GEOLOGIC TIME
HOW TO TELL TIME
GEOLOGICALLY
GEOLOGIC TIME
What is the concept of geologic time?
What are the ways we can constrain the time
of Earth events?
How do geologists work out the timing of
these events.
What are the basic concepts that are applied?
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2
Geology at a Glance
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3
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4
GEOLOGIC TIME
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GEOLOGIC TIME
The earth has a long and complex history that
has involved many events.
Geologists must be able to read the geologic
record in order to unravel earth history.
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6
Magnitude of Earth History
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7
GEOLOGIC TIME
You may not be a geologist, YET, but it is still
important and interesting to be able to
understand the earth record that you observe
every day.
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8
GEOLOGIC TIME
There are two basic ways to unravel a
geologic record:
1. Relative sequence of events.
2. Absolute ages of events.
What is the difference? (+4)
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GEOLOGIC TIME
Before about 1950, geologist relied on relative
methods of dating and correlation of units to
develop a geologic time scale.
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GEOLOGIC TIME
Relative methods involved understanding the
geologic events in a given area and recording
the observed sequence of events in the rock
record.
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GEOLOGIC TIME
Over time, geologists attempted to correlate
events and geologic units from different parts
of a region to construct a history.
This was then extend over larger regions to
attempt to establish a geologic time scale that
could be applied over the entire Earth.
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GEOLOGIC TIME
In the nineteenth century, geologists began to
assemble a geologic column, which is a
composite column containing, in chronological
order, the succession of known strata, fitted
together on the basis of their fossils or other
evidence of relative age.
The corresponding column of time is the
geologic time scale (refer to table 8.2 in the
textbook).
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13
Geologic Time Scale
Contrasting several dating techniques
chronicling Earth’s history to produce a
geologic Time Scale.
• Geologic Time Scale is divided into Eons,
Eras, Periods, and Epochs.
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Eons
An eon is the largest interval into which geologic time is
divided.
There are four eons:
The Hadean Eon is the oldest: some of the samples
brought back from the moon were formed during the
Hadean Eon.
The Archean Eon follows the Hadean: Archean rocks,
which contain primitive microscopic life forms are the
oldest rocks we know of on the Earth.
The Proterozoic Eon follows the Archean.
The Phanerozoic Eon is the most recent of the four
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16
eons.
Eras
• Each of the eons is subdivided into shorter
time units called eras.
• The Phanerozoic Eon is divided into the:
o Paleozoic era: (ancient life); mostly marine invertebrates
o Mesozoic era: (middle life) dominated by reptiles
o Cenozoic era: (recent life) dominated by mammals
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Eras
• In the Paleozoic Era, early land plants appeared,
expanded and evolved. Developing animal life
included
marine
invertebrates,
fishes,
amphibians,and reptiles.
• The Mesozoic Era saw the rise of the dinosaurs,
which became the dominant vertebrates on land.
Mammals first appeared during the Mesozoic Era
as did flowering plants.
• Mammals dominated the Cenozoic Era. Grasses
evolved during the Cenozoic Era, and became an
important food for grazing mammals.
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Periods
• The Eras of the Phanerozoic Eon are divided
into periods.
– The periods are defined on the basis of the
fossils contained in the equivalent rocks.
– The two Periods are the Quaternary Period and
the Tertiary Period
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Epochs
• Periods are further subdivided into epochs on the
basis of the fossil record.
• The Tertiary Period is divided into these epochs:
– Paleocene.
– Eocene.
– Oligocene.
• The Quaternary Period is divided into these
epochs:
– Holocene.
– Pleistocene.
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GEOLOGIC TIME
The
concept
that
most
geologic
processes happen very slowly was
proposed by James Hutton (1726-1797).
Hutton also proposed a very important
concept know as uniformitarianism.
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GEOLOGIC TIME
What
is
the
concept
uniformitarianism? (+3)
of
Why is it an important concept? (+2)
Is this concept really practical for us
to use? Explain! (+3)
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23
GEOLOGIC TIME
Modern view holds that processes that
operate today have shaped the Earth through
Geological Time, but rates may not have always
remained constant.
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24
Concepts of Relative Dating
Position in that sequence identifies relative age.
Basic principles:
•
Relative positions of layered rocks.
•
Relationships of rocks units.
