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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