geochronology, isotopes

Introduction to Geochronology

Have you ever wondered, “How do geologists know how old a rock is?” 

Geochronology is the science of determining how old rocks, sediments and fossils are. Whether they formed 4 billion years ago or within the historic record, geologists can employ geochronologic methods to determine either the relative or absolute ages of these materials.

Radiometric dating–which measures the amount of decay of a radioactive isotope with a known half-life–is the most commonly utilized method in geochronology. There are a bevy of different isotope systems, each with different half-lives that dictate the time frame over which they can be applied.

Isotopes–which are elements with the same number of protons but a different number of neutrons–can be radioactive and decay over time. A half-life is the amount of time it takes for one half of theirs atoms to decay. As their atoms decay from what is the parent (P), they become daughter atoms (D).


Once a rock crystallizes, the clock for the half-life begins; all the atoms are still the original parent. After 1 half-life, half of the parent atoms have decayed to daughter atoms. Accordingly, after each consecutive half-life, half of the remaining parent atoms will continue to decay to daughter atoms until you are only left with the daughter.

By knowing the half-life of the isotope system you are working with, you can measure the daughter/parent ratio (D/P) to determine how much time has passed since the rock was formed. The table below shows a simple ratio schematic based on the decay from parent to daughter in the rocks above.


Different isotope systems can have half-lives ranging from seconds to billions of years; you need to choose an isotope system that has a half-life that will cover the time frame of the expected age of your rock (you need an appreciable amount of both parent and daughter atoms to measure the ratio for an age).

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