Earth's Radioactivity: The Internal Energy of Our Planet

Diagram illustrating radiogenic heating of the Earth's mantle — *Illustration showing the distribution of radioactivity and internal heating of the Earth. Image source: astronoo.com (new window)*

Earth's radioactivity

The internal radioactivity of the Earth is mainly due to the decay of radioactive isotopes (natural radioactive isotopes such as uranium-238, thorium-232, and potassium-40). These decays produce alpha particles, beta particles, and gamma rays, thereby releasing thermal energy within the mantle and Earth's crust.

This energy, released by the radioactive decay of radioisotopes, plays a fundamental role in maintaining part of the Earth's mantle in a partially molten state, driving mantle convection that causes the movement of lithospheric plates. Without this internal heat source, plate tectonics, the formation of oceanic ridges, and volcanic activity would be greatly reduced, or even impossible in the long term.

The distribution of radioactive isotopes is not homogeneous: some elements, such as uranium and thorium, are concentrated in the continental crust, while potassium-40 is more present in the upper mantle. This heterogeneity influences the local distribution of heat flow and the geological dynamics of different regions of the Earth.

Table: Viscosity of the Earth's mantle by depth

Viscosity is not uniform and depends strongly on depth, temperature, pressure, and mineralogical composition. For comparison, honey has a viscosity of about 10 Pa·s, so even the upper mantle is about 10¹⁸ times more viscous than honey, but it can still flow slowly over millions of years.

Orders of magnitude of the Earth's mantle viscosity
Mantle zone	Depth (km)	Approximate viscosity (Pa·s)	Comment
Upper mantle	0 – 410	10¹⁹ – 10²¹	More ductile, allows convection on the scale of lithospheric plates
Intermediate mantle	410 – 660	10²¹ – 10²²	Slightly higher viscosity, influences the transfer of material between upper and lower zones
Lower mantle	660 – 2,890	10²² – 10²⁴	Very viscous, but over long timescales, it flows and drives global convection

Sources: Karato & Wu (1993), Rheology of the Upper Mantle, Science 260: 771–778; Mitrovica & Forte (2004), Earth and Planetary Science Letters 225: 177–191.

Impact on internal heating and mantle convection

Radiogenic energy generates internal heating (convective heat flow in the Earth's mantle) that causes convective movements in the mantle. These movements drive the displacement of lithospheric plates, fueling plate tectonics and volcanic phenomena. The amount of heat produced can be estimated at about 20 TW for the entire Earth.

Table: Main radioactive isotopes and their contribution to Earth's heating

Radioactive isotopes and associated heat flow
Isotope	Half-life (years)	Relative abundance (%)	Heat flow (TW)	Comment
Uranium-238	4.47 × 10⁹	0.3	8	Main contributor in the mantle and crust
Thorium-232	1.41 × 10¹⁰	1	8	Mostly present in the continental crust
Potassium-40	1.25 × 10⁹	0.012	4	Mainly contributes to the heat flow of the upper mantle

Sources: Jaupart et al. (2007), Physics of the Earth and Planetary Interiors 160: 3–30; Turcotte & Schubert (2014), Geodynamics, 3^rd edition.

Earth's Radioactivity: The Internal Energy of Our Planet