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Methods of Geological Dating: Numerical and Relative Dating

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  • 0:08 Introduction to…
  • 1:06 Relative Dating
  • 3:05 Numerical Dating
  • 5:50 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

Learn how scientists determine the ages of rocks and fossils. We'll explore both relative and numerical dating on our quest to understand the process of geological dating. Along the way, we'll learn how stratigraphic succession and radioactive decay contribute to the work of paleontologists.

Dating Dinosaur Fossils

Consider the following scenario: Paul the Paleontologist is a very famous scientist who has studied dinosaur bones all over the world. Recently, he appeared on the evening news to talk about a new dinosaur he just discovered. The dinosaur is called superus awesomus. Paul says he can tell from the fossils that superus awesomus lived on Earth about 175 million years ago. Paul is super awesome, so I'm going to take him at his word.

But really, how do scientists figure out how old their dinosaur bones are? And, what about other findings like fossil fish, plants and insects? Scientists are always spouting information about the ages of rocks and fossils. How do they know these ages? Well, they figure it out using two different methods: relative dating and numerical dating. Let's find out more about these geological dating methods in order to understand how Paul the Paleontologist can be so sure about the age of his dinosaur fossils.

Relative Dating

The first method that scientists use to determine the age of rocks is relative dating. In this method, scientists compare different layers of rock to determine an ordered sequence of events in geologic history. That means they don't really know how old their rocks actually are. The key in relative dating is to find an ordered sequence. Scientists piece together a story of how one event came before or after another. Relative dating cannot tell us the actual age of a rock; it can only tell us whether one rock is older or younger than another.

The most common form of relative dating is called stratigraphic succession. This is just a fancy term for the way rock layers are built up and changed by geologic processes. Scientists know that the layers they see in sedimentary rock were built up in a certain order, from bottom to top. When they find a section of rock that has a lot of different strata, they can assume that the bottom-most layer is the oldest and the top-most layer is the youngest. Again, this doesn't tell them exactly how old the layers are, but it does give them an idea of the ordered sequence of events that occurred over the history of that geologic formation.

Fossil succession can be used to determine the relative ages of fossils.
image showing rock layers with dinosaur fossils

Sort of an offshoot of stratigraphic succession is fossil succession, or a method in which scientists compare fossils in different rock strata to determine the relative ages of each. Let's say that Paul the Paleontologist found an iguanodon fossil in the light green layer shown above. And, he also found a coelophysis fossil in the yellow layer. Which fossil is Paul going to say is older? Of course, the coelophysis, which means that coelophysis came before iguanodon. In fact, Paul already knows that coelophysis lived around 200 million years ago, while iguanodon lived around 150 million years ago. So, what if Paul found that superus awesomus dinosaur fossil in this middle layer? He could be pretty confident that his super awesome dinosaur was about 175 million years old.

Numerical Dating

Stratigraphic and fossil succession are good tools for studying the relative dates of events in Earth's history, but they do not help with numerical dating. One of the biggest jobs of a geologist is establishing the absolute age, in years, of a rock or fossil. Unlike relative dating, which only tells us the age of rock A compared to rock B, numerical dating tells us the age of rock A in x number of years. If I told you that I was 30 years old, that number would be my numerical age. If I told you I was 32 years younger than my mother, that number would be my relative age. Which of these does a better job of describing my age? The numerical age, because it is exact. So, in both geology and paleontology, we want to be able to point to an object and say exactly how old it is. To do that, we have to learn a little bit about radioactive decay.

In 1896, a French physicist named Henri Becquerel discovered radioactivity in an element called uranium. He saw that it underwent radioactive decay, or emission of energetic particles to produce new elements. In 1905, Ernest Rutherford figured out that we could use radiation to establish the ages of rocks. By studying how the mass of uranium changed with radioactive decay, Rutherford was able to determine the age of a rock containing a uranium mineral. This was an amazing discovery. It meant that scientists could suddenly establish the actual ages of all their rocks and fossils!

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