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What is Relative Dating? - Law of Superposition, Principles of Original Horizontality & Cross-Cutting Relationships

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  • 0:05 Introduction to…
  • 1:09 Original Horizontality
  • 1:54 Law of Superposition
  • 3:44 Cross-Cutting Relationships
  • 4:47 Inclusions and Unconformities
  • 7:16 Lesson Summary
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Lesson Transcript
Instructor: April Koch

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

Discover how geologists study the layers in sedimentary rock to establish relative age. Learn how inclusions and unconformities can tell us stories about the geologic past. We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!

The Grand Canyon and Relative Dating

Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon. Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time. Now imagine that you come upon a formation like this:

Example of a rock layer that is not smooth or parallel
example of rock layer

What do you think of it? How do you study it? How can you make any conclusions about rock layers that make such a crazy arrangement?

Geologists establish the age of rocks in two ways: numerical dating and relative dating. Numerical dating determines the actual ages of rocks through the study of radioactive decay. Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another. Relative dating requires an extensive knowledge of stratigraphic succession, a fancy term for the way rock strata are built up and changed by geologic processes. In this lesson, we'll learn a few basic principles of stratigraphic succession and see whether we can find relative dates for those strange strata we found in the Grand Canyon.

Original Horizontality

In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. It's called the Principle of Original Horizontality, and it just means what it sounds like: that all rock layers were originally horizontal. Of course, it only applies to sedimentary rocks. Recall that sedimentary rock is composed of... sediments, which are deposited and compacted in one place over time. As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness. It sounds like common sense to you and me, but geologists have to define the Principle of Original Horizontality in order to make assumptions about the relative ages of sedimentary rocks.

Law of Superposition

Once we assume that all rock layers were originally horizontal, we can make another assumption: that the oldest rock layers are furthest toward the bottom, and the youngest rock layers are closest to the top. This rule is called the Law of Superposition. Again, it's pretty obvious if you think about it. Say you have a layer of mud accumulating at the bottom of a lake. Then the lake dries up, and a forest grows in. More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer. The forest layer is younger than the mud layer, right? And, the mud layer is older than the forest layer. When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history. The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.

How do we use the Law of Superposition to establish relative dates? Let's look at these rock strata here:

Example of rock with five layers
example of rock with five layers

We have five layers total. Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old. We're not so sure about the next layer down, but the one below it is 100 million years old. Can we tell how old this middle layer is? Not exactly, but we do know that it's somewhere between 70 and 100 million years old. Geologists use this type of method all the time to establish relative ages of rocks.

Now, what if instead of being horizontal, this rock layer was found in a tilted position?

Whatever caused this formation to tilt happened after the strata was formed.
image of rock with layers tilted

What could a geologist say about that section of rock? Following the Principle of Original Horizontality, he could say that whatever forces caused the deformation, like an earthquake, must have occurred after the formation of all the rock strata. Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.

Cross-Cutting Relationships

We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock. Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.

Whatever caused this igneous intrusion occurred after the strata formed.
rock with intrusion

We could assume that this igneous intrusion must have happened after the formation of the strata. If it had happened before the layers had formed, then we wouldn't see it punching through all the layers; we would only see it going through the layers that had existed at the time that it happened. The newer layers would have formed a cap overtop.

The Principle of Cross-Cutting Relationships states that rock formations that cut across other rocks must be younger than the rocks that they cut across. The same idea applies to fault lines that slide rock layers apart from each other; a fault that cuts across a set of strata must have occurred after the formation of that set. Geologists find the cross-cutting principle especially useful for establishing the relative ages of faults and igneous intrusions in sedimentary rocks.

Inclusions and Unconformities

Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock. These items are called inclusions - foreign bodies of rock or mineral enclosed within another rock. Because the sedimentary rock had to have formed around the object for it to be encased within the layers, geologists can establish relative dates between the inclusions and the surrounding rock. Inclusions are always older than the sedimentary rock within which they are found.

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