Geological Radiodating

Teo Camacho
May 24, 2017

Submitted as coursework for PH241, Stanford University, Winter 2017


Fig. 1: The physics of decay and origin of C-14 for the radiocarbon dating. (Source: Wikimedia Commons)

Geologist use radiodating to help determine ages of rocks and subsequently an estimate for the age of the Earth. It has been practiced and tried since 1953 when Clair Patterson first estimated the age of the Earth. [1] To understand and make use of radiodating, scientist use properties of radioactive chemical isotopes, half lives, and concentration percentages as markers to make these age estimates. Although radiodating can be a complicated topic, this essay looks to break down the basics of radiodating and examples of how radiodating is used in geology.

The Basics of Radiodating

The basis of understanding geological radiodating breaks down into Physics and Chemistry. First, isotopes of elements are atoms that have a different number of neutrons than other atoms of the same element. Elements will always have the same number of protons, however having different number of neutrons affects the molecular mass. For example, carbon will always have 6 protons. But the neutrons can vary among 6, 7, and 8, making C-12 versus C-13 versus C-14 are all isotopes of carbon. [2]

Some isotopes of elements are unstable. This instability means that the atom does not have enough energy to hold the nucleus together. When there is an unstable isotope, radioactive decay will occur. Radioactive decay is the spontaneous breakdown of an unstable atomic nucleus which subsequently releases heat and matter. [3] An example of radioactive decay can be seen in Fig. 1 where Carbon-14 is used as an example.

Although radioactive decay is spontaneous, predictions and estimates can be made about the rate of decay. A half- life is the time it takes for half of the unstable (radioactive) nuclei to undergo radioactive decay. Half-lives don't pertain to specific individual atoms. Rather, a half-life refers to a large number of atoms that are typical in samples used for radiodating and predicting that half of the radioactive atoms present will undergo radioactive decay. [3]

These basic chemistry and physics principles are all incorporated into the different methods used for radiodating. Instruments, like a mass spectrometer, a tool that separates ions of different mass and measures their relative quantities, and techniques, like using ratios of atoms found in sample rocks, are all used to study the age and history of our Earth. [3]

Examples of Radiodating

One of the most commonly practiced method of radiodating is measuring the decay of K-40 to Ar-40. [4] Potassium-Argon (K-Ar) dating is used to date rocks that are upwards of millions of years old. [4] In order to measure the K-Ar decay, geologist would either use mass spectrometers or measured by using of ratios K to Ar that also includes a reference sample. The process of using ratios involves decay constants, the half-life of Potassium, and ratios of Potassium to Argon. [4]

In order to date younger material, geologist date the unstable isotope C-14. This method is used primarily to date organic matter, like fossils, or sedimentary rocks. [4] To date these rocks, geologist again use one of two methods, either mass spectrometers or tracking the beta decay. Beta decay is when a radioactive atom emits a beta particle, a neutron split into an electron and proton. [3] Dating C-14 using beta decay involves using properties of radioactive nuclei, half-lives, and atomic ratios of C-14 and the particle it decays into, N-14. [4]

Another example of radiodating in geology is with the radioactive decay of Uranium to Lead. This method is used to estimate the age of the Earth. [4] It has been practiced by comparing different rock samples, like meteorites, moon rocks, and minerals, across the world to get as accurate as an estimate as possible. Geologist use ratios of different uranium decays to lead: Pb-207 over Pb-204 and Pb-206 over Pb-204, and graphs them against each other. The slope then represents the estimate of the age of the Earth. [4]


The development of radiodating throughout the 20th century has led to great discoveries about the history of the Earth. This essay was not able to go into great depth of different radiodating techniques meaning that does not mean that only the techniques highlighted are the only important methods. Rather, the purpose of this essay is to give a general overview of geological radiodating because of the information and insight that it could provide into knowing more about the galaxy.

© Teo Camacho. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] G. B. Dalrymple, "The Age of the Earth in the Twentieth Century: a Problem (Mostly) Solved," Geol. Soc. London Special Publication 190, 205 (2001).

[2] N. J. Tro, Principles of Chemistry: A Molecular Approach, 3rd Ed. (Pearson, 2015).

[3] F. Press and R. Siever, Earth: Fourth Edition (W. H. Freeman, 1985).

[4] M. A. Geyh and H. Schleicher, Absolute Age Determination: Physical and Chemical Dating Methods and Their Application (Springer, 1990).