•
Sequence of rock units.
•
Correlation of rock units and events from different
areas.
•
Relative-age Copyright
principles.
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Concepts of Relative Dating
Layered sedimentary or volcanic rocks contain
important clues about past environments at
and near Earth’s surface.
Their sequence and relative ages provide the
basis for reconstructing much of Earth’s
history. The study of strata is called
stratigraphy.
The use of layered volcanic and sedimentary
rocks is important for relative dating.
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26
Concepts of Relative Dating
Most sediment is laid down in the sea,
generally in relatively shallow waters, or by
streams on the land.
Ash from volcanic eruptions can also be
deposited on the surface.
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Concepts of Relative Dating
The fact that sediment and ash is deposited
in layers forms the basis for the principle
of original horizontality.
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Original Horizontality
What is the principle
horizontality? (+3)
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of
original
29
Original Horizontality
With this principle in mind, if you observe
layered rocks that are inclined or disrupted
in an outcrop, what can you infer? (+3)
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Original Horizontality
Describe a simple teaching method by which
you could illustrate this principle. (+3)
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31
Superposition
Another important concept for relative age
assignment is referred to as the principle
of superposition.
What is this principle? (+3)
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Superposition
Describe a simple teaching method by which
you could illustrate this principle. (+3)
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Superposition
Give me an example of an earth process
where the principle of superposition would
not apply in a layered sequence of rocks.
(+3)
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34
Lateral Continuity
What is the
Continuity? (+3)
principle
of
Lateral
Using snowfall as an example, explain why
lateral continuity must happen in layered
rocks. (+3)
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35
Principle of Inclusion for Sediments
When sediments are deposited the younger
units rest on top of then older units.
Sometimes fragments of one unit can be
entrained in another. This forms the basis
for the principle of inclusion.
Explain the basic idea of the principle of
inclusion (refer to page 206 in your
textbook). (+3)
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36
Principle of Inclusion for Intrusive
Rocks
When bodies of magma intrude into older
rocks they can also pull off pieces of the
rock and include them. These are called
xenoliths or foreign rocks. This allows us to
determine the relative ages of dikes, sills,
and other bodies of plutonic rock.
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Principle of Cross-Cutting
Relationship
A rock unit can be cut by another geologic
unit or feature.
This forms the basis of the principle of
cross cutting relationships.
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Principle of Cross-Cutting
Relationship
Describe two geologic processes that could
cause cross cutting in the geologic record.
Be specific and make certain your discussion
is concise and clear. (+6)
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Principle of Cross-Cutting
Relationship
Using the concept of cross cutting which
unit is cutting the other (dark unit or light
unit). (+2)
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40
Principle of Faunal Succession
Explain the principle of faunal succession.
(+3)
Why is this idea so important in working out
geologic history? (+ 3)
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41
Fossils
Fossils are a valuable tool for geologists
because they give us clues about past life
and environments preserved in a rock unit.
Fossils can also be used for rock correlation.
Index fossils make the best time markers.
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42
Fossils
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Index Fossils
What is an index fossil?
What are three criteria that are desired in an
index fossil to make it useful? (+4)
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44
Other Key Time Markers
A time marker in a geologic record is
especially useful if it is distinct, short in
duration, widespread, and allows accurate
correlation.
Besides index fossils, give me another example
of a geologic event or process that could
create this type of time marker. (+3)
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45
Unit Correlation
Geologists can use fossils, rock sequences, and
relative events to piece together the
geologic history of one area.
By comparing this record to other areas a
more extensive and regional record can be
developed.
This is done by correlating rock records and
looking for similarities in the different
records to connect them.
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46
Unit Correlation
A rock-stratigraphic unit is any distinctive stratum
that differs from the strata above and below.
The basis of rock stratigraphy is the formation.
A formation is a collection of similar strata that
are sufficiently different from adjacent groups
of strata so that on the basis of physical
properties they constitute a distinctive,
recognizable unit that can be used for geologic
mapping over a wide area.
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47
Unit Correlation
Each of the boundaries of a time-stratigraphic unit,
upper and lower, is uniformly the same age.
The primary time-stratigraphic unit is a system,
which is chosen to represent a time interval
sufficiently great so that such units can be used
all over the world.
