The inner core is solid mainly because pressure beats temperature at Earth’s center, forcing the material to stay locked in a solid state even though it is incredibly hot.

The basic idea in one line

Deep inside Earth, the temperature is high enough to melt iron, but the pressure is even higher, so iron is squeezed into a solid rather than allowed to be liquid.

How temperature and pressure compete

Think of the inner core as a battle between heat trying to melt iron and pressure trying to freeze it.

  • Temperature in the core is estimated to be similar to or hotter than the surface of the Sun’s photosphere (thousands of degrees Celsius), which would normally keep iron molten.
  • Pressure at the inner core is enormous because of the weight of all the overlying rock and metal, reaching millions of times the air pressure at Earth’s surface.
  • At such extreme pressures, the melting point of iron goes up, so the “temperature needed to melt” iron is much higher than it would be at the surface.

Result: at the actual conditions of the inner core, iron sits below its (pressure‑raised) melting point, so it stays solid, even though the absolute temperature is extremely high.

Why the outer core is liquid but the inner core is solid

A natural follow‑up to “why is the inner core solid” is “then why is the outer core liquid?”

  • As you move outward from the very center into the outer core, both pressure and temperature decrease, but pressure drops faster in terms of its effect on melting point.
  • That means in the outer core, the actual temperature ends up above the local melting point of iron alloys, so the material is molten.
  • In the inner core, the same material is under greater pressure, so its melting point is higher than the actual temperature; there it is solid.

So, inner core: “too much squeeze to melt”; outer core: “hot enough and not squeezed quite enough, so it stays liquid.”

What the inner core is made of

The inner core is not just any rock; it is mostly metallic iron with some nickel and light elements, in a crystalline form.

  • Seismic waves show that the inner core transmits shear (S‑) waves, which only solids can support, confirming its solid character.
  • The solid iron–nickel crystals appear to be partly aligned, which explains anisotropy: seismic waves travel faster in some directions than others.
  • Some studies even suggest an “inner inner core” with slightly different crystal orientation and properties, hinting at a complex growth and cooling history.

A simple everyday analogy

Imagine trying to boil water inside an ultra‑strong, shrinking metal sphere that squeezes the water from all sides. As you crank up the pressure enormously, the boiling point rises, and at some point, even at very high temperature, the water could no longer boil and would instead behave more and more like a tightly packed phase. For Earth, replace water with iron and the metal sphere with an entire planet’s weight.

Quick recap

  • The inner core is incredibly hot but under even more incredible pressure.
  • High pressure raises the melting point of iron so much that the inner core material is solid at those conditions.
  • Farther out, in the outer core, pressure is lower relative to temperature, so the iron‑rich material is molten.

Information gathered from public forums or data available on the internet and portrayed here.