Relative and Absolute Dating Techniques

Daily Dose of Dinos:  New Dinosaur Discovery Article

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Section 1: Relative Dating

Understanding Relative Dating SUSD5 Student Version

Purchase a Copy of the Relative Dating Lab from Teachers Pay Teachers for $1.00.  Includes answer key

In this section we will learn how scientists go about figuring out how old rocks, minerals, and fossils are.

The first method is called Relative Dating.  Relative dating does not give an exact date.  It tells us orders of sequence instead.  Which rock layer is older and which is younger?  In the image below, you will see the tracks of a bird, barefoot woman, a business man, a motorcycle, and a really small clown car.  Using deductive reasoning, tell a story in chronological order.  Click on the picture to enlarge it if needed.

Relative Dating

What are the order of events?

There are a few techniques used to help us relatively date rock strata.

Index fossils 

Index fossils are fossils that lived a relatively short period of time, were abundant, and were geographically widespread.  Index fossils help paint a picture on how organisms might have changed and evolved over time.  They can show how some species lived and what specie replaced them.  They also demonstrate that some species haven't changes all that much.  Using index fossils helps scientists determine the relative age of rock strata.

Index fossils

Index fossils

Here is an example of how valuable index fossil are when trying to relatively date some rock strata.  If you come across a Paradoxides pisus when looking at rocks, you can be assured that the layer in which you found this fossil is from the Cambrian Period or about 500 million years old, because the Paradoxides pisus lived 500 million years ago and it has never been found in any other rock strata that is either younger or older.

Law of Superposition

The Law of Superposition states that in undisturbed layers of rock, the oldest rocks are on the bottom and get young in age as you move up the layers.  We already know that sedimentary rocks are formed by sediments being horizontally deposited.  Every new layer is deposited on the older layer.  Bottom layers remain unseen to us until tectonic processes or weathering and erosion make them visible.  

Law of Superposition

Law of Superposition

Cross-Cutting  Relationships

Cross-cutting relationships states that any feature that cuts across a body of rock is younger than the rock that it cuts across.  Faults can offset layers which tells us that the fault happened after the layers was deposited.  If magma intrudes, creating a sill, we know that the sill is younger than the layer above and below, because the layers already had to exist in order for a sill to be formed.  You can tell that a batholith is younger than the layers that are on top because a batholith intrudes into current layers.

Cross Cutting Relationships 2.png

Cross-Cutting Relationships.

In the image above, you can see the relative order in which each even and layer occurred. Layers 4 and 8 represent periods of erosion by using squiggly lines.  Number 7 have to of come after 1-6 because it cut across all of the other layers.  Layer 10 represents a fault, in this case a normal fault caused by tension.    It happened last and we know that because all of the other layers had to have been there before the Earth faulted.


Sometimes rock layers are missing due to weathering and erosions.  These gaps in rock layers are called unconformities.  After layers have been eroded, other sediment will be laid on top.  These gaps represent a missing period in our relative time scale.

An angular unconformity occurs when rocks are tilted, then eroded and then new sediment is deposited on top.

Angular uncomformity

Angular Unconformity

A disconformity occurs when there is evidence that horizontal layers eroded and then were reburied, representing another gap in the age sequence.



A nonconformity occurs when igneous and metamorphic rocks are exposed due to uplift, eroded and then sediments are deposited on top.  An example would be an intrusive sill that was exposed due to weathering and erosion and then reburied by overlying sediment.  Again, this represents a gap in the rock record.  We don't know how long it took for the rock to be exposed because the evidence was weathered away.



Law of Inclusions

This law states that if a rock contains fragments of another rock, then it must be younger than the fragments.

law of inclusions

Law of Inclusions

Rock Correlation

Rock correlation is matching exposed layers in one area to exposed layers in another area.  Index fossils are used to connect and prove that these layers are the same.  Sometimes relative dating does not help with dating rocks so scientists use absolute dating instead.

Rock Correlation