Relative Age of Rocks

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Students will be able to infer the relative age of rocks and fossils from index fossils and the ordering of the rock layers.

Big Idea

Rocks, fossils, and other types of natural evidence tell Earth’s story.


5 minutes

As students settle down for the day's lesson, I present the following video and ask, "How do we know that all of these events have happened?"

The first answer I get from the students is usually along the lines of, "It is written down/from historians". I follow up with the question, "Well, how do we know what happened before there were people to write things down?", eliciting from the students the realization that the Earth's history is "written" in fossils and rocks that we can analyze.

Explain 1

10 minutes

I tell the students that today's lesson will help them understand several big ideas that scientists use to determine the history of the Earth, and begin the following presentation.

Throughout the presentation, there are several opportunities for the students to Think-Pair-Share answers to the embedded questions. 

Activity 1

5 minutes

When we reach slide 6 of the presentation, I distribute the "letter cards" (one set per table), and tell the students that their first task is to organize them in order, with oldest (CT) at the bottom. I explain that "new letters" cannot just appear, there has to be something linking each of the cards to the next.

I remind the students that as they are working, it is not enough to place the cards in the correct order, they must also be able to explain why they are placing each card in that particular order, and should be ready to explain their reasoning (SP7). Organizing the cards is a simple way for students to use identify patterns in data (CCC Patterns; SP4). 

The order of the cards is : CT, ACG, UA, NBU, NB, ON, DXO, MD.

Explain 2

15 minutes

I continue with the presentation (slides 7-15), stopping for quick Think-Pair-Share, at each of the embedded questions. By having several predetermined stopping points, I am able to give students time to interact with all these new concepts, and turn the presentation into a more interactive learning experience.

Activity 2

10 minutes

When we reach the last slide, I distribute the fossil cards (one set per table). I tell the students that, as before, they must organize the fossil record from oldest to newest, and be able to explain their choices. (SP4).

The letters on the bottom right of the cards spell the word RELATIVE, and are there to give me a quick check-in as I am circulating the room listening to the students' explanations.

Watch a quick video of the students working through both card activities.


5 minutes

To close this lesson I distribute the superposition assessment, and ask students to complete it before leaving the classroom. This quick assessment allows me to verify student understanding of the concept of superposition.

The class was able to identify the culprits, as well as indicate how the law of superposition applied. Here is an example of the student work.