Learning Goals
Time
How we achieved a modern sense
of time.
• Relative Geologic Time
– Superposition (oldest on bottom)
– Crosscutting and inclusion
• Intrusions younger than host
• Cobbles are older than host
• Radiometric dating (absolute time)
– Half lives and exponential decay
• Geologic Time
– Hadean, Archean, Proterozoic,
Phanerozoic
Yearly Calendars are Ancient
Clocks
• Sundials are relative (solar) clocks.
• Mechanical clocks developed late
• Stonehenge is 2000+ BC and
indicates that ancient cultures
counted days and knew precisely the
repeat cycle of the seasons.
Sundial: Solar Clock
13th century (pre Renaissance).
• Clocks spread through Europe in 1416th Centuries.
• Renaissance Italy was ‘obsessed’
with measurement, for painting,
sculpture, as well a practical
matters (commerce and war).
Clocks
• Mechanical
Clocks spread
through Europe
in the 14-16
Centuries.
• This elaborate
one is in
Prague.
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Deep Time
(Clocks in the rocks)
• Bishop Ussher 17th Cent. (biblical): 4004BC
• Buffon 18th Cent. (Cooling of spheres):
~50000 Y
• Hutton late 18th Cent. (Geological cycles):
Infinite
• Darwin late 19th Cent. (Biological changes):
Billions
• Kelvin late 19th C (Sun’s energy): 40 Million
Max (Kelvin was wrong!)
• Modern (Radiometric): 4.55 Billion
Superposition: Oldest on bottom
Relative Age of Rocks
• Original Horizontality
– Sediments were originally flat-lying
• Superposition
– The oldest ones are on the bottom
• Cross-cutting
– The disturbed (host) rocks are older
than disturbing rocks
Correlation of Layers
• Physical Continuity
– Horizontal tracing
• Similarity of rock types and
sequences
• Correlation of Fossils
– Faunal succession
Crosscutting:
Correlation
of Layers
Host rocks (red) are older than
the intruding rocks (black).
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Crosscutting
Principle of
Cross-cutting Relationships
Grand Canyon
Angular Unconformity, GCNP
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Angular Unconformity, GCNP
Angular Unconformity, GCNP
Angular Unconformity, GCNP
The Age of the Earth
• Bishop Ussher 17th Cent. (biblical): 4004BC
• Buffon 18th Cent. (Cooling of spheres):
~50000 Y
• Hutton late 18th Cent. (Geological cycles):
Infinite
• Darwin late 19th Cent. (Biological changes):
Billions
• Kelvin late 19th C (Sun’s energy): 40 Million
Max (He was wrong!)
• Modern (Radiometric): 4.55 Billion
Ice Ages
Mammals and
Flowering Plants
Dinosaurs
Fish
Trilobites
Time scale is NOT linear
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Relative Age of Rocks
• By the mid 19th century a relative time
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Event Sequence
scale had been worked out for the
sedimentary rocks of Europe (Phanerozoic).
They lacked an absolute time scale.
Kelvin and classical physicists advocated
40 million max.
Darwin and evolutionary biologists
advocated billions of years.
Discovery of radioactivity at about 1900
confirmed billions.
Event Sequence
1. Original horizontality
2. Superposition (oldest
on the bottom)
3. Crosscutting
(Intruding igneous
rocks are younger
than their hosts)
Radiometric Dating:
Establishing an absolute time scale
• Minerals contain naturally radioactive
elements
– K, U, Th, Rb, Sm
• These elements decay to stable daughter
elements
• When minerals crystallize from melt, they
contain parent only.
Radiometric Dating
Example:
40K
-
40Ar
• A K-feldspar (KAlSi3O8) crystallizes in
a granite and initially contains no Ar.
• Natural K is 0.012% 40K
• Atmospheric Ar has 3 stable isotopes
– 36K, 38K, 40K
– No 36K, or 38K in feldspar.
• If we measure the concentration of
daughter element in a mineral and we know
the decay rate, we can calculate when the
mineral crystallized.
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Radiometric Dating
Example:
40K
-
40Ar
• A K-feldspar (KAlSi3O8) crystallizes in
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a granite and initially contains no Ar.
Natural K is 0.012% 40K
40K decays to 40Ar with a half-life of
1.31 x 109 years (1.3 billion years).
If we measure the 40Ar content of the
feldspar, we can get a crystallization
date of the mineral.
Isotope measurements are made
with a mass spectrometer.
Inclusion:
Host rocks are younger than
the included rocks (cobbles).
Radiometric Dating
• Igneous and metamorphic rocks can
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be dated directly by radiometric
methods.
Sediments cannot be dated directly.
Igneous rock fragments in sediments
can be dated. (Sed must be younger)
Igneous rock intruding sediments can
be dated. (sed must be older)
14C can be used to date organic
matter less than ~50000 yrs old.
Radiometric Dating
• Igneous and metamorphic rocks can
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be dated directly by radiometric
methods.
Sediments cannot be dated directly.
Igneous rock fragments in sediments
can be dated. (Sed must be younger)
Igneous rock intruding sediments can
be dated. (sed must be older)
14C can be used to date organic
matter less than ~50000 yrs old.
