So having worked out the structure of the earth, we may also wish to know how old it is. The Big Bang which formed the universe is thought to have occurred at about 13.8 billion years ago .
What about the formation of Planet Earth?
By finding the oldest rocks on the Earth, we can find a minimum age for the planet itself. The technique used to measure the age of rocks is called radiometric (or radioactive) dating. Certain elements are unstable and undergo radioactive decay and this happens at a very stable, predictable rate. So by looking at the ratio of the original element to the ‘daughter’ element produced by the radioactive decay, we can tell how long ago that rock originally crystallised (i.e. cooled from liquid magma to solid rock).
The element most commonly used for this technique is uranium (U), which decays into lead (Pb). Uranium-lead (U-Pb) dating is one of the oldest dating methods, with an age range from about 1 million years to over 4.5 billion years. How does it work?
- Uranium has 2 'isotopes' which are chemically similar: 235U and 238U. Each decays to lead on a slightly different chemical path. These decays happen at a precisely known rate, called a half-life.
- The decay of 238U to 206Pb, has a half-life of 4.47 billion years, i.e. it takes that long for the original amount of uranium to turn into half that amount of uranium plus an equal amount of lead. The half-life of 235U is 704 million years. By using both of these methods, very accurate dates can be obtained for suitable rock samples.
- Sampling of various rocks shows a calculated age of the Earth of 4.6 billion years.
- Some of the oldest rocks are found in northwest Scotland, where the Lewisian gneisses are up to 2.5-3 billion years old (Allaby 2008; Huddart & Stott 2013).
Radioactive decay from the ‘parent’ element to the ‘daughter’ element e.g. uranium to lead.