus, with ΔV > 0 for iron). At the inner-core boundary (ICB), roughly 330 GPa, laboratory and computational estimates put the melting point of iron at about 5,700 K (± several hundred K).

Now compare the local temperature to that pressure-dependent melting curve:

• In the outer core, the actual temperature (the geotherm) lies above the melting curve → the iron alloy is liquid.
• At the inner-core boundary, the geotherm crosses the melting curve → iron begins to freeze solid.
• In the inner core, pressure pushes the melting point higher than the local temperature → solid, crystallizing as the hexagonal close-packed (hcp) ε-iron phase.

So the inner core is solid not despite being hottest, but because the crushing pressure raises iron's freezing point beyond the (still enormous) temperature. The outer core stays molten because Earth simply has not cooled far enough for the geotherm to drop below the melting curve throughout that shell.

Light elements: the antifreeze in the deep Earth

The dissolved light elements do more than fix the density deficit — they act as a freezing-point depressant, exactly like salt on an icy road or antifreeze in a radiator. This is a colligative effect: adding solute (S, O, Si) to the iron liquid lowers its liquidus temperature, so the alloy stays molten at temperatures where pure iron would already have frozen. That helps keep the outer core liquid across its full thickness.

The chemistry also drives the geodynamo through fractional crystallization. When iron freezes onto the growing inner core, the solid preferentially incorporates iron and excludes most of the light elements — much as freezing seawater rejects salt into brine. The consequences are:

• A buoyant, light-element-enriched liquid is released at the inner-core boundary. Being less dense, it rises through the outer core, driving compositional convection.
• Latent heat of crystallization is released as the liquid freezes, adding thermal buoyancy.
• Over time the outer core becomes progressively enriched in light elements while the inner core is comparatively depleted — a chemical stratification recorded in the density difference across the ICB.

Which light elements dominate matters for the story: oxygen and silicon point to equilibration with silicate magma during core formation, while sulfur reflects volatile delivery. Their partitioning between solid and liquid iron sets how efficiently compositional convection can power the field.

The geodynamo: how molten iron makes a magnetic field

A magnetic field does not require magnetized rock — the core is far too hot for permanent magnetism (iron loses its ferromagnetism above the Curie temperature, ~1,043 K, and the core is five times hotter). Instead the field is self-generated by a magnetohydrodynamic dynamo. The recipe needs three ingredients, all present in the outer core:

• A large volume of electrically conductive fluid — liquid iron is an excellent conductor.
• Vigorous convection — powered by heat loss to the mantle plus the compositional and latent-heat buoyancy from inner-core freezing described above.
• Rotation — Earth's spin organizes the flow via the Coriolis force into helical, column-like structures aligned with the rotation axis.

Moving conductor plus a seed field induces electric currents (Faraday's law), and those currents regenerate and amplify the magnetic field faster than it decays — a sustaining feedback. The result is a dipole-dominated field of about 25–65 microtesla at the surface. Without ongoing convection the field would ohmically decay in only tens of thousands of years, so the field's very existence is direct evidence that the outer core is still liquid and still churning today.

How fast it cools and how old the inner core is

The core cools by dumping heat into the overlying mantle across the core–mantle boundary (CMB), at roughly 4,000 K and 136 GPa. Estimates of the total heat flow out of the core are on the order of 5–15 terawatts — a large number, yet the core is so massive (about 1.9 × 10²⁴ kg, roughly a third of Earth's mass) and so hot that it cools only about ~100 K per billion years. That slow cooling is why the outer core has stayed molten for the full age of the Earth.

As the geotherm creeps downward, the point where it intersects the iron melting curve migrates outward, so the inner core grows. Current estimates put inner-core growth at roughly 0.5–1 mm per year in radius, adding on the order of ~10⁶ kg of solid iron each second across its whole surface. Working backward, the solid inner core likely began to crystallize somewhere between 0.5 and 1.5 billion years ago — a comparatively recent event, and its onset may have re-energized the geodynamo, possibly recorded as a jump in the strength of ancient magnetic fields preserved in rocks.

Why it matters: the shield that keeps Earth habitable

The molten, convecting core is not a geophysical curiosity — it is a precondition for the surface being livable. The magnetic field it generates carves out the magnetosphere, deflecting the bulk of the solar wind and cosmic rays. Without it, the steady erosion of the upper atmosphere by charged particles would, over hundreds of millions of years, strip lighter gases and water to space.

• Mars is the cautionary tale. It is smaller, so its core cooled faster and its dynamo shut down roughly 4 billion years ago. Loss of the global field left the atmosphere exposed; today Mars retains only ~0.6% of Earth's surface pressure and is cold and dry.
• The field also protects surface life and technology — shielding the biosphere from ionizing radiation and channeling the worst of solar storms toward the poles as auroras rather than across the whole planet.
• Core heat drives mantle convection, which in turn powers plate tectonics — the engine behind the carbon-silicate weathering thermostat, volcanic outgassing, and the long-term recycling of nutrients.

So a chain runs from deep chemistry to habitability: iron sank to form a core, pressure and light-element antifreeze keep the outer shell liquid, its convection sustains the geodynamo, and the resulting field lets Earth hold onto the atmosphere and oceans that life depends on.

Outer core vs inner core: the same iron alloy in two states

Property	Outer core	Inner core
Depth range	2,890–5,150 km	5,150–6,371 km
Physical state	Liquid Fe alloy	Solid Fe alloy (hcp ε-iron)
Temperature	≈4,000–5,400 K	≈5,400–5,700 K
Pressure	136–330 GPa	330–360 GPa
Light-element load	Higher (~10 wt%)	Lower (~3–5 wt%)
Role	Generates the geodynamo	Grows ~1 mm/yr, releases latent heat