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 Law of Superposition: When sedimentary rock layers are deposited, younger layers are on top of older deposits.

Rules of Relative Dating 2. Law of Original 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: 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, so you have a missing section of time in the rock record.

Unconformity

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

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 an age for the fossil, then you know that the rock the fossil is found in is also that age. Trilobite: Index fossil for the Paleozoic Era Age: 590-250 mya

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/glg103/relative_age_principles.htm