The Earth’s core lies beneath the thick, rocky mantle and is divided into two distinct regions: a solid inner core and a liquid outer core. Understanding the composition and dynamics of the core provides insight into the planet’s formation, internal heat engine, and the processes that make Earth habitable. The liquid outer core is a highly dynamic layer whose constituents and behavior dictate much of the planet’s geophysical function.
Defining the Outer Core: Location and Physical State
The outer core is located at depths ranging from approximately 2,889 kilometers to 5,150 kilometers below the surface. This layer is about 2,260 kilometers thick, situated above the solid inner core and directly beneath the mantle. Despite the immense pressure, the outer core exists in a fluid, molten state.
The temperature within this layer is extreme, estimated to range from 4,000°C to 6,000°C. This heat keeps the metallic components fully melted, allowing them to flow freely. Seismic data confirms the liquid state, showing that shear waves cannot pass through the outer core. The inner core remains solid because the greater pressure at the planet’s center compresses the material into a rigid form, even at comparable temperatures.
Elemental Composition: Iron, Nickel, and the Density Mystery
The outer core is predominantly an alloy of Iron (Fe) and Nickel (Ni), which together make up an estimated 85 to 90 percent of the material by weight. Iron is the most abundant element, with Nickel constituting roughly 5 percent of the overall mass. Geophysical measurements reveal an inconsistency between the core’s actual density, derived from seismic data, and the expected density of a pure Iron-Nickel alloy.
Seismic models show the outer core is approximately 5 to 10 percent less dense than a pristine Fe/Ni mixture would be. This “density paradox” requires the presence of lighter elements mixed into the metallic alloy. These elements, which have low atomic numbers, are necessary to lower the overall density of the core to match the observed seismic properties.
Scientists have identified several leading candidates for these lighter elements:
- Sulfur (S)
- Silicon (Si)
- Oxygen (O)
- Carbon (C)
- Hydrogen (H)
The exact proportions remain a subject of active research, but models suggest that the total amount of these light elements is between 5 and 10 percent by weight. Estimates often include about 1.7 percent Sulfur and up to 4.0 percent Silicon.
The presence of Oxygen is strongly supported by high-pressure experiments. Precise percentages are difficult to determine directly due to the core’s inaccessibility, but current estimates place Oxygen’s concentration between 0.8 and 5.3 percent by weight. The inclusion of these lighter elements is crucial for matching the density and provides insight into chemical processes during Earth’s formation.
The Outer Core’s Essential Function: Earth’s Magnetic Shield
The existence of the liquid, electrically conductive outer core is directly responsible for generating the planet’s magnetic field, a process known as the geodynamo. Convection currents within the molten iron-nickel alloy are driven by heat escaping from the inner core and the planet’s ongoing cooling. As the core material cools, it crystallizes onto the inner core, releasing lighter elements that rise and stir the liquid metal.
This buoyant motion of the liquid metal, combined with the Earth’s rotation, creates a powerful feedback loop. The movement of the conductive fluid generates electric currents, and these currents, in turn, induce a magnetic field. The Coriolis effect, caused by the planet’s spin, helps organize these currents into spiral patterns that sustain the field and give it a roughly dipolar, or two-pole, structure.
The resulting magnetic field extends far into space, forming a protective bubble called the magnetosphere. This shield aids planetary habitability by deflecting harmful charged particles, such as those from the solar wind and cosmic radiation. Without the dynamic liquid metal within the outer core, Earth would lack this defense, making the surface susceptible to intense radiation and the stripping away of its atmosphere over time.