Dating Rocks

Lesson Plan - Get It!

Audio:

Relative dating is the process of finding the age of things compared to other things. You can think of relative as meaning compared to.

Another way to describe height is to actually measure it.

If the mom in this image is 5'9", then that is referred to as her absolute height.

Rocks can also be dated using absolute dating, which measures the number of radioactive materials remaining in a sample. (Check out our lesson under Additional Resources in the right-hand sidebar to learn more about radiometric dating.)

You just practiced using relative height. Now, let's see if you can classify by relative age.

• Now, what about the relative age of these rock layers?

• How did you do?

You probably didn't realize when you answered these questions that you were using the law of superposition.

The law of superposition states that layers on the bottom are older than layers on the top. Think about it.

• If Layer A was already formed, how would Layer B form later underneath Layer A?

It would be nearly impossible.

It is important to note that the law of superposition assumes that very little folding or uplift of layers has occurred. If that happens, then relative dating becomes much trickier, as you can see in the image below.

• How did the rocks in this image form?

It's hard to tell exactly. However, we can assume that the layers initially formed in straight layers but were later folded and moved around; most likely because of movements of the earth's crust.

The assumption that rock layers were originally formed horizontally is called the law of original horizontality.

Watch the video below to see how most rock layers are formed.

Occasionally, a layer seems to disobey this rule and appears to cut through other layers:

Move on to the Got It? section to find out how this could possibly happen.