Download Chapter 24 Geologic Time

Chapter 24
Geologic Time
Physical Science II
Module 3, Part 4
Geologic Time
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Geologists study the rocks/minerals, the structures, and
the processes that occur on Earth today.
In addition, geologists look back and attempt to interpret
the history of the Earth over geologic time.
The Earth is very old, about 4.6 billion. (4560 million)
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Humans have only been around during the last 2 million years.
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Intro
Geologic Time

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In this chapter we will discuss geologic time and how
geologists are able to measure it.
Our discussion of geologic time should serve to:

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Heighten our sense of responsibility as present-day custodians of
the Earth;
Show the enormous complexity of the processes that have
resulted in today’s biota.
We will discuss fossils, relative geologic time,
radiometric dating, absolute geologic time, and the
geologic time scale.
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Intro
Fossils
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Fossil – any remnant or indication of past life that is
preserved in rock
Paleontology is the study of fossils.
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The study of fossils is of great interest to both geologists and
biologists.
Paleontologists combine present-day biologic
information with ancient fossil and rock data to make
an interpretation of past events and environments.
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Section 24.1
Fossil Preservation
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Fossils are preserved in rocks in a number of ways.
In general, the three most important factors that lead to
good preservation include: quick burial, lack of oxygen,
and the presence of hard material that can be preserved.
Under extremely rare circumstances, the soft parts of
organisms may be preserved in some manner.
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Section 24.1
Fossil Preservation – Original Remains
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Ancient insects and other organisms occasionally became
encased in sticky tree resin.
The resin hardens into amber and original organism is
perfectly preserved.
Intact wooly mammoths have been recovered in Alaska
and Siberia, encased in ice.
Shark’s teeth and marine shells may also be found in
original condition.
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Section 24.1
Fossil Preservation – Replaced Remains

Fossils are more commonly found composed of
replacement minerals.
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
Silica (SiO2), Calcite (CaCO3), and Pyrite (FeS2) are common
replacement minerals.
The hard parts (bones, shell, etc) of ancient organisms is
slowly replace by the circulation of mineralized
groundwaters after death/burial.
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Mode of Fossil
Preservation

Replaced Remains –
Dinosaur bones are
commonly composed
of silica. (SiO2)
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Section 24.1
Fossil Preservation – Carbonization
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Carbonization will occur when plant remains decay under
conditions of very low oxygen or anaerobic conditions.
Most elements except carbon are driven off as the plant
material decays, leaving behind a carbon residue.
In many cases the carbon residue will retain many of the
features of the original plant.
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Section 24.1
Fossil Preservation – Molds and Casts
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In many cases the entire embedded shell, bone, or
piece of wood is completely dissolved, leaving behind
a hollow void – a mold.
New mineral or sediment material may later fill the
mold, creating a cast of the original fossil.
Molds and casts only show the shape and size of the
original organism.

Internal details of the organism is not preserved.
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Section 24.1
Modes of Fossil Preservation
Molds and casts of marine organisms
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Section 24.1
Fossil Preservation – Trace Fossils
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Trace fossil – any type of imprint or trail made by the
movement of an ancient animal
Ichnology is somewhat broader and includes the study of
plant and animal traces.
Trace fossils include tracks, burrows, borings, and tooth
marks.
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Section 24.1
Modes of Fossil Preservation
Trace Fossils – Fossilized Burrows
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Section 24.1
Fossil Evidence of Life
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Fossilized blue-green algae is the earliest fossilized
remains of life on Earth – 3.5 billion years ago!
As time moved forward the fossil record indicates
that life became more complex.
Fossils serve as exceptional indicators of past
environments.

