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What Methods Were Used to Date the Oldest Rocks?

The science of dating rocks has expanded greatly in recent years with increased research. Broadly, scientists date rocks in either relative or absolute terms. A relative date tells the scientist how old the rock is compared to other rocks around it, while absolute dating gives a specific age, although the date is not necessarily accurate or precise.

Relative Dating
- When scientists date rocks relatively, they use geological facts to compare rocks and deduce relative ages from them. So, for example, if one layer of rock is below another layer, then the scientist knows that the lower layer is older. In addition, if the scientist finds a fossil or other artifact with a known date, the scientist can use that information to date the rocks. So if in a specific layer of rock he finds a fossil of a dinosaur that only lived 14-15 million years ago, he knows that the rocks above that layer are younger than 14-15 million years and the rocks in deeper layers are older.
Radiometric Dating
- Radiometric dating is an overarching term for the most common type of absolute dating. In radiometric dating, scientists use known facts about isotopes or variations of certain atoms to date rocks. Since some isotopes naturally transform into different isotopes over time, a scientist can measure the presence of these two isotopes to determine the age of the rock. Many types of atoms have isotopes that scientists measure in this manner, including carbon, potassium and uranium.
Radiocarbon Dating
- Many people have heard of carbon dating, which is a common form of radiometric dating. This type of radiometric dating measures the presence of the carbon isotope carbon 14, which naturally decays into carbon 12 and carbon 13 over time. Carbon dating has an effective age limit of around 60,000 years, after which carbon dating cannot give accurate information. This makes carbon dating useful for dating human artifacts, but less helpful for dating the oldest rocks on the planet.
Potassium-Argon Dating
- A type of radiocarbon dating that provides much better results for old rocks is potassium-argon dating. The potassium isotope Potassium 40 will naturally decay into the element argon 40. Because argon is a noble gas, it is unlikely to have been trapped in a rock outside of this process, making argon a reliable indicator of the rock's age. This radiometric dating method is most useful for rocks older than 100,000 years since rocks typically require at least that much time to collect enough argon to measure.

Rocks & Minerals