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

numerical dating method
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
factors affecting isotope dating results
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
enough number of atoms
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
other numerical dating method
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
geological time scale
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
sequence of time scale
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
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