Earth's outer core

Lore & Background

The outer core is liquid, unlike the solid inner core, and evidence for its fluid state comes from seismology, which shows that seismic shear-waves are not transmitted through it. Although having a composition similar to Earth's solid inner core, the outer core remains liquid because there is not enough pressure to keep it in a solid state.

Reader's Guide

Earth's outer core is significant because its turbulent convection, driven by high temperatures and low viscosity, generates Earth's magnetic field via the dynamo theory. The average magnetic field strength in the outer core is estimated at 2.5 millitesla, 50 times stronger than at the surface. This magnetic field protects life from interplanetary radiation and prevents atmospheric dissipation by the solar wind. The outer core also contains light elements such as hydrogen, carbon, oxygen, silicon, and sulfur, which lower its density compared to pure iron-nickel. As the core cools, the liquid at the inner core boundary freezes, causing the solid inner core to grow at an estimated rate of 1 mm per year, releasing light elements that float upward and contribute chemical convection to power the geodynamo. The outer core's composition and behavior provide constraints on Earth's accretion and core formation history, and its cooling rate suggests the core will not freeze for approximately 91 billion years, well after the Sun is expected to expand and sterilize Earth's surface.

Did You Know?

• Seismic shear-waves are not transmitted through the outer core, indicating it is liquid.
• The outer core is approximately 5 to 10 percent lower density than iron at core temperatures and pressures due to light elements.
• The average magnetic field strength in Earth's outer core is estimated to be 2.5 millitesla, 50 times stronger than at the surface.