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48
Figure 11.6
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49
Unit Correlation
The primary unit of geologic time is a geologic
period, which is the time during which a
geologic system accumulated.
Correlation
is
the
determination
of
equivalence in time-stratigraphic or rockstratigraphic units of the succession of
strata found in two or more different
places.
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50
Unit Correlation
Correlation involves two main tasks:
1. Determining the relative ages of units
exposed within a local area being studied
(identifying the same formation wherever
it crops out).
2. Establishing the ages of the local rock
units relative to a standard scale of
geologic time.
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Unit Correlation
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Unit Correlation
Describe a simple class room activity to
demonstrate the concept of correlation.
(+5)
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53
Gaps in the Geologic Record
No geologic
continuous.
record
is
There are gaps created in
the record by erosion
or lack of geologic
events in an area for a
given period of time.
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Gaps in the Geologic Record
The many unconformities exposed in rocks of Earth’s
crust are evidence that former seafloors were
uplifted by tectonic forces and exposed to
erosion.
Preservation of a surface of erosion occurs when
later tectonic forces depress the surface.
The surface, in turn, becomes a site of deposition
of sediment.
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Gaps in the Geologic Record
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Gaps in the Geologic Record
An unconformity is a substantial break or gap in a
stratigraphic sequence.
Three important kinds of unconformities are found in
sedimentary rocks:
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57
Gaps in the Geologic Record
1. Disconformity
2. Angular Unconformity
3. Non Conformity
Describe each of these unconformities and
discuss what they tell you about the geologic
history of a rock sequence. (+12)
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58
Gaps in the Geologic Record
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Gaps in the Geologic Record
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Gaps in the Geologic Record
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Angular Unconformities and Nonconformities
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Gaps in the Geologic Record
What type of unconformity is shown in the
figure below? (+2)
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63
Relative Dating vs. Absolute Time
Geochronology is the study of time in
relation to earth’s existence:
• Relative Dating
Determines how old a rock is in relation to
its surrounding
• Numerical Dating
Determines actual age in years
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Relative Dating vs. Absolute Time
If you examine a geologic time scale you will
notice that there are numbers in billions,
millions, or thousands of years on the chart.
This did not come from relative dating.
These numbers come from techniques that
give absolute ages of time.
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65
Relative Dating vs. Absolute Time
• Early attempts to measure geologic time numerically were
inaccurate.
– Edmund Halley suggested, in 1715, that sea salt might be
used to date the ocean.
– John Joly finally made the necessary measurements and
calculations in 1889. His determination of the ocean’s
age, 90 million years, was not correct.
• Salts are added both by erosion and by submarine
volcanism, but salts are also removed by solution.
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Relative Dating vs. Absolute Time
• Lord Kelvin, a physicist, attempted to calculate the
time Earth has been a solid body.
• By measuring the thermal properties of rock and
estimating the present temperature of Earth’s
interior, he calculated the time for the Earth to
cool to its present state.
– His estimate of 100 million years is incorrect.
– The Earth’s interior is cooling so slowly that it
has a nearly constant temperature over periods
as long as hundreds of millions of years.
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67
Radiometric Dating and the Geologic Column
• Through various methods of radiometric dating,
geologists have determined the dates of
solidification of many bodies of igneous rock.
• “Moon dust” brought back by astronauts, is 4.55
billion years old.
• The Earth was formed approximately 4.55 billion
years ago.
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68
Figure 11.15
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69
Continent-continent collision, Mountain building, and Mountain
unbuilding.
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70
Geochronologic Methods
Radiometric age methods:
•
Absolute Age Methods (e.g., U-Pb): based on radioactive decay.
•
Fission Track: High speed particles emitted during radiation may pass
through crystal leaving ‘tears’ within the crystal- the older the rock,
the more fission tracks.
•
Dendrochronology (Tree-Ring dating): Annual growth rings.
•
Varve- deposited layers of lake-bottom: Paired layers of sediments.
•
Lichenometry: Lichens grow at a fairly constant rate.
•
Cosmogenic isotopes: Used in dating land features.
•
Magnetic Polarity Time Scales: based on magnetization in rocks.
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Radioactive Age Determinations
• In 1896, the discovery of radioactivity provided
the needed method to measure the age of the
Earth accurately.
• Different kinds of atoms of an element that
contain different numbers of neutrons are called
isotopes.