A rock layer containing fossil coral indicates that it was deposited
in a shallow, warm sea.
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Section 24.1
Microfossils
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Just as life today occurs in all sizes and shapes, ancient
life did also.
Some rocks contain countless microfossils, so small
that they can only be studied with the aid of powerful
microscopes.
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Microfossils are particularly useful when drilling deep wells.
Due to their small size the entire fossil can be “collected” and
studied in the drill cuttings.
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Section 24.1
Relative Geologic Time
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Relative geologic time is determined by placing the
sequence of rocks and geologic events into sequential
order without knowing their actual dates.
Several common sense principles are used to help
determine the relative ages of the rocks and relative
sequence of geologic events:
The principle of superposition
The principle of cross-cutting relationships
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Section 24.2
Principle of Superposition
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Principle of superposition – in a sequence of
undisturbed sedimentary rocks, lavas, or ash the
oldest layer is on the bottom with each ascending
layer progressively younger
In other words, the bottom layer was deposited first
and is therefore the oldest layer; the top layer was
deposited last and is therefore the youngest layer.
If the layers have been disturbed (faulted or folded)
this must be taken into account.
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Section 24.2
Principle of Cross-Cutting Relationships
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Principle of cross-cutting relationships – an igneous
rock is younger than the rock layers that it has
intruded
This principle also applies to faults and folds, where
the fault or fold is younger than any rocks that are
affected.
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Section 24.2
Principle of Cross-Cutting Relationships

The igneous dike
is younger than
the layers that it
cuts across and
the fault is
younger than the
dike, since it cuts
across the dike.
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Section 24.2
Unconformities
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Unconformities represent gaps or breaks in the geologic
rock record.
Nowhere on Earth is the rock record for all geologic
time complete.
In any given area, there are missing layers due to nondeposition or due to erosion of some of the layers.
The amount of missing geologic time represented by an
unconformity is usually very difficult to determine.
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Section 24.2
Applying Principles of Relative Dating
An Example
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Using the principles of
relative dating, analyze
the figure below and
put the rocks marked 1
through 5 in order from
youngest to oldest.
Note that rock layer 5
is above rock layer 4,
rock layer 4 is above
rock layer 3, and rock
layer 3 is above rock 1.
Also notice that rock 2
cuts across rock 1.
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Section 24.2
Applying Principles of Relative Dating
An Example (cont.)
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The principle of
superposition indicates
that rock layer 5 is the
youngest, followed by rock
layers 4, then 3, then 1.
The principle of crosscutting relationships
indicate that rock 2 is
younger than rock 1.
Therefore the correct
order from oldest to
youngest is 1, 2, 3, 4, 5.
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Section 24.2
Finding an Unconformity
Confidence Exercise
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Using the figure below,
find the unconformity and
determine when
(relatively) it must have
been formed.
Note that the top of rock
2 is even with rock 1.
This indicates that both
rocks 1 & 2 underwent
erosion together.
Rock layer 3 was
deposited after this
episode of erosion.
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Section 24.2
Finding an Unconformity
Confidence Exercise (cont.)
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The time of erosion
occurred after rocks 1
and 2 were present but
before rock layer 3 was
deposited.
Thus the unconformity
was formed before rock
layer 3 and after rocks
1 and 2.
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Section 24.2
Correlation
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After geologists determine the relative ages of the
rocks at several separate localities they attempt to
match the layers by age.
Correlation is the process of determining age
equivalence between different localities.
For example, if the rock at location A is known and
the rock in location B is correlated to A, then the age
of the rock at B is the same as the age of the rock at
A.
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Section 24.2
Index Fossils
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Index fossils – fossils that are wide-spread in
distribution, easily identified, and limited to a
particular time segment of the Earth’s history
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These fossils can be of major assistance during the process
of correlation.
Once a particular index fossil has been thoroughly
established, geologists immediately know the age of
any rocks containing this index fossil anywhere in the
world.
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Section 24.2
Using the Process of Correlation
An Example

