R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CHAPTER 4
GEOLOGIC TIME: CONCEPTS AND PRINCIPLES
OUTLINE
INTRODUCTION
HOW IS GEOLOGIC TIME MEASURED?
EARLY CONCEPTS OF GEOLOGIC TIME AND EARTH’S AGE
RELATIVE DATING METHODS
ESTABLISHMENT OF GEOLOGY AS A SCIENCE—THE TRIUMPH OF
UNIFORMITARIANISM OVER NEPTUNISM AND CATASTROPHISM
Neptunism and Catastrophism
Uniformitarianism
Modern View of Uniformitarianism
LORD KELVIN AND A CRISIS IN GEOLOGY
ABSOLUTE DATING METHODS
Atoms, Elements, and Isotopes
Radioactive Decay and Half-Lives
Long-Lived Radioactive Isotope Pairs
Fission Track Dating
Radiocarbon and Tree Ring Dating Methods
GEOLOGIC TIME AND CLIMATE CHANGE
PERSPECTIVE Denver’s Weather—280 Million Years Ago!
SUMMARY
CHAPTER OBJECTIVES
The following content objectives are presented in Chapter 4:
 The concept of geologic time and its measurement have changed through human
history.
 The fundamental principles of relative dating provide a means to interpret geologic
history.
 The principle of uniformitarianism is fundamental to geology and prevailed over the
concepts of neptunism and catastrophism because it provides a better explanation for
observed geologic phenomena.
 The discovery of radioactivity provided geologists with a clock that could measure
Earth’s age and validate that Earth was very old.
 Absolute dating methods are used to date geologic events in terms of years before
present.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
 The most accurate radiometric dates are obtained from igneous rocks.
 Geologic time is an important element in the study of climate change.
LEARNING OBJECTIVES
To exhibit mastery of this chapter, students should be able to demonstrate comprehension
of the following:
 the differences between relative and absolute dating methods
 the concept of geologic time and its importance to the study of historical geology
 the manner in which the concept of geologic time has dramatically changed
 the fundamental principles of relative dating and their importance for reconstructing
Earth history
 the concepts of, and differences between, neptunism, catastrophism, and
uniformitarianism
 Lord Kelvin’s role in nearly destroying the foundation of uniformitarian geology
defined by Hutton and Lyell
 radioactive decay, and the principles involved in the use of radiometric dating for
absolute age dating of geologic events
 the various long-lived radioactive isotope pairs, and their limitations for use in terms
of absolute ages
 alternative absolute dating methods, including fission track and tree-ring dating
methods
 the carbon-14 dating method, and its differences from long-lived radioactive isotope
dating techniques
 methods of determining past climate changes in the geologic record, and the
importance of these data in climate change research and predictions
CHAPTER SUMMARY
1. Geologists use two frames of reference for geologic time: relative age dating
places geological events in a sequential order, while absolute age dating provides
specific ages for rock units or events. The geologic time scale uses both frames of
reference.
Figure 4.1
The Geologic Time Scale
2. Early Christian theologians were responsible for formulating the idea that time is
linear, and that Earth was very young. James Ussher calculated an age for Earth of
approximately 6,000 years based upon his interpretation of scripture.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Other attempts were made to determine Earth’s age on the basis of scientific
evidence, but the ages were too young.
3. Relative dating methods place events in sequential order, but do not tell us how
long ago the event took place.
4. Nicholas Steno developed three principles of relative age dating, including the
principle of superposition (oldest rock layers are on the bottom), the principle of
original horizontality (sediments are originally deposited horizontally), and the
principle of later continuity (sediment extends out or terminates against the edge
of a depositional basin).
Figure 4.2
The Principles of Original Horizontality, Superposition, and
Lateral Continuity
5. Hutton is credited for the principle of cross-cutting relationships, which states that
igneous intrusions or faults are younger than the rocks they intrude or displace.
