Stratigraphy, Age Dating
and Extinctions
How old is that rock?
!  1. Relative age: order of events
Relative dating tells us what order things happened,
but not how many years ago they happened.
!  2. Absolute age: age in years
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Determining Relative Geologic Time
1. 
2. 
3. 
4. 
Principle of Original Horizontality
Principle of Superposition
Principle of Cross-Cutting Relationship
Principle of Faunal Succession
1. Principle of Original Horizontality
states that most sedimentary layers of rock are
deposited in a horizontal position.
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2. Principle of Superposition
it states that in undeformed stratigraphic
sequences, the oldest strata will be at the bottom
of the sequence.
2. Principle of Superposition
Youngest
ab
Oldest
c
d
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3. Principle of Cross-Cutting Relationships
Sometimes magma pushes, or intrudes, into existing rocks. When the melted
rock cools and solidifies, the resulting feature is called an igneous intrusion.
The principle states that an igneous intrusion is
always younger than the rock it cuts across
Igneous intrusion
Faulted rock
4. Principle of Faunal (fossil) Succession
States that different strata each contain particular assemblages of
fossils by which the rocks may be identified and correlated over long
distances; and that these fossil forms succeed one another in a definite
and habitual order. This law, together with the law of superposition of
strata, enables the relative age of a rock to be deduced from its content
of fossil faunas and floras.
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I. Relative Dating " Unconformities
comformity
unconformity
Types of Unconformity:
1. Disconformity
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Types of Unconformity:
2. Angular unconformity
Types of Unconformity:
3. Nonconformity
Sedimentary
Igneous
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Order of Events?
Oldest
3
Youngest
1
4
2
2
1
3
4
Grand Canyon
Example
Order of Events?
Oldest
1
Youngest
2
4
3
3
4
2
1
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Complete the ordering of the geologic events
How old is that rock?
!  Relative age: order of events
Relative dating tells us what order things happened,
but not how many years ago they happened.
!  Absolute age: age in years
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II. Absolute Dating
Radioactive Decay: The spontaneous transformation of an
unstable atomic nucleus into a lighter one.
Half-life: the time required for one half the atoms of a given
amount of a radioactive substance to transform into other atoms
Half-life Example
•  You start with $100 in your bank account
•  Every week you remove half of the money
•  The half-life of your account = 1 week
1 week
(= 1 half-life)
1
Your Bank Account
1 half-life
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
Start: $100
Week 1: $50
Week 2: $25
Week 3: $12.5
Week 4: $6.25
Week 5: $3.125
Week 6: $1.56
Week 7: $0.78
Week 8: $0.39
Week 9: $0.19
Radioactive Decay Curve
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Isotopes & Half Lives
Young things
Old things
Mass Extinctions
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Mass Extinctions
Mass Extinction Theories:
1. Extraterrestrial Impacts
A Bad Day…. 65 Million Years Ago
Dinosaurs extinction
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Mass Extinctions theories :
2.Volcanic Eruptions
Fig. 4-2, p.75
Mass Extinction Theories:
3. Supercontinets and Earth’s Carbon Dioxide Budget
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ERAS
Cenozoic: 66 - 2.5 m.y.a
! 
Fo
re
xa
m
! 
! 
! 
PERIODS
Millions of
years ago
Mesozoic: 245 - 66 m.y.a.
Paleozoic: 544 - 245 m.y.a.
Precambrian: pre-544 m.y.a.
Fo
re
xa
m
Key events in the
evolution of life
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