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
How do we determine if layers separated by
large distances formed at the same time?
WY
UT
How would we recognize a “gap” in the rock
record – if part of the rock record is missing
and how much time the “gap” represents?
CO
Laws / Principles
of Stratigraphy
Timing of Geologic Events
1) relative-age dating (fossils,
stratigraphy, structure)
2) absolute-age dating (isotopes,
tree rings, etc.)
Nicolaus Steno (1669)
• Law of Superposition
• Principle of Original Horizontality
(1638-1686)
• Principle of Lateral Continuity
William Smith (1793)
• Principle of Fossil Succession
(1769-1839)
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Law of Superposition
Principle of Original Horizontality
Youngest rocks
Oldest rocks
In a sequence of undisturbed layered rocks,
the oldest rocks are on the bottom.
Use of Fossils to Correlate
Rock Formations
Layered strata are deposited horizontally
or nearly horizontally
Principle of Fossil Succession
William “Strata” Smith (1793)
• Recognized that different strata
contained different fossils
• Recognized an order or
succession of fossils and strata
• Used fossils to correlate
formations from different outcrops
Unconformity
A surface that represents a break
in the rock record due to erosion or
nondeposition.
Angular Unconformity
Types of Unconformities
Angular Unconformity
Nonconformity
Disconformity
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Siccar Point, Scotland
Nonconformity
Devonian
Old Red Sandstone
Older tilted strata
(shales and slates)
Several unconformities are
present in the Grand Canyon
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Disconformity
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2
1
Several unconformities are
present in the Grand Canyon
Cambrian Tapeats Sandstone
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3
2
1
Precambrian Wapatai Shale
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South rim of the Grand Canyon
250 million years old
South rim of the Grand Canyon
250 million years old
Paleozoic Strata
550 million years old
1.7 billion years old
Precambrian
Principle of
Cross-cutting Relationships
550 million years old
Nonconformity
1.7 billion years old
Vishnu Schist
Host rocks (red) are older than the
intruding rocks (black).
Your turn
Use the geologic principles to
place the events in order
What is this surface?
Lava Flow
(bed H)
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• By the mid 19th century a relative time
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.
Phanerozoic
How old is the Earth?
Geologic Time Scale
Geologic Time Scale
Time divisions (units) of Earth’s history
as recorded by rock formations – based
originally on relative-dating methods:
Fossil groups or assemblages
Fossil succession (order of fossils)
Stratigraphic relationships
Cross-cutting relationships
and later…
Absolute (isotopic) ages
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
• Modern - (Radiometric): 4.55 Billion
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Many isotopes of each element
occur naturally
Absolute-Age
Dating
Absolute ages of geologic events
and rock formations are based on
radioactive elements and the rates
at which they decay.
Rocks are composed of minerals,
Minerals are composed of atoms of different
elements
1. Proton: positive charge
2. Neutron: no charge
3. Electron: negative charge
Isotope: a variety of an element with the
“normal” number of protons, but
different number of neutrons
http://ie.lbl.gov/education/isotopes.htm
Isotope
The number of protons
determines the element
Æ “the atomic number”
The neutrons of a given element
may vary
ISOTOPE: variations of the same
element, with different # of
neutrons, and so different atomic
mass number
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Isotopes of Carbon
Radioactive decay: Spontaneous changes in
structure of atomic nuclei
• Types of radioactive decay
1. Alpha emission: Emission of 2 protons and 2 neutrons
Which of the following accurately describes alpha emission
A) Atomic number lower by 2; atomic mass unchanged
B) Atomic number lower by 2; atomic mass lower by 2
C) Atomic number lower by 2; atomic mass lower by 4
D) Atomic number lower by 4; atomic mass lower by 4
Example of alpha emission
U238 Æ Th234
2. Beta emission
–A neutron loses an electron and turns into a
proton; the electron is ejected from the
nucleus
Which of the following accurately describes beta emission
A) Atomic number unchanged; atomic mass unchanged
B) Atomic number increases by 1; atomic mass unchanged
C) Atomic number decreases by 1; atomic mass unchanged
D) Atomic number increases by 1; atomic mass dec by 1
What is change in
1) Atomic number? -2
-4
2) Atomic mass?
Th234 Æ Pa 234
3. Electron capture: An electron is captured,
combines with a proton to form a neutron
Parents and Daughters
• Parent: an unstable radioactive isotope
• Daughter product: isotopes resulting from
decay of parent
Keep track of the ratio
# of daughter (D) to
# of parents (N):
D/N
What is change in
1) Atomic number? -1
0
2) Atomic mass?
K40 Æ Ar40
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Radiometric Dating:
Establishing an absolute time scale
• Minerals contain naturally radioactive
elements
– K, U, Th, Rb, Sm
• These radioactive parent isotopes decay to
stable daughter isotopes
• When minerals crystallize from melt, they
contain parent only.
• If we measure the concentration of
daughter isotope in a mineral and we know
the decay rate, we can calculate when the
mineral crystallized.
Types of Radioactive Decay
• Particle composed of:
• alpha
2 neutrons+
2 protons
• betaelectron
• beta+
positron
• gamma
photon
nuclear
reactions
• neutron neutron
Mass# Atomic # Example
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2
U, Th,
0
0
0
1
-1
+1
0
0
40K
40K
all
235U
An Example: U238 to Pb206
Common types of radioactive decay
The half-life of a radioactive isotope is
the time required for half of the original
number of radioactive parent atoms to
decay to stable daughter products.
Fraction of elements present
Radioactive decay curve
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Naturally Radioactive Isotopes
Half-lives:
If the amount of radioactive isotope
(the parent) is ¼ the amount originally
present, how many half-lives have
gone by?
A.
B.
C.
D.
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2
3
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• Parent Daughter Half life
Decay
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β+
β8α, γ
7α, γ
6α, γ
β-
40K
40Ar
87Rb
87Sr
238U
206Pb
235U
207Pb
232Th
208Pb
14C
14N
1.3
4.9
4.5
7.1
1.4
5.7
x
x
x
x
x
x
109 y
1010 y
109 y
108 y
1010 y
103 y
Radiometric Dating
Example:
40K
-
40Ar
• A K-feldspar (KAlSi3O8) crystallizes in
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.
Some Major Events
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Latest warming 7000y
Ice age ~1.8 MY
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
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The era of dinosaurs is
subdivided into Triassic,
Jurasssic, and Cretaceous.
Together these are known as the:
Dike @ 66
million years
old
A. Archean
B. Proterozoic
C. Paleozoic
D. Mesozoic
E. Cenozoic
Ash bed @ 160 million years old
This surface represents:
A. A fault
B. A fold
C. An unconformity
D. The Phanerozoic
Why can’t 14C be used to
date limestones?
limestones?
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
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A. No carbon in limestone
B. 14C half-life too short
C. 14C half-life too long
D. Daughter 14N not retained by limestone
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