Radioactive Sources of Heat

Radioactive isotopes create the majority of Earth’s heat. To understand this radioactive origin of geothermal energy, we must examine nuclides.

Nuclides and Isotopes

All atoms have protons, neutrons, and electrons. A nuclide is a term used to characterize atoms by their number of protons, their number of neutrons, and their nuclear energy state. Isotopes describe nuclides in greater detail; they are nuclides of a particular element that differ from one another based on their number of neutrons. So, when a scientist uses the word, “isotopes,” they are referring to a set of nuclides with the same number of protons, but different numbers of neutrons.

For example, let’s take a look at the element, carbon. The figure below  illustrates the three common carbon isotopes, distinguished by the differences in the number of neutrons (brown-filled circles) in the nucleus: carbon-12 (¹²C), carbon-13 (¹³C), and carbon-14 (¹⁴C). 

Stable vs. Unstable Isotopes

Some isotopes are considered stable and others are considered unstable. Instability occurs as a result of an imbalance in the number of protons and neutrons in the atomic nucleus. The greater the imbalance, the more likely that an isotope will be unstable. For example, ¹²C and ¹³C are stable, whereas ¹⁴C is unstable. An unstable isotope has excess internal energy. The unstable isotope can shift to a more stable configuration by giving up some of its energy. The unstable isotope achieves a more stable form by emitting energetic particles. This process is called radioactive decay and the unstable isotopes emit radiation (energy in the form of electromagnetic waves) and are called radioisotopes.

Carbon Isotope Examples and Radioactive Decay

Considering the element carbon, ¹²C and ¹³C are stable isotopes and therefore are not considered radioactive. However, ¹⁴C is radioactive (unstable) due to the extra weight of its two additional neutrons, and therefore, experiences radioactive decay. The radioactive decay emits radiation and produces heat as the parent isotope decays into a stable form: the daughter isotope. With carbon, the radioactive parent isotope ¹⁴C will decay to the stable daughter isotope ¹⁴N. During this process, one of the eight neutrons in the ¹⁴C nuclide becomes a proton, decreasing the number of neutrons to seven and increasing the number of protons from six to seven. This radioactive decay has created the stable daughter isotope of nitrogen: ¹⁴N.

Some examples of parent and daughter isotopes are:

Radiogenic Heating

As previously mentioned, unstable isotopes become more stable by releasing energy during radioactive decay. The release of heat energy from radioactive decay is known as radiogenic heating. On Earth, four radioactive isotopes are responsible for the majority of radiogenic heat because of their enrichment relative to other radioactive isotopes: uranium-238 (²³⁸U), uranium-235 (²³⁵U), thorium-232 (²³²Th), and potassium-40 (⁴⁰K). 

Movement of Heat Through the Earth

As heat is released within the core, that energy moves. Temperatures at the core-mantle boundary can reach over 4,000 °C (7,200 °F).¹Lay, T., Hernlund, J., & Buffett, B. A. (2008). Core–mantle boundary heat flow. Nature geoscience, 1(1), 25-32. The core’s heat creates a convection current within the mantle layer that, not only assists in heating the crust to create geothermal energy, but also, moves Earth’s tectonic plates. The core heats magma causing the magma to rise towards the crust; while the magma closer to the crust cools and drops towards the core. The rising magma heats aquifers and rocks, creating steam vents, hot springs, geysers, underwater hydrothermal vents, and other features essential for harnessing geothermal energy.²National Geographic. (n.d.). Geothermal Energy. National Geographic Education. Although we frequently observe the movement of steam, water, or magma, most of Earth’s geothermal energy is dry and at depth. Many accumulations of high heat within Earth’s crust can only be accessed by drilling to the appropriate depth.