Exam 1 GG 101
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32.8125
40
60
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1
Relative Age Dating
Absolute Age Dating
Age Dating Using Magnetic Records
The Age of the Earth
Geologic Time
A major difference between
geologists and most other
scientists is their attitude about
time.
A "long" time may not be
important unless it is > 1 million
years.
2
Earth History
Two ways to date
geologic events
1) relative dating (fossils,
structure)
2) absolute dating (isotopic, tree
rings, etc.)
3
Relative vs Absolute Age
• Usually geologists first establish
relative ages then try to get absolute
age dates
• Determining relative age relies on a
number of geologic principles that
were developed during the 17th to
early 19th centuries
Steno's Laws
Nicolaus Steno (1669)
• Principle of Superposition
• Principle of Original Horizontality
• Principle of Lateral Continuity
Laws apply to both sedimentary and
volcanic rocks.
4
Principle of Superposition
In a sequence of undisturbed
layered rocks, the oldest rocks
are on the bottom.
Principle of Superposition
Sedimentary rocks are deposited in a
layer-cake fashion:
Layer 4
Layer 3
Layer 2
Layer 1
Each layer is older than the one
above and younger than the one
below
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Principle of Superposition
Youngest rocks
Oldest rocks
Principle of Original Horizontality
Layered strata are
deposited horizontal or
nearly horizontal or nearly
parallel to the Earth’s
surface.
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Principles of original
horizontality and superposition
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Principle of Lateral Continuity
Layered rocks are deposited in
continuous contact.
Principle of Lateral Continuity
Map view
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Principle of Lateral Continuity
Map view
Principle of Lateral Continuity
Map view
9
Correlation
• Process used to tie separated strata together
• Based on matching physical features such as
– Physical continuity - trace of rock unit
– Similar rock types - marker beds, coal
seams, rare minerals, odd color
Correlation
• Within sedimentary layers there are
often the remains of small animals
(fossils)
• Fossils are quite useful for correlating
between two sections that are not
laterally continuous
10
Ammonite Fossils
Petrified Wood
Correlation
• Fossils represent living creatures that
have evolved through time, so when we
find a fossil of the same type in two
different areas, we are pretty sure that
the rocks are about the same age
11
Correlating beds using
index fossils
Using Fossils to Correlate Rocks
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Unconformity
A buried surface of erosion
13
Layers of rock are said to be
conformable when they are
found to have been deposited
essentially without interruption
Unconformity represents missing
time in the geologic record
Sedimentation of Beds A-D
Beneath the Sea
14
Uplift and Exposure of D to
Erosion
Continued Erosion
Removes D and Exposes C
to Erosion
15
Subsidence and
Sedimentation of E over C
Unconformity:
a buried surface of erosion
Formation of a Disconformity
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First type of unconformity
South rim of the Grand Canyon
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South rim of the Grand Canyon
250 million years old
Paleozoic Strata
550 million years old
1.7 billion years old
Precambrian
South rim of the Grand Canyon
250 million years old
550 million years old
1.7 billion years old
Nonconformity
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Nonconformity in the Grand Canyon
Nonconformity in the Grand Canyon
Tapeats Sandstone
(~550 million years old)
Vishnu Schist
(~1700 million years old)
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Second type of unconformity
Siccar Point, Scotland
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Siccar Point, Scotland
Buried and tilted erosional surface
Sedimentation of Beds A-D
Beneath the Sea
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Deformation and Erosion
During Mountain Building
Erosional Surface Cuts
Across Deformed Rocks
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Subsidence and Subsequent
Deposition Buries Erosional Surface
Angular
Unconformity
Formation of an
Angular Unconformity
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Third type of unconformity
Principle of Cross-Cutting
Relationships
• A rock unit must always be older than
any feature that cuts or disrupts it
– If a rock unit is cut by a fracture
• The rock itself is older than the fracture that
cuts across it
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Cross-cutting Relationships
Relative Geologic Dating
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The Geologic time scale
• Divisions in the worldwide
stratigraphic column based on
variations in preserved fossils
• Built using a combination of
stratigraphic relationships, crosscutting relationships, and absolute
(isotopic) ages
The Geologic
Time Scale
26
Absolute geochronology
• Add numbers to the
stratigraphic column based on
fossils.
