The Earth’s core, the deepest and most inaccessible region of our planet, consists of two distinct layers: a solid inner core and a liquid outer core. These two layers exist at immense depths, beginning roughly 2,890 kilometers beneath the surface, and together they represent a realm of extreme pressure and temperature. While fundamentally separated by their physical state, the inner and outer cores share several characteristics that unify them as the planet’s metallic heart.
Common Material Makeup
The most significant similarity between the inner and outer cores is their shared chemical composition, primarily consisting of an iron-nickel alloy. This metallic mixture, often termed ferronickel, makes up the vast majority of both layers, distinguishing them from the silicate-rich mantle. The core is the main reservoir for iron on Earth, a fact deduced from seismic wave analysis.
Both layers experience temperatures ranging between 4,000°C and 6,000°C, approaching the heat of the Sun’s surface. Despite this intense heat, the core material is incredibly dense due to the weight of the overlying planet. The outer core has a density of approximately 11.0 to 12.0 grams per cubic centimeter, which increases to about 12.8 to 13.1 grams per cubic centimeter in the inner core. For comparison, this is more than twice the density of typical surface rock, reflecting the concentration of heavy elements like iron and nickel.
Seismic evidence suggests that the core includes lighter elements to account for the measured density deficit compared to laboratory models of pure metal. Elements like sulfur, oxygen, silicon, or carbon are believed to be alloyed with the iron and nickel in both layers. This inclusion modifies the physical properties of the metallic alloy and plays a role in the core’s dynamics. The intense pressure and temperature conditions are constant for both layers, setting them apart from all other layers of the Earth.
The Critical Divide of Physical State
The most profound difference between the two layers is their physical state, with the outer core being liquid and the inner core being solid. This distinction is not due to a temperature difference alone, but rather the effect of immense pressure on the melting point of the metallic alloy. The outer core, extending from the mantle boundary down to about 5,150 kilometers, is a fluid layer approximately 2,260 kilometers thick.
The inner core, a solid sphere with a radius of about 1,220 kilometers, is hotter than the outer core, yet the material remains solid. This phenomenon is known as pressure freezing. The pressure at the inner core boundary is overwhelming—reaching approximately 3.5 million times the atmospheric pressure—forcing the metallic atoms into a rigid, crystalline structure despite the extreme heat.
The liquid state of the outer core is confirmed by how seismic waves travel. Shear waves (S-waves) cannot propagate through liquids; they stop abruptly at the core-mantle boundary, confirming the outer core is molten. Conversely, the solid nature of the inner core is indicated by its ability to transmit compressional waves (P-waves) at a higher velocity than the liquid outer core.
Unique Functions Within the Earth System
The difference in physical state dictates the unique functions each core layer performs. The liquid outer core is the engine for the Earth’s magnetic field, a process known as the geodynamo. The molten iron and nickel, which are excellent electrical conductors, are constantly in motion due to thermal and compositional convection.
As this electrically conductive fluid churns and circulates, it generates powerful electric currents, which in turn create the planet’s vast magnetic field. This magnetic field extends into space, forming the magnetosphere that shields the Earth’s surface from harmful solar radiation and charged particles. Without the liquid motion of the outer core, this protective field would collapse, making the surface environment far less hospitable to life.
The solid inner core plays a supportive role in this system. Its growth, as the liquid outer core slowly freezes onto its surface, drives convection in the outer core. The exclusion of lighter elements during this freezing provides compositional buoyancy, which helps power the turbulent fluid motions necessary for the geodynamo. Furthermore, the inner core’s rotation and solid presence affect the dynamics of the liquid outer core and contribute to the overall stability of the planet. The transfer of heat outward also contributes to the internal heat flow that influences processes like plate tectonics on the surface.