The The Laws of Relative Dating

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Often when geologists examine rock outcrops like the one illustrated above they are interested in not only the types of rocks present, but the order in which they formed.  When they put events in chronological order like they use Relative Dating.

Relative Dating

  • putting events or objects in order based on when they happened compared to each other
  • do not need to know exactly when something happened, just if it happened before or after something else

We do this all the time in our regular day to day lives.  For example, lets put the following movies in order of relative age from oldest to youngest:  Twilight, Finding Nemo, the Avengers,  the Hunger Games.

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We don't need to know what year any of these movies came out to place them in order of relative age.  

Nemo is the oldest, followed by Twilight, Hunger Games, and the Avengers

Again, we have not stated anything about the actual age of these movies, we've just put them in order relative to one another.  The same thing can be done with geologic features in a rock outcrop.  To do this geologists use the Laws of Relative Dating.

The Law of Superposition

In any series of undisturbed sedimentary rocks, THE OLDEST ROCKS ARE ON BOTTOM AND THE YOUNGEST ROCKS ARE ON TOP.


The Law of Cross-Cutting Relationships

Igneous intrusions and faults are YOUNGER THAN ANY LAYER THEY CUT INTO.

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The Law of Included Fragments

Any fossil or rock fragment found inside a layer of rock MUST BE OLDER THAN THE ROCK IT IS IN.

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Mulroy's Law of Contact Metamorphism

Any layer that has been changed by contact metamorphism must be OLDER THAN THE INTRUSION THAT CAUSED IT.