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Chapter 8: Geological Times

Chapter 8: Geological Times. Geochronology is the study of time in relation to earth’s existence Relative Dating Determines how old a rock is in relation to its surrounding Numerical Dating Determines actual age in years. Geologic Time - Perspective. Differing Views of the Earth’s Age

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Chapter 8: Geological Times

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  1. Chapter 8: Geological Times Geochronology is the study of time in relation to earth’s existence • Relative Dating • Determines how old a rock is in relation to its surrounding • Numerical Dating • Determines actual age in years

  2. Geologic Time - Perspective • Differing Views of the Earth’s Age • James Ussher (1664): 5,668 years old • Biblical Perspective • Modern Scientific View

  3. Relative Dating Relies on Key Principles such as • Uniformitarianism- thepresent is key to the past • Original horizontality • Sediments deposited in horizontal layers • Superposition • Youngest rocks are on top (assuming no tectonic activity) • Cross-cutting relationships • Cut layer is older than ‘cutting’ rock • Faunal succession • Organisms succeed one another in recognizable reproducible pattern • Unconformity • Represents a break (gap) in the rock record

  4. Earth History

  5. Sedimentary Structures

  6. An organism buried in sediment

  7. 3 Types of unconformities

  8. 3 Types of unconformities-contd

  9. 3 Types of unconformities-contd.

  10. Hypothetical view of early earth

  11. Grand Canyon

  12. Sedimentary Rock Sequence

  13. Hypothetical Landscape

  14. Relative Dating-contd. • INDEX FOSSILS: Species that had wide geographical distribution, but lived for only a brief period of time • Unconformmities: Boundary between 2 layers that are of different age • Reasons: • Rock layers removed by erosion • No rock Layers deposited CORRELATION METHODS METHODS OF CORRELATION: Paleontological Similarities Mineralogical Similarities

  15. Numerical Dating Method Isotope Dating relies on the rate of decay of radioactive isotopes within a rock • Radioactive isotopes have nuclei that spontaneously decay emitting or capturing a variety of subatomic particles • Decaying radioactive isotope- parent isotopes decay to form daughter isotopes • Half-life- is the time it takes for half the atoms of parent isotope to decay • Some radioactive isotopes with daughter products • U-238 => Pb-206; K-40 => Ar-40; C-14 => N-14

  16. Unstable atomic nuclei decay

  17. Beta Decay – unstable atomic nuclei

  18. Electron Capture – unstable nuclei

  19. Radioactive Decay

  20. Bracketing Ages

  21. Factors affecting Isotope Dating Results • Isotope dating is more useful for igneous rocks • Clock is set when igneous rock crystallizes locking the radioactive isotopes within its crystal lattice • Rock/Mineral must be a closed system • Atoms of parent and daughter are still present in rock/mineral being dated • Condition of parent Material • Fracture, weathering and migrating ground water • Age of Substance • Enough measurable daughter isotope, use appropriate radioactive isotope

  22. Enough Number of Atoms?? • Case Study: Half-life = 10 days • Time (d) Number of atoms: • 0 1024 80 d 4 • 10 512 90 d 2 • 20 256 100 d 1 • 30 128 1000 atoms----- 1 atom • 40 64 in 10 half-life • 50 32 • 60 16 • 70 8

  23. Loss of daughter isotopes

  24. Loss of daughter isotopes-contd.

  25. Loss of daughter isotopes-contd.

  26. Loss of daughter isotopes-contd.

  27. Carbon-14 Method

  28. Other Numerical dating method • Fission Track • High speed particles emitted during radiation may pass through crystal leaving ‘tears’ within the crystal- the older the rock, the more fission tracks • Dendrochronology (Tree-Ring dating) • Annual growth rings • Varve- deposited layers of lake-bottom • Paired layers of sediments • Lichenometry • Lichens grow at a fairly constant rate • Cosmogenic isotopes • Used in dating land features

  29. Fission-Track Dating method

  30. Correlation of tree ring section

  31. Origin of Lake Varves

  32. Origin of Lake varves-contd.

  33. Age of Lichen

  34. Cosmogenic Isotopes-surface exposure dating

  35. Geology at a Glance

  36. Geological Time Scale Contrasting several dating techniques chronicling Earth’s history to produce a geologic Time Scale • Geologic Time Scale- divided into Eons, Eras, Periods, and Epoches • Phanerozoic Eon (evidence of life began) divided into three eras • Paleozoic (ancient life) dominated by marine invertebrates • Mesozoic (middle life) dominated by reptiles • Cenozoic (recent life) dominated by mammals

  37. Geological Time Scale

  38. Sequence of Time scale • Eons--- Eras---Periods---Epochs • Life on earth ~4.6 byr old • Lack of 4.6 byr old on earth – however, moon rocks and meteorite provide a handle on this

  39. Extinction Graph

  40. METAMORPHISM & METAMORPHIC ROCKS `

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