Rock Layers
Rocks tell us the story of the formation of an area. All we need are some guiding principles and we can see the story.
The following are the Principles of Relative Dating:
The Principle of Original Horizontality
The Principle of Superposition
The Principle of Crosscutting
The Principle of Inclusions
The Principle of Fossil Succession
The following are the Principles of Relative Dating:
The Principle of Original Horizontality
The Principle of Superposition
The Principle of Crosscutting
The Principle of Inclusions
The Principle of Fossil Succession
The Principle of Original Horizontality.
Original Horizontality - Sediments are deposited in horizontal layers.
Layers that are folded are usually older. They've been around longer and have had time to fold.
Layers that are folded are usually older. They've been around longer and have had time to fold.
The Principle of Superposition
Superposition - in a sedimentary sequence the OLDEST will be on BOTTOM and YOUNGER will always be on TOP (if undisturbed – flat and level)
The Principle of Cross-cutting
Cross-cutting - Anything that cuts a rock must be younger than the rock
a. Igneous intrusion - Magma that pushes up through the rock layers. It is younger than rock it has intruded (cut across). Dikes and Batholiths are examples of igneous intrusions (see side picture).
b. Faults – Rock shifts due to earthquakes. They are younger than rock displaced
a. Igneous intrusion - Magma that pushes up through the rock layers. It is younger than rock it has intruded (cut across). Dikes and Batholiths are examples of igneous intrusions (see side picture).
b. Faults – Rock shifts due to earthquakes. They are younger than rock displaced
The Principle of Inclusions
Inclusions - Pieces of rock found IN another rock must be OLDER (formed first) than the rock that they are in.