Four fossils, labeled A
through D, are shown in
the figure below, along
with their time ranges.
a)
b)
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Which fossil would be the
most useful as an index
fossil?
If a rock layer from a
certain locality contains
both fossils C and D, what
is the age of the rock?
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Section 24.2
Using the Process of Correlation
An Example (solution)
a)
b)
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Fossil A would be the
best index fossil due to
the narrow range of time
that it existed.
A rock that contains
both fossils C and D
must be Silurian in age.
Only during the Silurian
did both organisms live.
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Section 24.2
Using the Process of Correlation
Confidence Exercise
a)
b)
c)
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Primitia would be the
least use as an index
fossil. It lived in a wide
range of time.
The presence of Phacops
indicates that a rock is
either Silurian or
Devonian in age.
Phacops is a trilobite.
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Section 24.2
Relative Geologic Time Scale
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
As geologists all over the world worked to correlate
rocks over large areas, the relative ages of most rocks on
the Earths surface have been determined.
Using fossils and principles of relative dating techniques
geologists have established a relative time scale for the
Earth’s geologic history.
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Section 24.2
Relative Geologic Time Scale
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Eon – the largest unit of geologic time
The Phanerozoic Eon is the one we live in.
The time before the Phanerozoic Eon is
known as Precambrian time.
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Section 24.2
Relative Geologic Time Scale
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Eons are divided into eras.
There are three eras contained within the
Phanerozoic Eon:
Paleozoic Era – the oldest and “age of
ancient life”
Mesozoic Era – the “age of reptiles”
Cenozoic Era – the youngest and “age of
mammals”
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Section 24.2
Relative Geologic Time Scale
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Each era, in turn, is divided into several
smaller units of time called periods.
The Paleozoic is split into seven periods.
The Mesozoic is split into three periods.
The Cenozoic is split into two periods.
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Section 24.2
Measuring Absolute Geologic Time
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The development of the relative geologic time
scale was a significant achievement and provided
geologists with an excellent tool for the
interpretation of rocks.
In addition to knowing the order of geologic
events, geologist also wanted to know how long
ago these events occurred.
Geologists wanted a tool that would enable them
to know absolute ages.
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Section 24.3
Measuring Absolute Geologic Time
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
The need to determine absolute time became more
crucial as James Hutton’s concept of uniformitarianism
and Darwin’s theory of organic evolution became widely
accepted.
Both geologists and biologists concluded that these
processes were very slow and indicated that the age of
the Earth was much older than previously thought.
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Section 24.3
Radiometric Dating
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
Radiometric dating – the determination of age by the
measurement of the rate of decay of radionuclides in
the rocks
Recall that an atomic nuclei is said to be radioactive
when it will naturally decay.
The product of decay is generally called the daughter
nuclei or daughter product.
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Daughter products may themselves be stable or radioactive
(unstable.)
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Section 24.3
Radiometric Dating – Half-life
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
Half-life – the length of time taken for half of the
radionuclide in a sample to decay
This rate of decay has been found to always be
constant.
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

Unaffected by temperature, pressure, and chemical environment
The older the rock the less parent and the more
daughter product is present.
Different radioactive parents may have drastically
different half-lives.
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Section 24.3
Half-Life and Radiometric Dating
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As the parent nuclide decays the proportion of the parent decreases
and the proportion of the daughter increases.
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Section 24.3
Rock “Clocks” – Condition #1
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Radioactive decay can serve as a “clock” for dating
rocks, if the following conditions are met
Over the lifetime of the rock, no daughter or parent
has been added or subtracted.
This condition requires that there has been no
contamination of the rock.
If either parent or daughter nuclides are added or
subtracted by metamorphism or fluid movement, the
date obtained is not valid.
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Section 24.3
Rock “Clocks” – Condition #2
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The age of the rock is reasonably close to the halflife of the parent radionuclide.
If too many half-lives transpire it may become
impossible to measure the amount of the
remaining parent nuclide.
If only a small portion of one half-life transpires
then it may be impossible to measure the amount
daughter product present.
In either case, a valid date cannot be obtained.
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Section 24.3
Rock “Clocks” - Condition #3
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No daughter product was present when the rock
initially formed.
If daughter product was present when the rock
formed, later analysis of the rock will result in an
inaccurate parent to daughter ratio.
Sometimes it may be possible to determine the
amount of daughter nuclide initially present.
In order to use radiometric dating techniques at all,
the rocks must actually contain the appropriate
radionuclides.
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Section 24.3
Condition #3 – Sometimes a Problem
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In the case of uranium-lead dating, there are many
different isotopes of lead.
For example, we know that primordial lead consists
of 1.4% lead-204, 24.1% lead-206, 22.1% lead-207, and
52.4% lead-208.
We also know that lead-204 is never created from
radioactive decay.
Therefore if any lead-204 is present we know that the
other three lead isotopes are also present and we
know their ratios.
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Section 24.3
Primordial and Radiogenic Lead

Since lead-204 is present, we know how much of the other
isotopes are primordial and how much are radiogenic.
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Section 24.3
Major Radionuclides Used for
Radiometric Dating


