Relative DatingRelative Dating

I. Relative Dating

• Relative DatingRelative Dating is when you give the age of a rock or fossil comparedcompared to another rock or fossil.

– Example: Rock A is OLDEROLDER than Rock B.– An actual age in years is not determined.

II. Rules of Relative Dating1.1. Law of SuperpositionLaw of Superposition: When sedimentary

rock layers are deposited, younger layers are on top of older deposits.

Rules of Relative Dating2. 2. Law of Original HorizontalityLaw of Original Horizontality: Sedimentary

rock layers are deposited horizontally. If they are tilted, folded, or broken, it happened later.

Rules of Relative Dating3. 3. Law of Cross-Cutting RelationshipsLaw of Cross-Cutting Relationships: If an

igneous intrusion or a fault cuts through existing rocks, the intrusion/fault is YOUNGERYOUNGER than the rock it cuts through

III. III. CorrelationCorrelation• Rock layers in different places and be

correlated or matched upmatched up by matching up the rocks and fossilsrocks and fossils in the layers.

IV. IV. UnconformityUnconformity• When rock has been erodederoded, so you have

a missing sectionmissing 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 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 EraPaleozoic EraAge: Age: 590-250590-250 mya mya

Graptolite • Index Fossil of the Ordovician PeriodOrdovician Period

• Age: Age: 500-335 500-335 myamya

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://northonline.sccd.ctc.edu/gel101tb/images/geocross3a.jpg

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