In the grand scheme of things, one can forget about how everything little thing that exists in our Universe has an age. While we consistently talk about planets and stars having ages of billions of years, we sometimes forget to think about not only how the rocks and geographical formations on Earth are formed but also how old they actually are! Thus, to help us better understand how the different parts of Earth was formed, scientists developed methods to date rocks and other sorts of similarly dense pieces of Earth.
Before actually dating the rocks, scientists decided to define the age of a rock based on how long ago the rock actually solidified (how long since the rock has been in its present arrangement of densely packed atoms). With this definition, we perform radiometric dating, or a method to carefully measure the different proportions of the rock’s different atoms and isotopes. We’re able to use radiometric dating because of how some isotopes of these elements are radioactive — knowing the half-life (how long it takes for half of a radioactive nuclei to decay) and the different proportions of isotopes in the given rock, we’re able to calculate how old the rock is (with relative accuracy). At the end of the day, the math to actually calculate age based on half-life and current amount isn’t too difficult, as it only involves proportions and logarithms (since the decay rate isn’t linear).
All in all, even though radiometric dating seems relatively complicated (involving concepts like isotopes, half-life, decay, etc.), it all boils down to something relatively simple. In essence, after knowing how many different isotopes of elements there are in a rock, we calculate how long it must have taken to reach those different amounts, leading to an age for the rock. Pretty cool stuff!