Note that since the half-lives vary the range of ages also varies.
Not all rocks can be radiometrically dated, only those with the appropriate mineral
present.
24 | 44
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Section 24.3
Major Radionuclides Used for
Radiometric Dating


Note that since the half-lives vary the range of ages also varies.
Not all rocks can be radiometrically dated, only those with the
appropriate mineral present.
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Section 24.3
Potassium-Argon Dating
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
Potassium (K) is one of the most abundant elements
in minerals of the Earth’s crust.
A very small percentage (0.012%) is the radioactive
isotope, potassium-40.


Potassium-40 has a half-life of 1.25 billion years and decays to
Argon-40.
K-Ar dating can be used in a variety of minerals
including orthoclase, muscovite, biotite, hornblende,
and others.
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Section 24.3
Potassium-Argon Dating
Limitations
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Recall that Ar is an inert gas.
When K-40 decays to Ar-40 the minerals are
susceptible to Ar-40 leakage, especially if the mineral
has been heated.
If some of Ar-40 (the daughter product) leaks out, the
resulting date will not be valid.
K-Ar dating may reveal the last time the rock was
heated and not the time of original crystalliztion of
the mineral.
24 | 47
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Section 24.3
Rubidium-Strontium Dating
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
Rubidium-87 is a common constituent of many
crustal minerals.
Rubidium-87 has a half-life of 49 billion years and
decays to Strontium-87.
A significant portion of Sr-87 is primordial and
therefore corrections are necessary.
Rb-87 is found in many of the same minerals as K-40.
Therefore Rb-Sr dating is commonly used as a check
against K-Ar age determinations.
24 | 48
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Section 24.3
Using Radiometric Dating
An Example
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The ratio of U-235 to its daughter, Pb-207, is 1 to 3 in
a certain rock. That is, only 25% of the original U-235
remains. (The half-life of U-235 is 704 x 106 years.)
How old is the rock?
To decay from 100% to 25% takes 2 half-lives.
100%  50%  25%
(2) x (704 x 106 years) = 1408 x 106 years
or 1.41 billion years = age of the rock
24 | 49
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Section 24.3
Using Radiometric Dating
Confidence Exercise
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
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The ratio of U-235 to its daughter, Pb-207, is 1 to 7 in
a certain rock. That is, only 12.5% of the original U235 remains. (The half-life of U-235 is 704 x 106
years.) How old is the rock?
To decay from 100% to 12.5% takes 3 half-lives.
100%  50%  25%  12.5%
(3) x (704 x 106 years) = 2112 x 106 years
or 2.1 billion years = age of the rock
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Section 24.3
Carbon Dating
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Developed in 1950 by American, Willard Libby
Carbon-14 (14C) dating is the only radiometric dating
technique that can be used to date once-living
organisms.
14C is a radionuclide with a half-life of 5730 years.
The age of an ancient organic remain is measured by
comparing the amount of 14C in the ancient sample
compared to the amount of 14C in modern organic
matter.
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Section 24.3
Carbon Dating
 14C
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


24 | 52
is a natural product
formed in the atmosphere.
About one in a trillion C
atoms in plants is 14C.
14C is incorporated into all
living organisms.
Living matter has an activity
of about 15.3
counts/minute/gram C.
At death the 14C present
begins to decay.
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Section 24.3
Carbon Dating – Modern Methods
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
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In the newest carbon dating techniques, the amounts
of both 14C and 12C are measured.
The ratio of these two isotopes in the ancient sample
is compared to the ratio in living matter.
Using this method only very small samples are
needed and specimens as old as 75,000 years can be
accurately dated.
Beyond 75,000 years, the amount of 14C still not
decaying is too small to measure.
24 | 53
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Section 24.3
Carbon Dating - Limitations



Carbon dating techniques assumes that the amount
of 14C in the atmosphere (and therefore in living
organisms) has been constant for the past 75,000
years.
We now know that the amount of 14C in the
atmosphere has varied by (+) or (-) 5%.
These variations in 14C levels have been due to
changes in solar activity and changes in the Earth’s
magnetic field.
24 | 54
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Section 24.3
Carbon Dating - Limitations



