How to teach Relative Age

What is Relative Age?

Relative dating is determining if one rock or fossil is older or younger than another rock or fossil. To determine the actual age of a rock or fossil, scientists must use absolute dating which uses radiometric decay. Sometimes, rock layers are disturbed by forces such as pressure, eruptions, and earthquakes. When rock layers are disturbed by these kinds of forces, scientists use the second law of relative age which is the law of cross cutting relationships. The law of cross cutting relationships says that disturbances that happen to a rock layer have to be younger than the rock layer. In other words, a fault that crosses a rock layer happened after the rock layer was already present. The law of cross cutting relationships applies to folds, tilts, faults, and intrusions. These 4 disturbances are all younger than the rock layers they cross – the rock layers were present first and then the disturbance happened.

Absolute age is the actual age that a rock or fossil is. It is determined by comparing the amount of parent and daughter isotopes in a rock or fossil. Because parent isotopes radioactively decay at an exact pace, a precise age can be calculated by examining how much of the parent has decomposed into daughter.

Absolute or Relative Age?

Test your knowledge of the difference between relative and absolute age by dragging each of the statements to the correct column.

Why teach Relative Age?

Relative dating is a perfect opportunity for students to see how scientists combine what they can observe with clues about what they can’t observe to form theories about events in the past. It provides a definitive demonstration of science being a combination of facts and critical thinking rather than someone’s opinion. Perhaps more now than ever, students need to be able to separate fact from fiction and understand that science isn’t just made up.

When should you teach Relative Age?

Teaching relative dating unit supports ESS1-4 which includes the geologic time scale and is grouped with the solar system topics. However, in order to understand relative dating, students need the prerequisite skills of the rock cycle, so teach relative dating after ESS2-1.

Relative dating could successfully be taught either before or after plate tectonics.

What to include in a Relative Age unit

Using the 5E lesson planning format, a relative age unit will include:

Day 1 – Engage 

Here you can present students with rock layers and ask them to predict how the rock layers got into the positions they were in. Most students have a fundamental knowledge of superposition – younger rocks are on top of older rocks – but will need to puzzle through more complicated rock blocks. Do this as a picture walk or a think-pair-share activity. It’s also great to do a puzzle activity. In our unit, we included a “Who got there first” puzzle in which students must use footprints to determine the order that people arrived. From there, it’s an easy extension to superposition.

Day 2 – Explore 

In the exploration phase of relative dating, students get to play with rock layers and try to figure out how they were formed. In our unit, we give students puzzle pieces that can be assembled to create models of rock layers and ask them to demonstrate how the rock layers were formed either in a video or a slideshow.

Day 3 – Explain

At this point, students are ready to learn the vocabulary of relative age. In a standard slideshow and cloze notes activity, students fill out their cloze notes either during a lecture or by a carousel around the room.

Day 4 – Elaborate

Give students another day to play with rock layers, this time using the correct vocabulary. I use task cards and self-checking worksheets for practice and an escape room activity for reinforcement.

Day 5 – Evaluate

Students can demonstrate their understanding of relative age with either a summative or a formative assessment. A standard quiz-like assessment is great, or a problem based activity would be even better.

Relative Age Masterclass

Have you ever felt overwhelmed by the sheer amount of material that science teachers are expected to have mastered? Most middle school science teachers are pretty well versed in life science, earth science, or physical science, but few are well versed in all three. Covering the standards becomes difficult when you only have a passing understanding of broad concepts. You need to be shown the ropes by a master veteran teacher who has taught the subject before and understands what students need.

In our master class for middle school teachers, we will look at the best practices for teaching relative dating and outline a 5 day unit to cover the topics of superposition and cross cutting relationships. We’ve also included downloads to slideshows, activities, and worksheets for your students to practice what they’ve learned. Proceed through the course at your own pace and apply what you learn to your classroom.

Click here to join the masterclass!