Intrusion:
Host rocks (red) are older than
the intruding rocks (black).
14C
system is different from
other radiometric dating
systems.
• 14C in the atmosphere comes from 14N
• Plants take 14C from atmosphere
• 14C has half-life of 5730 years
• There is no 14C in rocks.
• 14C can be used to date plant and animal
matter that is younger than about 50,000
years.
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Exponential Decay
Types of Radioactive Decay
• Particle composed of: Mass# Atomic # Example
• alpha
2 neutrons+
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2
U, Th,
• beta• beta+
• gamma
2 protons
electron
positron
photon
nuclear
reactions
• neutron neutron
Naturally Radioactive Isotopes
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Geologic Time Scale
Period
Began (My ago)
Quaternary
0.01
Pleistocene
1.6
Pliocene
5.3
Miocene
23.7
Oligocene
36.6
Eocene
57.8
Paleocene
66.4
Cretaceous
144
Jurassic
208
Triassic
245
Permian
286
Pennsylvanian
320
Mississippian
360
-1
+1
0
1
0
40K
40K
all
235U
Naturally Radioactive Isotopes
Parent
• Eon
Era
• Phanerozoic Cenozoic
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Tertiary
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Mesozoic
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Paleozoic
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0
0
0
Daughter Half life
Decay
40K
40Ar
238U
206Pb
+
8, 
7, 
6, 
-
87Rb
235U
232Th
14C
87Sr
207Pb
208Pb
14N
1.3 Gy
49 Gy
4.5 Gy
0.7 Gy
14 Gy
5700 y
Geologic Time Scale
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Phanerozoic Cenozoic
Mesozoic
Paleozoic Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Proterozoic
Archean
Hadean
286
320
360
408
438
505
550
2500
4000
4550
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Some Major Events
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Latest warming 7000y
Ice ages ~1.8 MY to 7000 years ago
Dinosaur extinction 66 MY
Dinosaurs ~245 MY
Vertebrates ~400 MY
Multi-cell life forms ~550 ‘Cambrian Explosion’
‘Snowball earth’ 600 MY
Free O2 ~ 2.5 GY (CH4 and NH3 decline)
Single cell life forms ~3.7 GY
Oceans: at least by 4.3 GY
Accretion: 4.55 GY
Tree of Life
Ice Ages
Mammals and
Flowering Plants
Dinosaurs
Fish
Trilobites
Geologic Time Terms
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Hadean
Archean
Proterozoic
Phanerozoic
Paleozoic
Mesozoic
Cenozoic(Tertiary)
Cambrian
Unconformity
Angular unconformity
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Half-life
Alpha particle
Beta particle
Gamma ray
Neutron
A conglomerate contains some
granite cobbles. K-Ar date on
the granite gives 180 MY.
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A conglomerate contains some
granite cobbles. K-Ar date on
the granite gives 180 MY.
It is only rarely possible to use
radiometric methods to directly
date sedimentary rocks.
But igneous and metamorphic rocks
can be dated directly.
A conglomerate contains some
granite cobbles. K-Ar date on
the granite gives 180 MY.
Host rocks (red) are older than
the intruding rocks (black).
• A. The conglomerate is older than 180
MY
• B. The conglomerate is younger than
180 MY
• C. The conglomerate is 6000 y old
• D. No age inference can be made.
A shale is intruded by a basalt dike that
has an age of 1100 MY.
Clicker
A shale is intruded by a basalt dike
that has an age of 1100 MY.
• A. The shale is older than 1100 MY
• B. The shale is younger than 1100 MY
• C. The shale is less than 1 million
years old.
• D. No relative age inference can be
made.
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Exponential Decay
Geologic Time Scale
• Eon
Era
• Phanerozoic Cenozoic
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Tertiary
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Mesozoic
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Paleozoic
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The era of dinosaurs is
subdivided into Triassic,
Jurasssic, and Cretaceous.
Together these are known
as the:
• A. Archean
• B. Proterozoic
• C. Paleozoic
Period
Began (My ago)
Quaternary
0.01
Pleistocene
1.6
Pliocene
5.3
Miocene
23.7
Oligocene
36.6
Eocene
57.8
Paleocene
66.4
Cretaceous
144
Jurassic
208
Triassic
245
Permian
286
Pennsylvanian
320
Mississippian
360
Why can’t 14C be used to
date limestones?
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A. No carbon in limestone
B. No 14C in limestone
C. 14C half-life too long
D. 14C half-life too short
E. Daughter 14N not retained by limestone
• D. Mesozoic
• E. Cenozoic
Half-lives: If the amount of
radioactive isotope is ¼
the amount originally
present, how many halflives have gone by?
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A.
B.
C.
D.
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3
4
Half-lives: If the amount of
radioactive isotope is ¼
the amount originally
present, how many halflives have gone by?
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A.
B.
C.
D.
1
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3
4
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Event Sequence
1. The basaltic dike is older than the granite
A. True
B. False
Event Sequence
2. The basaltic sill is older than the granite.
A. True
B. False
Event Sequence
Event Sequence
3. Sediment layer ‘a’ is older than the granite
A. True
B. False
4. Sediment layer ‘q’ is older than the granite
A. True
B. False
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