Aim How can we explain relative dating Relative

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 (1)Lived over a wide geographic area (2)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: Uplift erosion submergence deposition