The Rules for Relative Dating
Learn how inclusions and unconformities can tell us Relative dating cannot establish absolute age, but it can establish whether one rock is. relative dating. Chapter 17 absolute dating. Chapter 17 principle of superposition . Chapter 17 principle of original horizontality. Chapter 17 principle of lateral. We can absolute date materials but it will always have an uncertainty range, we can never know the age with infinite precision. Relative dating.
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?
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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.
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: 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. 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.
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The forest layer is younger than the mud layer, right? For purposes of relative dating this principle is used to identify faults and erosional features within the rock record. The principle of cross-cutting states that any geologic feature that crosses other layers or rock must be younger then the material it cuts across.
Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses. Once a rock is lithified no other material can be incorporated within its internal structure.
In order for any material to be included within in the rock it must have been present at the time the rock was lithified. For example, in order to get a pebble inside an igneous rock it must be incorporated when the igneous rock is still molten-- such as when lava flows over the surface.
Therefore, the piece, or inclusion, must be older than the material it is included in.
Lastly the Principle of Fossil Succession. Aside from single-celled bacteria, most living organism reside at or very near the Earth's surface either in continental or oceanic environments.
As these organisms die they are deposited on the surface along with all other sediments.