When we watched a programme about the evolution of life on Earth, Mumsy asked how geologists knew how old the rocks with the fossils in them were. I said ‘isotopes’, she said ‘what?’ This is my attempt to explain radiometric dating and some of it’s uses.
We’re going to have to start at the beginning, with atoms.
Periodic Table – Royal Society of Chemistry – it might help for you to have a quick look at the RSC’s interactive periodic table.
Isotopes and half-lives
Atoms are made up of protons, neutrons and electrons. Each atom of a chemical element has a specific number of protons and electrons, but the number of neutrons can vary. Protons + neutrons = atomic weight, so for example Carbon-12 has six protons, [six electrons] and six neutrons, but Carbon-14 has six protons, [six electrons] and EIGHT neutrons. Atoms of the same element with different atomic weights are called isotopes.
Isotopes have varying degrees of stability, so Carbon-14 will decay into Carbon-12 eventually; this is called radioactive decay and is a spontaneous process. The parent isotope loses particles from it’s nucleus to form the daughter isotope, which could be an isotope of the same element or could be a different element, for example Thorium-232 decays to Lead-208. Decay happens at a steady, and known, rate, called its half-life. The half-life is the amount of time it takes for half the sample of the parent isotope to decay to its daughter isotope. Some have short half-lives, and decay rapidly, such as Carbon-14 which has a half-life of 5730 years; others have very long half-lives, such as Rubidium-87 which has a half-life of 48.8 billion years.
|Parent Isotope||Daughter Isotope||Half-Life Values|
|Uranium-238||Lead-206||4.5 billion years|
|Uranium-235||Lead-207||704 million years|
|Thorium-232||Lead-208||14.0 billion years|
|Rubidium-87||Strontium-87||48.8 billion years|
|Potassium-40||Argon-40||1.25 billion years|
|Samarium-147||Neodymium-143||106 billion years|
What happens once you have a rock sample
When igneous rocks solidify, from magma, the amount of the parent isotope in the rock is fixed and starts to decay in to the daughter isotope. If a sedimentary or igneous rock is subject to metamorphosis it resets the clock to that point in time. Using sensitive instruments the amounts of the parent and daughter isotopes are determined. The amounts of the chosen isotopes are then put in to the following formula:
I know, it looks scary but it isn’t really; lambda can be determined from the half-life of the chosen parent isotope, which is well-known, and then it’s just a case of plugging in the amount of the daughter and parent isotopes.
The most common isotope used is Potassium-40 so at least two sets of parent/daughter isotopes will be measured if the sample is suitable. Because there are problems due to natural affects, for example if a sample is heated then the daughter isotope (Argon-40 in the case of Potassium-40) may be released and the clock reset to zero, which will give a false, younger age.
What’s radiometric dating used for?
Radiometric dating is used in dating human and other hominid samples and sites.
For young samples, such as organic remains of hominids, Carbon-14 is used. By young I mean samples under 50,000 years old. This time period covers about a quarter of the history of modern humans, Homo sapiens, who have been around for 200,000 years. Earlier hominid species arose around 6 million years ago, although the Homo genus is only just over 2 million years old.
As you can see from the above graphic, H. sapiens shared the Earth with H. neanderthalensis and H. heidelbergensis. Recent genetic work done on bones from around Europe shows the effectiveness of Carbon dating:
For older samples, like rocks and older hominid samples, the other isotopes are used, especially Potassium-40. The organic material on these samples have long since fossilised and cannot be directly dated so the rocks around the fossils have to be dated, just as any ordinary rock would. This allows the fossilised material to be put in a known sequence.
Radiometric methods are also used for dating geological formations.
However, a variety of other methods are used such as luminescence dating and paleomagnetic dating. Together these different methods are called geochronology and produce more accurate dates. This helps to understand the way the planet has changed and how plate tectonics has moved the continents around.
This is Pangaea, in the last 145 million years the continents have moved about a bit.
The position of tectonic plates now.
Knowing when and where the tectonic plates have been helps to explain geological features, such as mountain ranges, like the Himalayas, or volcanic islands such as Iceland or Hawaii.
Does any of that help? I’m sure there are better writers out there who could have explained it better but I’ve tried.