Aim: How can we explain relative dating?
Relative Dating Determines age of a rock or event by comparing it to other rocks or events
The Principle of Superposition The youngest rock layer is found on top and the rock ages increase with depth
Youngest Oldest
Exceptions to P. of S. When rock layers are folded, overturned, or faulted, older layers are moved over younger layers.
Younger Older
Intrusion Molten rock squeezes into preexisting rocks and solidifies. YOUNGER than any rock it cuts through.
Extrusion Molten rock flows on surface and solidifies. Younger than rock beneath it, older than any layers above it.
Inclusion A body of older rock within younger rock. Ex. Pebbles held together by cement – pebbles are older than the cement
Contact Metamorphism Both intrusions and extrusions form contact metamorphism with layers they touch. If you see contact metamorphism, it is YOUNGER than that layer.
Folds, faults, and joints These rock features are YOUNGER than the layers faulted or folded
Using Relative Dating
Correlation Used to reconstruct geologic history of a location Determines if rock layers or events in 2 separate areas are the same
1. Similarity of Rocks Rocks matched on similarity in appearance, color, and composition. Ex. Two sides of a valley
2. Fossil Evidence Index fossils useful in determining if two rocks are from same time period They must have Lived over a wide geographic area Lived for a relatively short time
3. Volcanic Time Markers Volcanic eruptions are short and eject layer of ash over a large area Useful as time markers
Unconformities Eroded surface that has been buried – part of rock record is missing Rocks above unconformity are younger, rocks below are older Associated with orogenies
Commonly caused by: Uplifterosion submergencedeposition