– Most Isotopes of the chemical elements found in Earth
are generally stable and not subject to change.
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73
Radioactive Age Determinations
• A few isotopes, such as 14Carbon, uranium, rubidium
potassium and samarium, are radioactive.
o Radioactivity arises because of instability within an
atomic nucleus.
o If the ratio of the number of neutrons (n) to the number
of protons (p) is too high or too low, the atomic nucleus
of a radioactive isotope will transform spontaneously to a
nucleus of a more stable isotope of a different chemical
element.
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Radioactive Age Determinations
• The process is called radioactive decay.
o An atomic nucleus undergoing radioactive decay is said
to be the parent.
o The product arising form radioactive decay is called a
daughter.
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75
Figure 8.19: Bracketing ages.
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Radioactive Decay
• Radioactive decay can happen in five ways:
1. Beta decay: emission of an electron from the nucleus.
2. Positron emission: emission of a particle with the same
mass as an electron but with a positive charge.
3. Electron capture: by capture into the nucleus of one of
the orbital electrons, a process that decreases the
number of protons in the nucleus by one.
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77
Radioactive Decay
4. Alpha decay: emission from the nucleus of a
heavy atomic particle consisting of two neutrons
and two protons called an α (alpha) particle.
5. Gamma ray emission: emission of γ rays (gamma
rays), which are very short-wavelength, highenergy electromagnetic rays.
• Gamma rays have no mass, so gamma ray emission
does not affect either the atomic number or the
mass number of an isotope.
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Figure 8.17: Unstable atomic nuclei decay (continued).
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80
Rates of Decay and the Half-Lives of Isotopes
• The rate at which radioactive decay occurs varies
among isotopes.
• Decay rates are unaffected by changes in the
chemical and physical environment.
• The decay rate of a given isotope is the same in the
mantle or in a sedimentary rock.
• In radioactive decay, the proportion—fraction or
percentage—of parent atoms that decay during each
unit of time is always the same.
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81
Figure 8.17: Unstable atomic nuclei decay.
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Figure 8.17: Unstable atomic nuclei decay (continued).
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83
Rates of Decay and the Half-Lives of Isotopes
• The rate of radioactive decay is measured in
terms of half-life, the amount of time
needed for the number of parent atoms to
be reduced by one half.
• At the end of each unit of time (half-life),
the number of parent atoms has decreased
by exactly one-half.
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84
Rates of Decay and the Half-Lives of Isotopes
COMPLETE CLASS EXERCISE
1. Obtain two types of colored beads.
2. Assume the RED beds are Parent atoms and Blue beads are
daughter atoms.
3. Assume that each half life of decay is 30 seconds.
4. Exchange beads every 30 seconds and tell me how many of
each of the two colors of beads you have.
5. How many half lives have occurred before you can no longer
exchange beads.
6. How much time has elapsed. (+6)
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85
Radioactive Decay
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86
Figure 11.13
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87
Using Radioactivity to Measure Time
Radioactivity in a mineral is like a clock.
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Using Radioactivity to Measure Time
The length of time this clock has been ticking
is the mineral’s radiometric age.
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89
Using Radioactivity to Measure Time
• Many natural radioactive isotopes can be
used for radiometric dating, but six
predominate in geologic studies:
o Two radioactive isotopes of uranium plus radioactive
isotopes of thorium, potassium, rubidium and carbon
are used.
o In practice, an isotope can be used for dating
samples that are no older than about six half-lives of
the isotope.
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90
Figure 8.18: Radioactive decay.
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91
Figure 8.20: Loss of daughter isotopes.
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92
Figure 8.20: Loss of daughter isotopes (continued).
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Figure 8.20: Loss of daughter isotopes (continued).
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94
Figure 8.20: Loss of daughter isotopes (continued).
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95
Radiogenic Isotope Methods
What types of rocks or earth materials
is radioactive isotope (not carbon
dating) age methods most useful? (+3)
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96
Radiogenic Isotope Methods
What are some important assumptions
we make when we use these methods?
