# Relative Dating I Relative Dating Relative Dating is

• Slides: 21

Relative Dating

I. Relative Dating • Relative Dating is when you give the age of a rock or fossil compared to another rock or fossil. – Example: Rock A is OLDER than Rock B. – An actual age in years is not determined.

II. Rules of Relative Dating 1. Law of Superposition: Superposition When sedimentary rock layers are deposited, younger layers are on top of older deposits.

Rules of Relative Dating 2. Law of Original Horizontality: Horizontality Sedimentary rock layers are deposited horizontally. If they are tilted, folded, or broken, it happened later.

Rules of Relative Dating 3. Law of Cross-Cutting Relationships: Relationships If an igneous intrusion or a fault cuts through existing rocks, the intrusion/fault is YOUNGER than the rock it cuts through

III. Correlation • Rock layers in different places and be correlated or matched up by matching up the rocks and fossils in the layers.

IV. Unconformity • When rock has been eroded, eroded so you have a missing section of time in the rock record.

Unconformity

Why are some of the layers tilted? What happened to the top of the folded rocks? The missing rock layers is called an _____.

What type of rocks formed above? What events might have caused the rock layers to become tilted? Why is the top flat? What caused the angular unconformity?

Index Fossils • Some species of organisms only lived for a short period of time before they became extinct. If you use radiometric dating to get Trilobite: Index fossil for the an age for the fossil, then you know that the rock the Paleozoic Era Age: 590 -250 mya fossil is found in is also that age.

Graptolite • Index Fossil of the Ordovician Period • Age: 500 -335 mya

Fossil B lived 10 million years ago – 2 million years ago. Fossil A lived 20 million years ago- 8 million years ago. What do you know about the age of the rock containing both fossils?

• What is “B”? • Place the layers in order from oldest to youngest

Put the events in order

Put the events in order

http: //reynolds. asu. edu/glg 103/relative_age_principles. htm