Geologic Time

Our planet is an astounding 4.55 billion years old. We know this using a technique called Absolute Dating, in which we can look at the elements within small particles of rock. Some of these elements have extra neutrons which make them unstable – they are called isotopes, and because they are unstable, they decay into a more stable element over long, long periods of time.

In the case of a Uranium (U-238) Isotope, the time for just half of it found in a rock to decay to more stable Lead (Pb-206) takes 4.5 billion years old. We found just a tiny bit over half Pb-206 in some old minerals called zircon and a little bit less than half U-238. Therefore, the scientists reasoned, that U-238 had to be around for around 4.55 billion years – the oldest age of our planet! We have also dated particles in some asteroids using the same method to get a very similar age for the Solar System. This is why we are so sure of this age – it makes sense.

This is one story of how scientists have determined the age of something – Our Earth. But how do we figure out the age of rock layers, of dinosaur fossils, of the oldest continents out there? Sometimes we use Absolute Dating in the example of Earth, above. However, sometimes we use educated guesses based on the order of rock layers, called “Relative Dating”.

In this section, you will review the rules of both Relative Dating and Absolute Dating to get a better idea of how scientists figure out the age of rocks, and the general history of our planet.

And what a history it is! It’s been 4.55 billion years in the making, and there are certainly some exciting events that happened way before we were around…