Using relative dating principles

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. 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 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 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. Original Horizontality In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. Of course, it only applies to sedimentary rocks. Recall that sedimentary rock is composed of 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: This rule is called the Law of Superposition.

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?

Example of rock with five layers We have five layers total. Can we tell how old this middle layer is? 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. 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. Whatever caused this igneous intrusion occurred after the strata formed. We could assume that this igneous intrusion must have happened after the formation of the strata.

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.


7 Geologic Time. KEY CONCEPTS. Explain the difference between relative time and numeric time as applied to interpreting Earth history. Section shows how Earth history is understood using relative dating principles without actually knowing the numerical age of events. This was how scientists of that time interpreted Earth history, . Quiz & Worksheet - Relative Dating Use this quiz and worksheet to assess your understanding of the techniques geologists use to establish relative dating and the relationships that occurred.

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