1Lecture 26: Chapter 36A Brief History of EarthPlan•How old is Earth‐Eras and periodsHome Work:•Chapter 36•Key Terms•Review questions: 2, 7, 8, 9, 11, 12, 18,21How old is Earth?•Uniformitarianism is a geological doctrinethat states that the natural laws (like laws ofphysics) have been constant over geologicalpastpast.•The scientific theories and tools we havetoday can be used to learning about the past.•The Earth’s history is recorded in rocks!Relative DatingSedimentary rock layers and lava flowsprovide good evidence of relative rockagesFive principles to determinethe relative ages of rocks1. Original horizontality:each new layer is laid down nearlyhorizontally over older sediments2. Superposition:in an undeformed sequence of sedimentaryrocks, each layer is older than the one above and older than theone below.3. Cross‐cutting:an igneous intrusion or fault that cutsthrough preexisting rock is younger.4. Inclusion:Any inclusion is older than the rock containing it.5. Faunal succession:Fossil organisms follow one another in adefinite, irreversible time sequence.
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