The Age of a Rock

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).

This picture represents how an element’s half-life might be represented. We can see that for every half-life, the number of atoms is halved.

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!


2 thoughts on “The Age of a Rock

  1. I really enjoyed your post! Thank you for offering a simplified, concise, and helpful explanation of radiometric dating. It would be interesting to investigate the role that radiometric dating has played in astronomy, specifically our journey to discover the age of the universe. Because we know the universe is at least as old as the rocks on Earth, radiometric dating provides a lower limit to how old the universe can be. I am wondering when radiometric dating was first discovered and how much we knew about the universe at that point. If it was discovered at a time when relatively little was known about the universe, this could have been an extremely revolutionary and important tool for astronomers to have.


  2. Radiometric dating really shows the temporary nature of everything around us. It is crazy to think that every substance contains some information about the age of its components. Carbon dating has taught us so mush about human history, I am excited to think of all that we will learn about our solar system with samples from other worlds.


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