These slight variations in 14C abundance have been
corrected by careful analyses of California’s 5,000
year-old bristlecone pines.
An extremely accurate calibration curve has been
developed for 14C dates back to about 5000 B.C.
Carbon dating is widely used in archaeology, and has
been used to date bones and other organic remains,
charcoal from fires, beams in pyramids, the Dead Sea
Scrolls, and the Shroud of Turin.
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Section 24.3
Lord Kelvin & the Age of the Earth
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

In the middle to late 1800’s Lord Kelvin was the most
distinguished physicist in the world.
He attempted to determine the Earth’s age by using
the rate of heat loss from its interior.
As a basic assumption, Lord Kelvin considered that
the entire Earth began in a molten state and slowly
became solid as it lost this residual heat from the still
hot interior.

He was trying to determine the elapsed time from the
molten “beginning” to the present day.
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Section 24.4
Lord Kelvin & the Age of the Earth



Lord Kelvin’s calculations and measurements led him
to conclude that the Earth was between 20 and 40
million years old.
Many of the geologists and biologists of the time
thought that the slow pace of geologic and organic
evolution indicated a much older age.
Due to his considerable prestige and actual data, Lord
Kelvin’s conclusion was difficult to argue against.
24 | 57
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Section 24.4
Radioactivity & the Age of the Earth



Radioactivity was discovered in 1896 and it soon
became apparent that all the heat in the Earth’s
interior was not residual.
This discovery invalidated Lord Kelvin’s basic
assumption and it became evident that his estimate of
the Earth’s age was in error.
We now know that most of the Earth’s interior heat
is due to radioactive decay.

With the addition of radioactive-generated heat, there is
much more heat to account for and much more time
needed
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Section 24.4
Age of the Earth


After many years of study, geologists are now confident
that the Earth’s age is 4.56 billion years. (4560 million
years)
There are three main lines of evidence that support this
age for the Earth:



The age of Earth rocks
The age of meteorites
The age of the Moon rocks
24 | 59
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Section 24.4
Age of Earth Rocks


With the advent of radiometric dating techniques, it
is possible to put absolute dates on many igneous and
metamorphic rocks,
To date, the oldest rocks yet analyzed are zircon
crystals from northwest Australia.


Dated at approximately 4.3-4.4 billion years
Other exceedingly old rocks on Earth include 4.0billion-year-old rocks in Canada, 3.8-billion-year-old
granites in Greenland, and 3.4-billion-year-old granites
in South Africa.
24 | 60
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Section 24.4
Age of Meteorites
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
Meteorites from our solar system are thought to have
formed at the same time as Earth.
These meteorites have been reliably dated at 4.56 billion
years using both U-Pb and Rb-Sr methods.
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Section 24.4
Age of Moon Rocks


A number of Moon rocks have been meticulously
analyzed.
Rocks from the lunar highlands are the oldest, yielding an
age of 4.55 billion years.
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Section 24.4
Age of the Earth




Several different lines of evidence strongly suggest
that the planets, the moons, and the asteroids of our
solar system were all formed approximately 4.56
billion years ago.
It is doubtful that rocks on Earth will ever be found
that are fully 4.56 billion years old.
In its early history the Earth’s surface was likely
molten for several hundred million years.
Plate tectonics, weathering, metamorphism, and other
processes have destroyed many ancient rocks.
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Section 24.4
Geologists and Scripture

Some people argue for a very young Earth.




In the range of 5,000 to 10,000 years, according to their
scriptural interpretation
Geologists are consistently drawn into the “young
Earth versus old Earth” debate.
Ultimately, it really doesn’t matter to most geologists
how old the Earth is. They simply want to reliably
know how old it is.
At the present time the physical and biological
evidence on Earth points overwhelmingly to a “long
Earth” (4.56 billion years) perspective.
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Section 24.4
Geologic Time Scale


The modern geologic time scale has been constructed
using both relative geologic time and absolute geologic
time.
Most of the accepted dates are estimated values and are
subject to minor changes as new data is acquired.
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Section 24.5
Geologic Time
Scale

Time is given in millions
of years before present,
along with major
geologic and biologic
events
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Section 24.5
Construction of the Geologic Time Scale

Sedimentary rocks have been used primarily to
establish the relative time scale.



