The Earth’s internal structure is layered, consisting of the crust, the thick silicate mantle, and the metallic core. The core is divided into the liquid outer core and the solid inner core, located at the planet’s center. This solid state is counterintuitive because the inner core is the hottest region of the planet. However, the physical state is not determined by temperature alone. The factor that forces the inner core material into a solid state is the crushing weight of all the overlying rock and metal, resulting in colossal pressure.
Anatomy and Composition of the Inner Core
The inner core is the innermost layer of the Earth, a dense ball with a radius estimated to be about 1,230 kilometers, slightly smaller than the Moon. This sphere is suspended within the liquid outer core, located approximately 5,150 kilometers beneath the surface. Scientists cannot sample this deep layer directly, so its composition and characteristics are inferred primarily through the study of seismic waves generated by earthquakes.
These seismic studies indicate that the inner core is composed predominantly of an iron-nickel alloy. Iron accounts for the vast majority of the mass, but nickel is significant, and models suggest that several lighter elements, such as silicon, oxygen, or sulfur, are also mixed into the alloy. This specific metallic composition is highly dense and provides the base material whose physical state is governed by the extreme conditions at the planet’s center.
The Intense Heat of the Earth’s Core
The Earth’s core is a region of thermal energy, with temperatures estimated to be between 5,400 and 6,000 degrees Celsius. These temperatures are high enough to melt or vaporize any known substance at surface pressures. This makes the inner core the planet’s most intense heat source.
The heat within the core originates from multiple sources. Primary is the residual thermal energy left over from the planet’s formation approximately 4.5 billion years ago. Another element is the latent heat released as the liquid iron alloy of the outer core slowly crystallizes and freezes onto the surface of the growing inner core. Also, heat is continually generated through the slow process of radioactive decay of elements within the Earth’s interior, though this occurs more prominently in the mantle.
Considering this immense heat, the iron-nickel alloy of the inner core should naturally exist in a liquid state. If this material were brought to the Earth’s surface, its high temperature would cause it to instantly become molten metal.
Extreme Pressure Overcomes Temperature
The factor that overcomes the intense heat is the overwhelming hydrostatic pressure exerted by the overlying layers of the Earth. The entire weight of the mantle and the outer core presses inward, generating pressures estimated to be between 3.3 and 3.6 million atmospheres. This pressure is millions of times greater than the atmospheric pressure at sea level.
This extraordinary compression drastically alters the physical properties of the iron-nickel alloy. Increasing the pressure also increases the temperature required for materials to transition from a solid to a liquid phase. This relationship is known as the melting curve, demonstrating that the melting point of the core’s iron alloy rises steeply under increasing pressure. The melting point of iron is elevated to match or exceed the actual temperature of the inner core when subjected to millions of atmospheres of pressure.
While the inner core’s temperature is near 6,000 degrees Celsius, the pressure-induced melting point of the alloy at that depth is even higher. Because the actual temperature is lower than this pressure-dependent melting point, the material cannot melt. It is instead forced into a tightly packed, crystalline solid structure known as epsilon-iron. The immense pressure effectively locks the atoms into place, preventing them from moving freely as they would in a liquid.
The Difference Between the Inner and Outer Core
The stark physical difference between the solid inner core and the liquid outer core results from the delicate balance between temperature and pressure. Both layers share a similar composition, primarily a mixture of iron and nickel. However, the outer core experiences a lower level of pressure because it lies above the inner core.
At the boundary between the two layers, the lower pressure causes a corresponding drop in the melting point of the alloy. Although the outer core is incredibly hot, its temperature exceeds the melting point for the material at that lower pressure. This allows the metal atoms to move freely, creating the liquid state responsible for generating the Earth’s magnetic field through convection currents.
Moving inward, the temperature continues to rise slightly, but the pressure increases much more dramatically. This steep pressure gradient forces the melting point to jump above the actual temperature of the material.