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
 James Ussher (1664): 5,668 years old
 Biblical Perspective
 Modern Scientific View

Relative Dating
Relies on Key Principles such as
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Uniformitarianism- the present 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
Earth History
Sedimentary Structures
An organism buried in sediment
3 Types of unconformities
3 Types of unconformities-contd
3 Types of unconformities-contd.
Hypothetical view of early earth
Grand Canyon
Sedimentary Rock Sequence
Hypothetical Landscape
Relative Dating-contd.
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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
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
Unstable atomic nuclei decay
Beta Decay – unstable atomic
nuclei
Electron Capture – unstable nuclei
Radioactive Decay
Bracketing Ages
Factors affecting Isotope Dating Results
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Isotope dating is more useful for igneous rocks
 Clock is set when igneous rock crystallizes locking the
radioactive isotopes within its crystal lattice
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Rock/Mineral must be a closed system
 Atoms of parent and daughter are still present in rock/mineral
being dated
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Condition of parent Material
 Fracture, weathering and migrating ground water
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Age of Substance
 Enough measurable daughter isotope, use appropriate
radioactive isotope
Enough Number of Atoms??
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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
Loss of daughter isotopes
Loss of daughter isotopes-contd.
Loss of daughter isotopescontd.
Loss of daughter isotopes-contd.
Carbon-14 Method
Other Numerical dating method
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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
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Dendrochronology (Tree-Ring dating)
 Annual growth rings
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Varve- deposited layers of lake-bottom
 Paired layers of sediments
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Lichenometry
 Lichens grow at a fairly constant rate
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Cosmogenic isotopes
 Used in dating land features
Fission-Track Dating method
Correlation of tree ring section
Origin of Lake Varves
Origin of Lake varves-contd.
Age of Lichen
Cosmogenic Isotopes-surface exposure dating
Geology at a Glance
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
Geological Time Scale
Sequence of Time scale
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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
Extinction Graph
METAMORPHISM & METAMORPHIC
ROCKS