Geologic Time:
How do we determine age (relative & absolute)
What is the age of the earth? How do we know?
What is the age of the Earth?
A. 4.44 million years
B. 1 million years
C. 4.55 billion years
D. 10000 years
Hutton’s Outcrop at Siccar Point
Hutton looked at this outcrop and reasoned that the Earth was very old.
He formulated the Principle of Uniformitarianism and opened the door for Darwin’s
Origin of Species.
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Uniformitarianism:
The Earth processes that act to shape the Earth today are the same processes that
acted to shape the Earth in the past.
“The present is the key to the past.”
Relative age dating - How do we place rocks in sequence?
Let’s review the principles…
Principle of Original Horizontality
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Steno: 1600s
Sediment (& lava) deposited horizontally in sheets.
Thus, inclined/folded beds are disturbed.
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Principle of Superposition:
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Steno - 1600s
If sediment is deposited in layers,
then…
Oldest layers are on bottom
provided they have not been
disrupted (e.g. faulted)
Principle of Continuity
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The layers of sediments
(and lavas) are continuous
over great distances.
Thus, can infer that they
continue across eroded
distances.
Principle Cross-cutting relationships
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Faults & intrusions cut
older rocks (i.e., faults and
igneous intrusions are
younger than the rocks they
cut)
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Principle of Inclusion
• If portions of pre-existing rocks are entrained in a rock (inclusions),
• Then the inclusions are older than the rock containing them.
Which is older, A or B?
A. A is older
B. B is older
A
B
Principle of Fossil Succession
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Fossils are remnants of previously
living organisms preserved in rock or
sediment
Smith - late 1700s noticed:
– Fossils occur in groups of species
(assemblage)
– Fossil organisms succeed one another
in definite order vertically through a
sediment column.
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He correctly reasoned that one could
group/correlate rock units over distance
based on their common fossil
assemblage
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Loss of a fossil organism from an
assemblage = extinction (Curvier, early
1800s)
– i.e., Fossil assemblages are distinct in
time
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Fossil succession & relative time
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Given that fossil assemblages change
over time due to evolution and
extinctions,
And, fossils assemblages correlate over
distances
Then, distinct fossil assemblages can be
used to assign relative age of the rocks
in which they are preserved.
Unconformities = Gap in record or loss of recorded time
animation!
Disconformity
Nonconformity
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Angular Unconformity:
What is implied or what can you
infer from unconformities?
Loss of recorded time!
Buried rocks uplifted to surface and
overlying rocks eroded (lost time).
Given Principles and Unconformities we can apply relative age
dating to rocks in a given area.
Can you list these numbered units
according to relative age?
(relative age animation)
Absolute Age
• Animation
– Kelvin’s calculation
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Periodic Chart of the Elements & Decay
U and Th - in the Ac contraction series
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Elements defined by number of protons in nucleus
Some elements have ‘unstable’ nuclei and spontaneously ‘decay’: Protons
turn into Neutrons and vise versa.
How does isotopic age dating work?
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Unstable isotope nuclei decay
Rate of decay is a function of the
number of Parent isotopes present
– This is what allows us to use it as a
clock!
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Emit particle and Heat!
– This is a major source of heat in
the crust and mantle
Now for some fun math…
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Half-life & Daughter- Parent ratio
animation
Isochron
animation
• As time proceeds, number of Daughters grow, parents diminish
• Thus the ratio of parents to daughters (D/P) is a function of time!
• Need lots of Parents and Daughters to measure D/P
– Not all rocks can be dated: need to have enough P and D. Which is to say
they need to have the right composition and the right age….
What happens to the Daughter/Parent ratio after many half-lives?
A.
It becomes very large
B.
It becomes very small
So, can very ‘young’ rocks be easily age dated?
A.
Yes
B.
No
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Why can’t all rocks be dated?
b/c not all rocks are attractive
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Not all rocks contain ‘appropriate’ isotopes (composition)
Age must be within magnitude of half-life or there won’t be enough parent or
daughter isotope present to measure.
Many rocks are Open Systems - they loose parent or daughter during their
long history.
Dating Sedimentary rocks = age of sediment NOT age of the rock.
So, what is the age of Earth?
We cannot date rocks from
Earth to obtain an age of the
Earth because tectonics
resurfaces the Earth over
time!
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Slope of isochron is a function of time
Iron meteorites = sample w/ very low abundance of parent isotope
Stony meteorites ~ Earth (terrestrial planet) closed to isotope loss
Marine Lead = homogenous Earth Pb! Shows that Earth falls on
meteorite isochron and validates the argument!
What we learn from isotopes:
date events in earth history by dating the rocks that record those events!
• Age of Earth
• When Earth differentiated
into layered structure
– 4.4 Ga detrital Zircon
• = oceans & crust
– 4.03 Ga Acasta gneiss
• = plate tectonics
• Age and timing (rates) of
mountain building events
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Absolute age date = number age
Relative age = sequence of events
Chicxulub impact site: 6 km
diameter meteorite impact.
Produced Ir-rich clay layer
world wide at K-T boundary
(relative dating). Ar-Ar dating
of igneous rocks from impact
melting give age = 65 Ma
• All rocks cannot be dated. So, how do we know when
something happened?
– Combine isotopic age dating with relative age to determine age
of events!
We then use principles to correlate rocks over regional
distances
Geologic Time Scale
• Some cool results of
absolute age
dating…
Oxygen atmosphere at ~2.4-1.8 Ga
•Constructed using…
–Absolute age of rocks
•Radiometric date
–Relative age dating
•Sequence of events
First continental crust and oceans at 4.4 Ga
Planets form in 1st 0.2% of SS history
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Geologic Time Scale:
Current Ice
Age
Major
Extinction
Major
Extinction
Cambrian
Explosion of
macroscopic
life
• How do we know when something happened?
• Combine isotopic age dating with correlations (relative
time scale) to determine age of events!
More interesting events
Humans
(Australopithecus)
diverge from apes
~ 8-5 Ma
Early apes
~ 24 Ma
90% of Earth
History in
Precambrian
Dinosaurs
evolve
from
reptiles
3.2 Ga
4.4 Ga
Time Scale compressed to 1 year
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More fun examples of geologic time
• Register roll: if 1 Ma = 1 inch,
– then earth history = 379 feet of paper or 126 yards.
• Driving time: if 1 year = inch
– then earth history = 71,811 miles
– At 75 mph, it would take 957 hours or 40 days of continuous
driving. (driving at 1320 inches/second).
• Counting: 1 yr = 1 sec
– 4.55e9 sec = 7.6E7 min = 1.3E6 hr = 5.3E4 days = 144 years
Review Questions
• Given a sequence of sediments, how do you know the
rocks at the base of the sequence are oldest (what
principles apply)?
• Compare the principle of cross-cutting relationships to the
principle of inclusion - how do they indicate relative age?
• Consider Fossil Succession:
– Who first recognized this principle?
– What is an index fossil
– How is this principle used to indicate relative age?
Review Questions
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What is the age of the Earth, who determined it and how?
Can the age of the Earth be determined using Uniformitarianist clocks?
Who were the scientists who developed our understanding of radioactivity and
what were their contributions?
Why can radioactive decay be used to determine age?
Why can’t all rocks be dated by radiogenic decay methods?
Important Dates:
– When did plate tectonics begin on Earth and thus producing oceans and continents?
– When did the atmosphere become oxygen rich?
What is the age of…
– The first evidence of life on Earth
– The first evidence of vertebrate animals (those with backbones)
– The first evidence of Mammals
– The divergence of Humans from Apes
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