2Timing the Geologic Record Chapter 8:Clocks in Rocks:Timing the Geologic Record
3About the Clocks in Rocks Geologists have uncovered deep time by studying the “clocks in rocks.”These clocks can be used to measure the duration of geologic processes and cycles of the Earth system.Geologists refer to both relative age and absolute age.
4Lecture OutlineReconstructing geologic history from the stratigraphic record2. Geologic time scale: relative ages3. Measuring absolute time with isotopic clocks4. Geologic time scale: absolute ages5. Advances in timing the Earth system
5● Principles of stratigraphy ● original horizontality ● superposition Reconstructing Geologic History fromthe Stratigraphic Record● Principles of stratigraphy● original horizontality● superposition● faunal succession
6Original Horizontality and Superposition YoungerOlderExample: Marble Canyon, Arizona
7Original Horizontality and Superposition YoungerOlderLeft undisturbed, the youngest layers always remain above the oldest.
8Original Horizontality and Superposition sedimentation inlake or sea
9Original Horizontality and Superposition Sediments are deposited in horizontallayers and slowly change into rock.
37Thought questions for this chapter As you pass by an excavation in the street, you see a cross section showing paving at the top, soil below the paving, and bedrock at the base. You also notice that a vertical water pipe extends through a hole in the street into a sewer in the soil. What can you say about the relative ages of the various layers and the water pipe?Why did 19th-century geologists who constructed the geologic time scale find sedimentary strata deposited in the sea more useful than strata deposited on land?
38Thought questions for this chapter In studying an area of tectonic compression, a geologist discovers a sequence of older, more deformed sedimentary rocks on top of a younger, less deformed sequence, separated by an angular unconformity. What tectonic process might have created the angular unconformity?A geologist documents a distinctive chemical signature caused by Proterozoic organisms. Would you consider this chemical signature to be a fossil?
39● Divisions of geologic time ● eras ● periods ● epochs 2. Geologic Time Scale:Relative Ages● Divisions of geologic time● eras● periods● epochs
58Key terms and concepts Eon Epoch Era Geologic time scale Half-life Absolute ageEonEpochEraGeologic time scaleHalf-lifeIsotopic datingMass extinctionPeriodPrinciple of faunal successionPrinciple of original horizontalityPrinciple of superpositionRelative ageStratigraphic successionUnconformity