Radiometric dating - internal clocks in rocks

Geochronology: the science of dating geologic materials


Basics of radioactive decay and isotopic dating

Radioactive decay occurs at an exponential rate, meaning that it can be described in terms of a half life. After one half live, half of the original radioactive isotope material in the system under consideration decays. Another half life and half of the remaining material decays, and so on. This is for unforced decay. Forced decay is when the isotopic material is packed densely enough that a decay in one unstable atom sends out a particle that hits another atom and causes it to decay. If it is packed too densely there is a run away reaction and one of those unpopular mushroom clouds or meltdowns. Normal concentrations of radioactive material on earth are well below the levels where forced decay occurs so we can use the relatively simple mathematics of exponential decay to describe the process. the rate of decay is constant for earthly P-T conditions.

A major assumption is that the rock or mineral being dated has been a closed system so that no parent isotope or daughter product has escaped or been added. This assumption can be tested for.

Systems commonly used for radiometric dating, with half lives.

For more information on radioactivity and geochronology - USGS site - .

What event is being dated?

What event sets the clock, or more succinctly, when is the system closed?

Carbon-14 dating

Diagram focusing on some short-lived radioactive isotopes, including carbon-14. Some of these other isotope systems are also used for dating purposes. Diagram from USGS website:

What geologic materials can be dated?

This depends on what system is being used, which determines what type of event is being dated.


Accuracy, error and testing the technique

Some thoughts on accuracy and error:

Testing the technique:

Other dating techniques:

The earth is about 4.5 billion years old, the oldest rocks are just shy of 4 billion years - welcome to deep time!

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