(+3)
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97
Radiogenic Isotope Methods
What are the strengths of these
methods? (+3)
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98
Radiogenic Isotope Methods
What are the weaknesses of these
methods? (+3)
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99
Factors Affecting Isotope Dating Results
• Isotope dating is more useful for igneous rocks:
Clock is set when igneous rock crystallizes locking the
radioactive isotopes within its crystal lattice
• Rock/Mineral must be a closed system: Atoms of
parent and daughter are still present in rock/mineral being
dated
• Condition of parent Material: Fracture, weathering and
migrating ground water
• Age of Substance: Enough measurable daughter isotope,
use appropriate radioactive isotope
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100
Radiocarbon Dating
•
14C
is especially useful for dating geologically young
samples.
• The half-life of radiocarbon is short—5730 years—
by comparison with the half-lives of most isotopes
used for radiometric dating.
• Radiocarbon is continuously created in the
atmosphere through bombardment of 14C by
neutrons created by cosmic radiation.
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101
Figure 11.14
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102
Radiocarbon Dating
• Though some variations have been identified,
the proportion of 14C is nearly constant
throughout the atmosphere and biosphere.
• Living organisms have the same proportion of
14C In their bodies as exists in their
environment.
• No carbon is added after death, so by
measuring the radioactivity remaining in an
organic sample, we can calculate how many
half-lives ago the organism died.
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103
Radiocarbon Dating
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104
Figure B01
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105
Figure B02
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106
Magnetic Polarity Time Scale
• Certain rocks become permanent magnets as a
result of the way they form.
• Magnetite and certain other iron-bearing minerals
can become permanently magnetized.
• Above a certain temperature (called the Curie
point), the thermal agitation of atoms is such that
permanent magnetism is impossible.
• Below that temperature, however, the magnetic
fields of adjacent iron atoms reinforce each other.
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107
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108
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109
Magnetic Polarity Time Scale
• As solidified lava cools, the temperature will
drop below 580oC, the Curie point for
magnetite.
• When the temperature drops below the
Curie point, all the magnetite grains in the
rock become tiny permanent magnets with
the same polarity as Earth’s field.
• All lava formed at the same time records the
same magnetic polarity information.
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110
Figure 11.18
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111
Magnetic Polarity Time Scale
• The Earth’s polarity has shifted in the past. A period in
which polarity remains stable is called a magnetic chron.
• The four most recent chrons have been named for
scientists who made great contributions to studies
of magnetism. The four chrons below occurred
during the last 4.5 million years. From the most
recent to the oldest:
–
–
–
–
Brunhes.
Matuyama.
Gauss.
Gilbert.
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112
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113
Primordial Gasses
• Studies of volcanic gases provide other clues
to the age of the Earth.
– Three gases, 40Ar (daughter of 40K), 3He, and 36Ar
(both primordial gases trapped in Earth from the
solar nebula), are being released, but they are not
being recycled.
– Because they accumulate in the atmosphere, their
growing proportion can be used to estimate the age
of the Earth.
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114
Numerical Age
•
•
•
•
Isotope Dating relies on the rate of decay of
radioactive isotopes within a rock
Radioactive isotopes have nuclei that spontaneously
decay emitting or capturing a variety of subatomic
particles
Decaying radioactive isotope- parent isotopes decay
to form daughter isotopes
Half-life- is the time it takes for half the atoms of
parent isotope to decay
Some radioactive isotopes with daughter products
 U-238 => Pb-206; K-40 => Ar-40; C-14 => N-14
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Figure 8.4b: Sedimentary structures.
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Figure 8-h-1: An organism buried in sediment.
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Figure 8.10: Three types of unconformities.
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Figure 8.10: Three types of unconformities (continued).
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Figure 8.10: Three types of unconformities (continued).
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Figure 8-h-2: Hypothetical view of early earth.
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Figure 8.12: Grand canyon.
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Figure 8.13: Sedimentary rock sequences.
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Figure 8.16:
Hypothetical
landscape.
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Figure 8.24: Fisson tracks.
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Figure 8.25: Correlation of tree-ring sections.
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Figure 8.26: Origin of
lake varves.
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Figure 8.26: Origin of
lake varves (continued).
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Figure 8.27: Age of lichen.
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129
Figure 8.28:
Cosmogenic isotopes.
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Figure 8.31: Rocks
underlying hypothetical
landscape.
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Figure 8.32: The
geologic time scale.
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132
Figure 8-h 03: Extinctions graph.
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133
Figure 8-eoc-1: Hypothetical landscape.
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134