Core
The core is Earth's innermost region, made mostly of iron and nickel, with a liquid outer core and a solid inner core.
Definition
The core is the very center of Earth, made mostly of iron and nickel and extremely hot, with a liquid metal outer part and a solid metal ball at the inner part. It begins about $2{,}900\ \text{km}$ below the surface and extends to Earth's center at $6{,}371\ \text{km}$; the liquid outer core is about $2{,}200\ \text{km}$ thick and the solid inner core has a radius of about $1{,}220\ \text{km}$. Convection in the liquid outer core generates Earth's magnetic field, which shields the planet from the solar wind. The core makes up about $32\%$ of Earth's mass but only $16\%$ of its volume, with densities from about $10$ to $13\ \text{g/cm}^3$. Temperatures reach roughly $5{,}000$ to $6{,}000\ \text{K}$ at the inner core boundary, where pressure of about $330\ \text{GPa}$ keeps iron solid despite the heat. The outer core contains a few percent of lighter elements (sulfur, oxygen, silicon, or hydrogen), and the inner core grows about a millimeter per year as the planet cools, releasing latent heat and light elements that help drive the geodynamo.
Example
The inner core is about as hot as the surface of the Sun, but the crushing weight of the whole planet keeps it solid. Scientists know the outer core is liquid because S-waves from earthquakes, which cannot travel through liquid, disappear on the far side of Earth, leaving a shadow zone. The magnetic field generated in the outer core reverses polarity at irregular intervals averaging a few hundred thousand years, and these reversals are recorded in seafloor basalt as magnetic stripes symmetric about mid-ocean ridges. Seismic anisotropy shows that the inner core transmits P-waves faster along the rotation axis than across it.
Key Insight
The swirling liquid metal of the outer core is what makes Earth a giant magnet, and that is why a compass points north. No one has ever drilled more than about $12\ \text{km}$ into Earth, yet we know the core's size, state, and density from earthquake waves that pass through it. Core formation happened within the first tens of millions of years of Earth's history, when molten iron sank through a magma ocean. That event set the planet's chemistry: siderophile elements such as gold and platinum are rare in the crust because most of them went down with the iron.