geologic time

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Transcript geologic time

Chapter 24
Geologic Time
Sections 24.1-24.5
Geologic Time
• 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)
– Humans have only been around during the
last 2 million years.
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Intro
24 | 2
Geologic Time
• In this chapter we will discuss geologic time
and how geologists are able to measure it.
• Our discussion of geologic time should serve
to:
– Heighten our sense of responsibility as presentday 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
24 | 3
Fossils
• Fossil – any remnant or indication of
past life that is preserved in rock
• Paleontology is the study of fossils.
– 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
24 | 4
Fossil Preservation
• 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
24 | 5
Fossil Preservation – Original Remains
• 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
24 | 6
Fossil Preservation – Replaced Remains
• Fossils are more commonly found
composed of replacement minerals.
– 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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24 | 7
Mode of Fossil
Preservation
• Replaced Remains –
Dinosaur bones are
commonly
composed of silica.
(SiO2)
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Section 24.1
24 | 8
Fossil Preservation – Carbonization
• 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
24 | 9
Fossil Preservation – Molds and Casts
• 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
24 | 10
Modes of Fossil Preservation
Molds and casts of marine organisms
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Section 24.1
24 | 11
Fossil Preservation – Trace Fossils
• 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
24 | 12
Modes of Fossil Preservation
Trace Fossils – Fossilized Burrows
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Section 24.1
24 | 13
Fossil Evidence of Life
• 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
24 | 14
Microfossils
• 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.
– 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
24 | 15
Relative Geologic Time
• 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
24 | 16
Principle of Superposition
• 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
24 | 17
Principle of Cross-Cutting Relationships
• 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
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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
24 | 19
Unconformities
• 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 non-deposition 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
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Applying Principles of Relative Dating
An Example
• 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
24 | 21
Applying Principles of Relative Dating
An Example (cont.)
• 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
24 | 22
Finding an Unconformity
Confidence Exercise
• 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
24 | 23
Finding an Unconformity
Confidence Exercise (cont.)
• 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
24 | 24
Correlation
• 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
24 | 25
Index Fossils
• Index fossils – fossils that are wide-spread in
distribution, easily identified, and limited to a
particular time segment of the Earth’s history
– 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
24 | 26
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) Which fossil would be
the most useful as an
index fossil?
b) 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
24 | 27
Using the Process of Correlation
An Example (solution)
a) Fossil A would be the
best index fossil due to
the narrow range of
time that it existed.
b) 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
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Using the Process of Correlation
Confidence Exercise
a) Primitia would be the
least use as an index
fossil. It lived in a
wide range of time.
b) The presence of
Phacops indicates that
a rock is either Silurian
or Devonian in age.
c) Phacops is a trilobite.
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Section 24.2
24 | 29
Relative Geologic Time Scale
• 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
24 | 30
Relative Geologic Time Scale
• 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
24 | 31
Relative Geologic Time Scale
• 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
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Relative Geologic Time Scale
• 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
24 | 33
Measuring Absolute Geologic Time
• 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
24 | 34
Measuring Absolute Geologic Time
• 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
24 | 35
Radiometric Dating
• 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.
– Daughter products may themselves be
stable or radioactive (unstable.)
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Section 24.3
24 | 36
Radiometric Dating – Half-life
• 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.
– 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
24 | 37
Half-Life and Radiometric Dating
• 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
24 | 38
Rock “Clocks” – Condition #1
• 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
24 | 39
Rock “Clocks” – Condition #2
• The age of the rock is reasonably close to the
half-life 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
24 | 40
Rock “Clocks” - Condition #3
• 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
24 | 41
Condition #3 – Sometimes a Problem
• 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
24 | 42
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
24 | 43
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
24 | 44
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
24 | 45
Potassium-Argon Dating
• 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
24 | 46
Potassium-Argon Dating
Limitations
• 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.
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Section 24.3
24 | 47
Rubidium-Strontium Dating
• 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.
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Section 24.3
24 | 48
Using Radiometric Dating
An Example
• 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 halflives.
• 100%  50%  25%
• (2) x (704 x 106 years) = 1408 x 106 years
• or 1.41 billion years = age of the rock
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Section 24.3
24 | 49
Using Radiometric Dating
Confidence Exercise
• 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 U-235 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 halflives.
• 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
24 | 50
Carbon Dating
• 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
24 | 51
Carbon Dating
•
•
•
•
•
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14C
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.
Section 24.3
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Carbon Dating – Modern Methods
• 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.
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Section 24.3
24 | 53
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.
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Section 24.3
24 | 54
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
24 | 55
Lord Kelvin & the Age of the Earth
• 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
24 | 56
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.
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Section 24.4
24 | 57
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
24 | 58
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
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Section 24.4
24 | 59
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
4.0-billion-year-old rocks in Canada, 3.8billion-year-old granites in Greenland, and
3.4-billion-year-old granites in South Africa.
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Section 24.4
24 | 60
Age of Meteorites
• 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
24 | 61
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
24 | 62
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
24 | 63
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
24 | 64
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
24 | 65
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
24 | 66
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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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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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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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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Using Igneous Rocks to Date Sedimentary
Rocks - Confidence Exercise
• 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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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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Proterozoic Supercontinent – Rhodinia
• Rhodinia formed in the late Proterozoic.
It broke apart during the early Paleozoic Era.
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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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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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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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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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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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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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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
24 | 80