What is Relative Dating? - Law of Superposition, Principles of Original Horizontality & Cross-Cutting Relationships
As a member, you'll also get unlimited access to over 55, lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized relativw to help you succeed. Log in or sign up to add this lesson to a Custom Course. Login or Sign up. 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:. What do you think of it? How do you study it? How can you make any what are the key principles of relative dating 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. Od dating requires an extensive knowledge of stratigraphic successiona 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 relatige we found what are the key principles of relative dating the Grand Canyon.
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 Horizontalityand it just means what it sounds like: What are the key principles of relative dating 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 principless 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.
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.
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:.
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 million years old. Can we tell prrinciples old this middle layer is? Not exactly, but we do know that it's somewhere between 70 and million years old.
Geologists use this type of method all the time to establish relative ages of rocks. 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 kye strata. Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.
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. 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 dating royal worcester bone china 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 tje across. The same idea applies to fault lines that slide rock layers apart what are the key principles of relative dating 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.
Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock. These repative 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. Other times, geologists discover patterns in rock layers that give them confusing information. There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock. These interfaces between discontinuous layers of rock are called unconformities.
They peinciples the task of relative dating, because they don't give an accurate picture of what happened in geologic history. For example, say we have a layer missing from the rock strata. That layer may have eroded away before the next layer was built upon the exposed surface. So, we'll never know what oc of rock used to be there or what fossils it may have held.
One famous example of an unconformity is the Great Unconformity of the Grand Canyon. It clearly shows the interface between two types of rock: The sandstones lie horizontally, just as they did when they were originally laid down. But, the shales are all deformed and what are the key principles of relative dating up. The tops of their folds are completely gone where the sandstones have replaced them.