Relative dating rules

In an undeformed sequence of sedimentary rock, older than the one above, younger than the one below. Layers of sedimentary rock a generally deposited in a horizontal position. When magma intrudes and crystallizes, we know intrusion is younger than the rocks affected.
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Documents Flashcards Grammar checker. Science Earth Science Geology Unit 5: Relative Dating Rules advertisement. Sedimentary rocks are deposited in flat, horizontal layers. An unconformity is a break in the sequence of the relative time scale… Part of the record in the rock has been removed by erosion then BURIED. An unconformity is a buried erosion surface. If the rock layers were disturbed by faulting, the faulting occurred AFTER the rocks were already there. Cross-section D Igneous Intrusions: If an intrusion of magma occurs within a group of rock layers, the intrusion happened AFTER the other rocks were already present.

Cross-section E Intrusions Intruding Intrusions: Cross-section F Faults and Intrusions: 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.

The Rules for Relative Dating

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.


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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 of 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.

The Rules for Relative Dating

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 Relationships, 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.

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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.

Inclusions , or 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.

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Unit 5: Relative Dating Rules

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Explore over 4, video courses. Find a degree that fits your goals. What is Relative Dating? 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! Try it risk-free for 30 days. An error occurred trying to load this video. Try refreshing the page, or contact customer support. Register to view this lesson Are you a student or a teacher? I am a student I am a teacher. What teachers are saying about Study.

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  • Want to watch this again later? Relative Dating with Fossils: Index Fossils as Indicators of Time. Numerical and Relative Geological Dating. Methods of Geological Dating: Numerical and Relative Dating. What is Relative Age?

    How to Interpret Events from Natural Phenomena. Conditions of Fossil Preservation: Absolute Time in Geology. Major Eons, Eras, Periods and Epochs. Theories of Geological Evolution: Classification of Metamorphic Rocks: Ocean Drilling as Evidence for Plate Tectonics. Introduction to Physical Geology: Intro to Natural Sciences. Middle School Earth Science: Weather and Climate Science: UExcel Weather and Climate: Guns, Germs, and Steel Study Guide. Holt McDougal Introduction to Geography: 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.

    The Grand Canyon and Relative Dating

    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? Original Horizontality In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. Law of Superposition Once we assume that all rock layers were originally horizontal, we can make another assumption: Let's look at these rock strata here: Example of rock with five layers We have five layers total.

    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. Cross-Cutting Relationships We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.