Principles of Relative Dating 1 - Superposition, Horizontality, Cross-cutting
As a member, you'll also get unlimited access to over 70, lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching 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 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 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 continue reading strange strata we found in the Grand Canyon. In order please click for source 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: 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. 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 how 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 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. 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 more info. We could assume that this igneous intrusion must have happened after the formation of the strata.
If How Can The Principles Of Stratigraphy Be Used To Do Relative Age Dating 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.
Basics--Stratigraphy & Relative Ages
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 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.
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 complicate 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 type 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 How Can The Principles Of Stratigraphy Be Used To Do Relative Age Dating rock: The sandstones lie horizontally, just as they did when they were originally laid down. But, the shales are all deformed and folded up. The tops of their folds are completely gone where the sandstones have replaced them.
Often, the sedimentary basin is within rocks that are very different from the sediments that are being deposited, in which the lateral limits of the sedimentary layer will be marked by an abrupt change in rock type. There are other means by which we can determine numeric age, although most of these methods are not capable of dating very old materials. We call such a break in the stratigraphic record a hiatus a hiatus was identified in our trash pit example by the non-occurrence of the Ceramic Cups layer at the Zoo site. Volcanic vs Plutonic Igneous Rocks: During this interval, erosion may have occurred before more deposits of sediments covered the surface.
What can we make of this giant unconformity? Can we establish any relative ages between the rock strata or the cause of their formations? Well, following the Principle of Cross-Cutting See more, we can tell that whatever deformed the shales - probably an earthquake - must have occurred before any of the upper sandstones were deposited.
In fact, we can put together a timeline. The shales were deposited first, in a horizontal position, and then there was an earthquake that made them all fold up. Then, the tops were eroded off until the rock was basically flat, and then the sandstones were deposited on top of everything else. With only a few geologic principles, we've established the relative dates of all the phenomena we see in the Great Unconformity. Geologists establish the relative ages of rocks mostly through their understanding of stratigraphic succession.
The Principle of Original Horizontality states that all rock layers were originally horizontal. The Law of Superposition states that younger strata lie on top of older strata. The Principle of Cross-Cutting Relationships states that intrusions and faults that cut across rock are necessarily younger than that rock.
Inclusionsor foreign bodies, found inside rock are necessarily older than that rock. And, unconformities show a discontinuity in the strata, which can only be understood by following the principles of stratigraphy.
Geologists utilize all of these laws and principles to establish the relative ages of rocks and the relationships between events that occurred throughout geologic time. To unlock this lesson you must be a Study. Did you know… We have over 95 college courses that prepare you to earn credit by exam that is accepted by over 2, colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.
To learn more, visit our Earning Credit Page. Not sure what college you want to attend yet? The videos on Study. Students in online learning conditions performed better than those receiving face-to-face instruction. Explore over 4, video courses. Find a degree that fits your goals. What is Relative Dating? Learn how inclusions and unconformities link tell us stories about the geologic past. We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!
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Stratigraphic Formations and Their Correlation A Formation is a a rock or group of rocks that differ from rocks that occur above or below and have distinctive characteristics and fossils such that the rocks can be recognized over wide areas. Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon. The direction of the earth's magnetic field, which can be normal polarity or reversed polarity. Some examples of isotope systems used to date geologic materials.
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