You cannot tell how old this rock is just by looking at it.
It does not have growth rings like a tree. It does not receive a senior citizen discount at fast food restaurants. It also doesn’t have millions of bruises caused by its Aunt Barbara pinching it to grow an inch every time it had a birthday. Instead, it and many other rocks have radioactive “clocks” inside of them that tell how old the rocks are. These are not clocks that you, or anyone else, can actually see (you can’t even see them with a microscope). Also, these “clocks” don’t actually display time. But by using some fancy lab equipment and by doing some math, scientists can look at these “clocks” and can tell you how long ago that particular bit of rock was formed.
Like everything else in the universe, including the electronic device you are looking at now and your elbow, rocks are made up of incredibly tiny particles called atoms. There are 118 different kinds of atoms that have been observed so far. Each different kind of atom is an element (you can think of an element as like a species of atom). Elements include many things you have heard of before, like:
And everyone’s favorite element, livermorium.
Most elements occur naturally and stay the same way for billions of years, but some natural elements, like uranium and potassium, have unstable versions of themselves (called radioactive isotopes) that change over time.
Radioactive isotopes have what scientists call a half-life. A half-life is how long it usually takes half of a bunch of radioactive isotopes to disintegrate. For example, the radioactive isotope uranium-238 has a half-life of 4.47 billion years. So what that means is if you had ten pounds of uranium-238 and you waited around 4.47 billion years, then five pounds of it would have disintegrated by then.
We know the half-lives of radioactive isotopes because scientists have been able to observe and measure the disintegration over and over [and over] again, and it is pretty consistent. To measure a half-life, the scientists don’t have to sit there and watch the atoms disintegrate for billions of year until half of it is gone. Instead they can start with a sample in their laboratories of, say, a trillion uranium-238 atoms, and see how many uranium-238 atoms disintegrate over the course of five years. Then they can use this rate of disintegration to figure out how long it would take half of all the uranium-238 atoms to disintegrate. That time period is its half-life.
When we talk about radioactive isotopes “disintegrating,” that does not mean they just disappear like Obi-Wan Kenobi did when he fought Darth Vader in the original Star Wars. When radioactive isotopes disintegrate, they turn into other elements. For example potassium-40 tends to turn into argon-40 as it disintegrates. This is the radioactive clock in rocks. When scientists find argon-40 atoms in a rock sample, they know that all of them must have come from disintegrating potassium-40 atoms. Argon is basically a loner element that doesn’t mix well with other elements. So if argon is in a rock, it must have come from the much more social element potassium-40, which does regularly mix with other elements and could help make a rock. By then comparing the number of potassium-40 atoms to the number of argon-40 atoms in a rock sample, scientists can figure out how many potassium atoms disintegrated into argon-40 atoms. They can then use the half-life of potassium-40 to figure out how long ago this rock was first formed.
To give you a very simplified idea of how this works, lets say that these are radioactive isotopes:
As these radioactive chocolate chips decay, they turn into peanut butter chips. Let’s also say the chocolate chips have a half-life of ten years. Knowing this, you find a box in your grandma’s pantry labeled “chocolate chip cookies.” You want to figure out how long ago these cookies were baked. So, you pull out a cookie and count the chips in it. You find the chocolate chip cookie has five chocolate chips and five peanut butter chips in it. This means half the chocolate chips stayed the same and half disintegrated into peanut butter chips. Because it would take ten years for half of the chocolate chips to turn into peanut butter chips, you now know the cookie had time to go through one half-life. Therefore, it is probably ten years old.
To make sure there was not something weird about that particular cookie, though, you check other cookies in the box to see if they all produce the same age for the cookies. Another cookie has 12 chips, six of which are chocolate and the other six are peanut butter. Another cookie has eight chips, four of which are chocolate and the other four are peanut butter. Pretty much all the cookies in the box have half chocolate chips and half peanut butter chips. Because of this, you feel confident the box of cookies must be ten years old, but then you go ahead and eat it anyways.
Scientists can use any ratio of parent elements (such as uranium-238 and potassium-40 [or chocolate chips in my goofy analogy]) to their daughter elements (lead-206 and argon-40 [or peanut butter chips]) to determine how old a rock is. So, for example, if a scientist measures the total amount of uranium-238 and lead-206 in a rock crystal, and it had 23 lead-206 atoms and 77 uranium-238 atoms, the scientist can plug these numbers into a formula to figure out how long ago that rock first solidified. The formula looks like this:
Since there is a lot more uranium than lead in my example, that rock would be a lot younger than the 4.47 billion year half-life (though a scientist would actually take the time to calculate the age).
These radioactive isotopes and their daughter elements are not found in everything, but they are found in igneous and metamorphic rocks. When measured in rocks, they tell us when the rock turned from a molten state into a solid state (in other words when it became a rock). When magma becomes rock, the radioactive isotopes in it get locked in like they’re in radioactive isotope prison. They can’t escape and new radioactive isotopes can’t break in to hang out with them. Because the radioactive isotopes are stuck, as they disintegrate into their daughter elements, their daughter elements get stuck too. The ratio of parent to daughter elements can then be like a stopwatch ticking off the time since the rock first formed.
Along with rocks, radioactive isotopes also get trapped in wood, bones and the shells of ocean creatures, so these also contain clocks that can help scientists learn the ages of events in Earth’s history.
This post actually reprints and revises a post I wrote for the JOIDES Resolution blog in 2011. I decided to reuse it because a lot of people have been asking me how scientists know the Chicxulub impact happened 66 million years ago. Radiometric dating of rocks and fossils is just one of the ways they know. I'll write about some of the other ways sometime in the near future, maybe.
To learn more about what we can learn from rocks, read my free eBook Uncovering Earth’s Secrets.
Online references and resources:
Nature Education. "Dating Rocks and Fossils Using Geologic Methods."
Smithsonian Institute. "Absolute Dating."
USGS. "How do we know the Age of the Earth? Radiometric Dating."
Photos and Images:
Click the photos and images used above to find their sources. If the photo does not link anywhere, it was taken by Kevin Kurtz.