Pick up an ordinary-looking rock and you could be holding something that formed hundreds of millions—or even billions—of years ago.
But rocks don't come with birthdays stamped on them.
So how can geologists possibly know how old they are?
Determining the age of rocks is one of the most fascinating parts of geology. Scientists use clues hidden within minerals, rock layers, fossils, and even radioactive elements to reconstruct events that happened long before humans existed.
Some methods tell geologists whether one rock is older or younger than another, while others can provide an estimate of its actual age.
Together, these techniques have allowed scientists to build a timeline of Earth's extraordinary 4.5-billion-year history.
Relative Dating: Putting Earth's Story in Order
Before scientists could determine numerical ages for rocks, geologists learned how to place geological events in the correct sequence.
This is called relative dating.
Imagine looking at a stack of sedimentary rock layers. In a sequence that hasn't been overturned or heavily disturbed, the layers near the bottom generally formed before the layers above them.
This idea is known as the Law of Superposition.
It doesn't tell us that a particular layer is exactly 200 million years old. Instead, it tells us that the layer is older than the material sitting above it and younger than the material beneath it.
It's a little like finding a stack of old photographs without dates. You might not know exactly when each photograph was taken, but other clues can help you work out which came first.
Fossils Can Provide Important Clues
Fossils are another valuable tool for understanding the relative ages of rocks.
Certain organisms lived during particular periods of Earth's history. When their fossils are found within rock layers, geologists can use them to help compare the ages of rocks found in different locations.
Some particularly useful fossils are called index fossils.
For a fossil to be useful in this way, the organism generally needs to have existed for a relatively limited span of geological time while also being widespread enough to appear in different locations.
Finding the same type of index fossil in two separate rock formations can help geologists determine that those layers likely formed during a similar period.
It's one of the ways Earth's geological story can be connected across enormous distances.
Absolute Dating: Putting a Number on the Past
Relative dating tells us the order in which events happened.
But what if we want to know approximately when they happened?
That's where techniques such as radiometric dating come in.
Certain elements contain unstable isotopes that naturally change, or decay, into other elements over time. This happens at predictable rates.
Scientists can measure the proportions of these isotopes and their decay products within suitable minerals. Using their known decay rates, they can calculate an estimate of how much time has passed since a geological event occurred.
Instead of simply saying:
"This rock is older than that one,"
scientists may be able to determine that a mineral crystallized hundreds of millions or even billions of years ago.
What Is a Half-Life?
One of the key ideas behind radiometric dating is the half-life.
A half-life is the amount of time required for half of a particular radioactive isotope in a sample to decay.
Different radioactive isotopes have different half-lives. Some are relatively short, while others last for billions of years.
That makes different isotopes useful for dating different kinds of geological materials and different periods of Earth's history.
For extremely ancient rocks, scientists can use isotope systems with very long half-lives.
And here's where our Learning Centre topics start connecting again: the natural radioactivity we talked about in Why Are Some Minerals Radioactive? is also one of the tools that allows scientists to investigate the immense age of our planet.
Tiny Minerals Can Hold Enormous Stories
Sometimes geologists don't date an entire rock directly.
Instead, they study specific minerals within it.
One especially important mineral is Zircon.
Zircon crystals can incorporate uranium when they form, making certain isotope systems within them useful for determining ages. They're also extremely durable and can survive geological processes that destroy or alter many other minerals.
Some ancient Zircon crystals have survived for more than four billion years.
Imagine that: a crystal small enough to hold in your hand—or even smaller—can preserve information from nearly the beginning of Earth's history.