The Earth’s core is a region where intense heat meets immense compression. At the very center lies the inner core, a solid sphere of metal confirmed by seismic data. This presents a paradox: the inner core reaches temperatures of approximately 5,200 to 5,500 degrees Celsius, comparable to the heat found on the surface of the sun. Under normal conditions, such intense heat would vaporize any known material, yet the inner core remains solid. Understanding how this metallic sphere resists melting reveals the complex physics governing the deep Earth.
Defining the Inner Core’s Intense Conditions
The solid inner core, with a radius of about 1,220 kilometers, is primarily composed of an iron and nickel alloy. Temperatures at the boundary between the inner and outer core are estimated to be around 5,100 to 5,400 degrees Celsius. This heat is generated by a combination of energy left over from the planet’s formation and the decay of radioactive elements. Under surface-level atmospheric pressure, iron melts at only about 1,538 degrees Celsius. The core’s metallic components are subjected to temperatures three to four times this amount, showing that temperature alone does not dictate the physical state of matter deep within the Earth.
The Overriding Force of Planetary Pressure
The factor that overrides the inner core’s extreme temperature is the colossal pressure exerted by the overlying layers of the planet. This pressure originates from the sheer weight of the Earth’s mantle, crust, and the liquid outer core pressing down on the innermost layer. The pressure at the inner core boundary is estimated to be between 330 and 360 gigapascals (GPa). This is equivalent to about 3.3 million times the atmospheric pressure at sea level.
This immense compression is maximized at the Earth’s center. The pressure is the cumulative result of all the mass above it, integrated throughout the planet’s radius. This overwhelming force squeezes the metallic atoms into a tightly packed crystalline structure. This extreme burden fundamentally alters the physical properties of the iron and nickel alloy.
How Extreme Pressure Raises the Melting Point
Pressure maintains the inner core’s solidity by restricting the movement of atoms. Melting occurs when thermal energy overcomes the attractive forces holding atoms in a fixed crystal lattice structure, allowing them to move freely and transition into a liquid state. The extreme pressure deep within the core physically forces the iron and nickel atoms closer together. This makes it far more difficult for them to break free from their fixed positions.
This relationship is described by the pressure-temperature phase diagram, which shows that a material’s melting curve increases dramatically as pressure rises. For the inner core’s iron and nickel alloy, the tremendous pressure elevates the melting point above the core’s actual temperature. Experiments suggest the melting temperature could be as high as 6,100 to 6,700 Kelvin at the inner core boundary. Since the core’s actual temperature is slightly lower than this pressure-dependent melting point, the metal remains in a solid state.
Why the Outer Core Remains Liquid
The liquid outer core provides a comparative example that reinforces the dominance of pressure in determining the physical state of the Earth’s interior. The outer core is composed of a similar iron and nickel alloy and is only slightly cooler than the inner core, with temperatures ranging from roughly 4,500 to 5,500 degrees Celsius. Despite this heat, the outer core is liquid because the pressure is significantly lower than that of the inner core.
The pressure at the outer core’s upper boundary is less than half the pressure at the inner core boundary. In this region, the existing temperature is hot enough to exceed the metal’s melting point at that lower pressure. As a result, the metallic atoms gain enough kinetic energy to move freely, and the material remains a turbulent liquid. The solid inner core only begins to form where the pressure increases enough to push the melting point of the iron-nickel alloy above the local temperature.