4.5: The Core

The Core

At the planet’s center lies the core, which is dense and metallic. Calculations indicate that the core is about 85% iron metal with nickel metal making up most of the remaining 15%. Geologists know the core is metal because of proxies, like metallic meteorites, and from calculations of the overall density of the planet. Moving deeper into the interior of the Earth, temperature and pressure increase. Although the inner core Links to an external site. is very hot (5200°C or 9392°F), it is solid because it is experiencing very high pressure (3.6 million atm). The pressure in the outer core Links to an external site. is not high enough to keep it solid. Much of the heat here is produced by the breakdown of radioactive elements in the inner core.

Earthquake waves are used by geologists to “see” Earth’s internal structure, similar to how a doctor uses a CT scan to look inside your body. This imaging of the Earth’s interior by earthquake waves is seismic tomography Links to an external site., which highlights distinct boundaries, or discontinuities Links to an external site., between layers, where composition and matter changes occur. Because the inner core of the Earth is a solid metallic sphere made mostly of iron and nickel and surrounded entirely by liquid, we can visualize it as a giant ball bearing spinning in a pressurized fluid. Seismic tomography of the inner core has found evidence that it is spinning - rotating - just slightly faster than the rest of the Earth.

Earth’s magnetosphere Links to an external site. is produced by the interaction of the rapidly rotating solid inner core and the convection and electrically-charged nature of the liquid outer core (the geodynamo Links to an external site.). This causes instability, and every several hundred thousand to several million years, Earth’s magnetic field becomes unstable to the point that it temporarily shuts down. When it restarts, its north and south magnetic poles switch; this is called a magnetic reversal Links to an external site..