Dating Rocks

Contributor: Lindsey Congalosi. Lesson ID: 13121

No one can tell the future, but geologists can see the past. Learn how you too can read rock formations to determine what happened to create them.

categories

Earth Science, Geography, Physical Science

subject
Science
learning style
Auditory, Kinesthetic, Visual
personality style
Beaver, Golden Retriever
Grade Level
Middle School (6-8), High School (9-12)
Lesson Type
Dig Deeper

Lesson Plan - Get It!

Audio:

In this lesson, you will learn how to be a geologist.

geologist

You may be surprised to learn that a big part of geology is dating. No, not that kind of dating! We aren't going to take the rocks out to a romantic dinner. This is a different kind of dating!

When geologists discuss dating rocks, they are not referring to social dating but rather determining the age of the rock… usually.

rock dating

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.

family

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?

rock layers diagram

  • How did you do?

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

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:

folded limestone

  • 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.

Law of Original Horizontality

The assumption that rock layers were originally formed horizontally is called the Law of Original Horizontality.

mountains in Paria River Valley

Watch GCSE Science Revision - Formation of Sedimentary Rock layers, by JamJarMMX, to see how most rock layers are formed and why we can make this assumption:

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.

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