Earth formed over 4.5 billion years ago from the gravitational collapse of planetesimals. That process released a large amount of energy, raising Earth’s initial temperature significantly. But surprisingly, even after billions of years, Earth’s interior is still hot and geologically active. Why?
This can be explained using three key physical ideas:
The temperature of Earth immediately after its formation is estimated by:
$$ T = \frac{4\pi G R_p^2 \rho}{5 C_p} $$
Where:
Using typical values, this gives $T \approx 41000\,\text{K}$ — hot enough to melt rock. So, Earth started as a molten body.
Heat escapes from Earth’s interior primarily by conduction through the lithosphere. The depth to which heat can escape through conduction and radiation over time $\tau$ is given by:
$$ L_{\text{max}} \sim \sqrt{ \frac{K_T \tau}{\rho C_p} } $$
Where:
This evaluates to about 300 km. Since Earth’s radius is over 6000 km, most of its interior lies beneath the conductive zone and remains well insulated. This means cooling is very slow.
Earth’s interior contains radioactive isotopes such as:
These decay slowly, releasing energy continuously. Because their half-lives are so long, these isotopes are still active today, providing a steady internal heat source.
The combination of initial heat, slow cooling, and radioactive heating powers:
Without these heat sources, Earth would be geologically inactive — like the Moon or Mercury.
| Source of Heat | Role in Earth’s Interior |
|---|---|
| Gravitational Accretion | Heated the early Earth to molten temperatures |
| Slow Thermal Conduction | Prevents deep interior from cooling quickly |
| Radioactive Decay | Continually replenishes internal heat |
→ Together, these processes explain why Earth still has a hot interior and remains geologically active.