Earth’s outer core is liquid because temperatures there are high enough to melt iron‑nickel alloys, while the pressure is not quite high enough to force them into a solid structure, unlike in the deeper inner core where pressure wins and keeps similar material solid.

Where the outer core sits

  • The outer core lies beneath the mantle and surrounds the solid inner core, starting at about 2,900 km below Earth’s surface and extending down to roughly 5,150 km.
  • This zone is a fluid shell about 2,200 km thick, composed mainly of molten iron with some nickel and lighter elements like sulfur, oxygen, or silicon.

Temperature vs pressure balance

  • Temperatures in the outer core are extremely high (thousands of degrees), easily above the melting point of iron at those depths, so the material tends to melt.
  • However, the pressure there, while enormous, is still lower than at the very center, so it cannot “squeeze” the iron‑nickel alloy into a rigid crystal lattice; farther in, in the inner core, the higher pressure outweighs the melting effect and the same material becomes solid.

How we know it is liquid

  • Seismic shear waves (S‑waves) generated by earthquakes do not pass through the outer core, and instead disappear at the core–mantle boundary, which is exactly what happens when such waves meet a liquid layer.
  • Compressional waves (P‑waves) slow and bend in a way that matches a low‑viscosity, liquid metal layer, confirming that this part of Earth’s interior is molten.

Role in Earth’s magnetic field

  • The moving, electrically conducting liquid metal in the outer core sets up electric currents as it circulates, driven by heat escaping from the core and compositional differences in the fluid.
  • These currents act like a giant dynamo: the flows, organized in part by Earth’s rotation, generate and sustain the planet’s global magnetic field, which helps shield the atmosphere and surface from the solar wind.

Simple way to picture it

  • Think of the inner core and outer core as made of almost the same stuff placed under different “settings” of heat and squeeze: in the inner core the “squeeze” (pressure) dominates, so it’s solid, while in the outer core the “heat” dominates, so it stays liquid.
  • This delicate balance between temperature and pressure with depth explains why the outer core is liquid even though it sits just above a solid inner core made of similar iron‑rich material.