The internal heat of Earth drives the planet’s geological activity, sustaining plate tectonics and mantle convection. This thermal energy also maintains the molten outer core, which generates the magnetic field that shields the planet from solar radiation. Understanding Earth’s thermal history requires looking back to its earliest stages. Three distinct mechanisms combined to generate this immense initial heat, known as primordial heat, shaping the planet we inhabit today.
Heat Generated by Accretion and Impacts
Earth began approximately 4.54 billion years ago through accretion, the gradual accumulation of smaller rocky debris and planetesimals within the solar nebula. These bodies collided at high velocities, converting their kinetic energy into thermal energy upon impact. This shock-heating process was particularly intense during the Hadean Eon, the planet’s first half-billion years.
The energy released by these collisions heated the accumulating material significantly. Unlike smaller impacts where heat radiates quickly into space, the massive size of the growing Earth allowed the heat to be buried deep beneath the surface. The continuous influx of hot material and the insulating effect of the surface debris retained this heat, leading to widespread melting. This intense heating likely resulted in the formation of a global “magma ocean” on the young Earth.
Heat Released During Core Formation
The second major source of primordial heat came from planetary differentiation, an event that occurred shortly after accretion. As the planet’s temperature rose, its constituent materials began to separate based on density. The denser, iron-rich metals that were mixed throughout the early Earth started to sink toward the center.
This massive global rearrangement was driven by gravity, releasing vast amounts of gravitational potential energy as heavy material sank. This energy was converted into heat due to the friction and viscous dissipation of the moving materials. The process effectively heated the entire planet, with estimates suggesting this release could have raised the internal temperature by thousands of degrees Kelvin. The descent of the molten iron created the distinct core and mantle layers, establishing the planet’s fundamental internal structure.
Energy from Radioactive Materials
Radioactive decay provided a third, and ultimately sustained, source of heat that has powered Earth’s interior throughout its history. This mechanism involves the spontaneous breakdown of unstable isotopes into more stable forms, releasing energetic particles that strike surrounding material, converting their energy into heat. Two types of radioactive isotopes contributed to Earth’s early thermal budget.
The earliest and most intense contribution came from short-lived isotopes, such as Aluminum-26 (Al-26), which have half-lives of only a few million years. These isotopes were present in the material that formed the solar system, and their rapid decay generated a powerful but brief pulse of heat in the young Earth and its precursor planetesimals. They contributed significantly to the initial melting of the planet but are no longer a factor in its current heat budget.
The sustained heat source comes from long-lived radioactive isotopes, primarily Potassium-40 (K-40), Thorium-232 (Th-232), and the Uranium isotopes, U-235 and U-238. These elements have half-lives on the order of billions of years, allowing them to continuously generate heat in the mantle and crust today. Because radioactive decay is exponential, these isotopes were far more abundant in the early Earth than they are now.
For instance, the concentration of K-40 was more than 12 times higher when Earth formed 4.5 billion years ago, meaning the radiogenic heating rate was much greater in the past. This intense early decay, combined with the heat from accretion and core formation, ensured the planet was hot enough to undergo massive differentiation and begin internal convection. Even today, the decay of these long-lived isotopes accounts for approximately half of the heat flowing out of Earth’s interior.