Relative Dating

Important Figures in Geologic Time James Hutton (1726-1797): Native of Edinburgh, Scotland. Father of modern Geology. Published “Theory of the Earth” in 1785 in which he outlined that geological features and ancient rocks could be explained by present-day physical and chemical processes. Charles Lyell (1797-1875): Rebelled against prevailing thought, which was rooted in Biblical interpretation and Catastrophism. His main contribution was the development of Uniformitarianism (Actualism). “The present is the key to the past…” Modern view holds that processes that operate today have shaped the Earth through Geological Time, but rates may not have always remained constant.

The Key to the Past Relative Time- “this rock is older than that” Principles Used to Determine Relative Age Unconformities Correlation The Standard Geologic Time Scale Index Fossils Absolute Time- “this rock is 28 million years old” Principles of radioactive decay Instruments The age of the Earth

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 can 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

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

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?

http://reynolds.asu.edu/glg103/relative_age_principles.htm

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

Put the events in order

Put the events in order

Youngest _____ Oldest

Youngest_____ _____ _____ _____ _____ _____Oldest

Youngest ___________________ Oldest

Youngest ____ ____ ____ ____ ____ ____ ____ ____ _____ _____ Oldest

Make your own Strata Sequence Requirements: One of each: Igneous intrusion, faulting, folding, erosion Each rock type needs the correct symbol Neatness – use a ruler Key for rock types Separate Answer key