Earth's outer core

Lore & Background

The outer core is a fluid layer approximately 2,260 kilometers thick, composed mainly of iron and nickel, lying between Earth’s solid inner core and its mantle. It begins about 2,890 kilometers beneath the surface at the core-mantle boundary and ends roughly 5,150 kilometers down at the inner core boundary. Unlike the solid inner core, the outer core remains liquid because the pressure at that depth is insufficient to solidify its material, despite a similar composition. Seismic evidence confirms this liquid state, as shear waves (S-waves) cannot travel through it. The outer core’s radius is constrained by seismic inversions to 3,483 kilometers, with an uncertainty of 5 kilometers. Temperatures range from about 4,137 to 4,300 K at the core-mantle boundary to 5,400–6,300 K near the inner core. Due to these high temperatures, the outer core is a low-viscosity fluid that convects turbulently. This turbulent motion, driven by eddy currents in the nickel-iron fluid, is the principal source of Earth’s magnetic field via the dynamo theory; the average magnetic field strength within the outer core is estimated at 2.5 millitesla, 50 times stronger than at the surface. As the core cools, liquid at the inner core boundary freezes, causing the solid inner core to grow by about 1 millimeter per year, equivalent to roughly 80,000 tonnes of iron per second. The outer core’s density is 5 to 10 percent lower than pure iron at core temperatures and pressures, indicating the presence of light elements such as hydrogen, carbon, oxygen, silicon, sulfur, and nickel.

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.