• Based on the regular
radioactive decay of some
chemical elements.
Isotopes
Different forms of the same
element containing the same
number of protons, but varying
numbers of neutrons.
i.e.:
i.e.
235U, 238U
87Sr, 86Sr
14C, 12C
27
Naturally Occurring
Isotopes of Carbon
Half-life
The half-life of a radioactive
isotope is defined as the time
required for half of it to decay.
28
Proportion of
Parent Atoms
Remaining as a
Function of
Time
Isotopic dating
• Radioactive elements (parents) decay to
nonradioactive (stable) elements
(daughters).
• The rate at which this decay occurs is
constant and known.
• Therefore, if we know the rate of decay
and the amount present of parent and
daughter, we can calculate how long this
reaction has been proceeding.
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Radioactivity and Absolute Time
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31
Radioactive
Decay of
Rubidium to
Strontium
by β emission
Production and Decay
of Radiocarbon
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Production and Decay
of Radiocarbon
Major Radioactive Elements Used
in Isotopic Dating
33
Geologically Useful Decay Schemes
Parent
235U
Daughter
207Pb
Half-life (years)
4.5 x 109
238U
206Pb
0.71 x 109
40K
40Ar
1.25 x 109
87Rb
87Sr
47 x 109
14C
14N
5730
Radiometric Dating
• We can calculate geologic age if
– The half-life of a radioactive isotope is
known
– The parent/daughter ratio can be
measured
– There is no loss of an isotopes from the
system
• e.g., 222Rn is an intermediate daughter product
in the 238U decay series to 208Pb
34
Oldest rocks and minerals on Earth
Narryer Gneiss, Western Australia
• Zircons in a metamorphosed sandstone dated
at 4.35 to 4.40 Ga
Acasta Gneiss, Northwestern Canada and the
Nuvvuagittuq greenstone belt, Northern
Quebec
• Rocks dated at 3.80 to 4.28 Ga
Several other regions dated at 3.8 Ga by various
methods including Minnesota, Wyoming,
Greenland, South Africa, and Antarctica.
Age of the Earth
Although the oldest rocks found on Earth
are 4.4 Ga, we believe that the age of the
Earth is approximately 4.6 Ga. All rocks
of the age 4.6 to 4.4 Ga have been
destroyed (the rock cycle) or are
presently covered by younger rocks.
35
Age of the Earth
This is based on the age of rocks brought
back from the Moon (4.4 Ga), and
meteorites (4.6 Ga), that are thought to
be good representatives of the early solar
system.
These data suggest that the present
chemical composition of the crust must
have evolved for more than 4.5 Ga.
The geologic
timescale
and absolute ages
Isotopic dating of intebedded
volcanic rocks allows
assignment of an absolute age
for fossil transitions
36
The big assumption
The half-lives of radioactive
isotopes are the same as they
were billions of years ago.
Test of the assumption
Meteorites and Moon rocks (that are
thought to have had a very simple
history since they formed), have been
dated by up to 10 independent isotopic
systems all of which have given the
same answer. However, scientists
continue to critically evaluate this data.
37
Bracketing ages
Radiometric
dates
provide
absolute
ages to the
Geologic
Column
38
Earth’s
Magnetic
Field
Magnetostratigraphy
• Technique that works best in volcanic
rocks
• Time scale based on polarity reversal of
Earth's magnetic field
• Major problem is that Earth's magnetic
field has been constant for the past
700,000 yrs (no reversals), so this
doesn't work for very young rocks
39
Magnetization
of
Magnetite
Lavas
record
magnetic
reversals
40
Magnetic
reversals
over the
past 20
million
years
The Geologic time scale
• Divisions in the worldwide
stratigraphic column based on
variations in preserved fossils
• Built using a combination of
stratigraphic relationships, crosscutting relationships, and absolute
(isotopic) ages
41
The
Geologic
Column
and
Time Scale
42