Sedimentary rocks are rarely suitable for radiometric dating
since they are composed of erosional debris.
Igneous rocks have been used primarily to attain
radiometric absolute dates.
In some cases metamorphic rocks have been used to
attain radiometric absolute dates for deformational
(marking intense heating and recrystallization) events.
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Section 24.5
Dating Sedimentary Rocks



In many cases the only way to attain an absolute date
on sedimentary rocks is to relate the sedimentary
rocks to igneous rocks.
This process is called ‘bracketing.’
The absolute dates that are obtainable from several
igneous rocks serve to bracket the minimum and
maximum age of the sedimentary layers of interest.
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Section 24.5
Using Igneous Rocks to Date
Sedimentary Rocks - An Example




In the figure below, the two
igneous dikes have been
dated: X = 400 My and Y =
350 My. What can be said
about the age of the
Devonian stratum labeled
B?
Igneous dike X intruded
Silurian strata.
Part of dike X and the
Silurian strata were eroded.
Strata B was deposited
later, therefore it is younger
than 400 My.
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Section 24.5
Using Igneous Rocks to Date
Sedimentary Rocks - An Example (cont.)



Strata B must be older
than the age of dike Y.
(350 My)
The Devonian strata B
is between 350 – 400
My.
We have therefore
‘bracketed’ the age of
the Devonian stratum.
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Section 24.5
Using Igneous Rocks to Date
Sedimentary Rocks





What can be said about the
absolute age of the sedimentary
rock layer A from the
Mississippian period?
Part of dike Y was eroded
before layer A was deposited.
Thus, rock layer A was
deposited after Y. (350 My)
From the Geologic Time Scale
we know that the Mississippian
Period extended from 360-320
My.
Layer A is younger than 350 and
older than 320 My.
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Section 24.5
Highlights of Geologic Time



The beginning of the Archeon eon, at 4000 m.y.a., marks
the date of the oldest Earth rocks.
The Protoerozoic eon began 2500 m.y.a. and coincides
with the formation of the North American continental
core.
Phanerozoic eon began at about 545 m.y.a.
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Section 24.5
Proterozoic Supercontinent – Rhodinia

Rhodinia formed in the late Proterozoic.
It broke apart during the early Paleozoic Era.
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Section 24.5
Highlights of Geologic Time



The Phanerozoic eon is divided into three eras: Paleozoic,
Mesozoic, and Cenozoic.
Hard-shelled marine invertebrates first became abundant
at the start of the Paleozoic.
Life began to proliferate during the Cambrian Period and
continued throughout the Paleozoic.
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Section 24.5
Highlights of Geologic Time




The Paleozoic came to an abrupt and possibly
catastrophic end 245 m.y.a.
The end of the Paleozoic is sometimes called the “Great
Dying.”
90% of the ocean species and 70% of the land species
became extinct.
Perhaps a huge asteroid hit the Earth.
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Section 24.5
Highlights of Geologic Time




During the Mesozoic era the global climates were mild.
Corals grew in what is now Europe.
Dinosaurs were common in the western U.S. and Canada,
as well as many other areas.
The Mesozoic came to a catastrophic end at about 65
m.y.a.
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Section 24.5
Highlights of Geologic Time


About 70% of the world’s plant and animal species,
including the dinosaurs became extinct.
Evidence strongly indicates that an asteroid struck the
Earth on the NW side of the Yucatan peninsula of Mexico.
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Section 24.5
Highlights of Geologic Time




The Cenozoic era began at 65 m.y.a. and is commonly
known as the ‘age of mammals.’
Our present period is called the Quaternary.
It began about 2 m.y.a. with the first appearance of the
genus Homo.
The Cenozoic is subdivided into epochs. The last two are
of special interest.
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Section 24.5
Highlights of Geologic Time


The Pleistocene epoch is also known as the ‘ice age’ and
is marked by significant worldwide glaciation.
Our present epoch, the Holocene began about 10,000
years ago when the glaciers retreated from Europe and
North America.
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Section 24.5
Geologic Time in Perspective – A Timeline


Each kilometer represents about 1 million years.
Note how long geologic time is compared to what we call
‘recorded history.’
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Section 24.5