Figure 4.3
The Principle of Cross-Cutting Relationships
6. Considering the religious, political, and social climate of the 17th, 18th, and 19th
centuries, it is easy to see why concepts such as neptunism, catastrophism, and a
very young Earth were eagerly embraced. As geologic data accumulated, it became
apparent that these concepts were not supported by evidence and that Earth must be
much older than 6,000 years.
Table 4.1
Werner’s Subdivision of the Rocks of Earth’s Crust
7. James Hutton thought present-day processes operating over long periods of time
could explain all the geologic features of Earth. He also viewed Earth history as
cyclical and through Earth to be very old. Hutton’s observations were instrumental
in establishing the principle of uniformitarianism.
Figure 4.4
Angular Unconformity at Siccar Point, Scotland (Active Figure)
8. Uniformitarianism, as articulated by Charles Lyell, soon became the guiding
principle of geology. According to the principle of uniformitarianism, the laws of
nature have been constant through time, and the same processes operating today
have also operated in the past, although not necessarily at the same rates.
Enrichment Topic 1. Scientific Principles and Scientific Laws
The late Stephen J. Gould argued that a scientific law is not the pinnacle of a hierarchy
that begins with observation and ends with a "proven” theory or a mathematical
expression of a "truth.” Rather, a scientific law is nothing more than a fundamental
assumption used as a basis for the scientific method. The assumption is unquestioned
because no unexplained instance of phenomena violating the assumption has ever been
documented. As such, the principles of historical geology are actually its scientific laws.
37
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Enrichment Topic 2. The Golden Age of Geology
The “Golden Age of Geology” describes the early years (1798-1840) when the discipline
of geology was evolving as a separate science, and the constructs of plutonism and
uniformitarianism were being proposed and debated. These proposals weren’t readily
accepted, and the discipline only evolved through numerous debates and conflicts.
Several articles and books by researchers of the history of science provide some insight
into the cultural, political, and social influences of 19th century upon the evolving science
of geology and/or ideas for classroom incorporation of the history of science (Clary,
Rudwick, Thackray, and others). The 1830s were particularly important for the
development of geology, since it was in the 1830s that a working class emerged in Great
Britain, and printing techniques had advanced to allow easier replication and greater
affordability of books. The great geologists of the day (De la Beche, Buckland, Lyell,
Mantell) wrote geological texts for this working class, and the dissemination of
geological thought became more widespread among the lay population.
9. The modern view of uniformitarianism assumes that the laws of nature are constant,
but the rates and intensities of change have varied throughout time. Some geologists
prefer the term “actualism” rather than “uniformitarianism.”
10. Lord Kelvin precipitated a crisis in geology when he used the heat of Earth, the
melting temperatures of rocks, and the rate of heat loss to calculate an age for Earth
of 20-400 million years. The discovery of radioactivity destroyed Kelvin’s
arguments.
11. Radioactivity was discovered during the late 19th century, and soon thereafter
radiometric dating techniques allowed geologists to determine absolute ages for
geologic events. Radioactive decay results in the change of atomic structure; types
of radioactive decay include alpha decay, beta decay, and electron capture. Several
steps may be involved in an element’s decay.
Figure 4.5
Three Types of Radioactive Decay
12. Absolute-age dates for rock samples are usually obtained by determining how
many half-lives of the radioactive parent element have elapsed since the sample
originally crystallized. A half-life is the time it takes for one-half of the radioactive
parent element to decay to a daughter element.
Figure 4.6
Radioactive Decay Series for Uranium 238 and Lead 206
Figure 4.7
Uniform, Linear Change Compared to Geometric Radioactive
Decay (Active Figure)
13. The most accurate radiometric dates are obtained from long-lived radioactive
isotope pairs in igneous rocks. The five common long-lived radioactive isotope
pairs are uranium 238-lead 206, uranium 235-lead 207, thorium 232-lead 208,
rubidium 87-strontium 87, and potassium 40-argon 40. The most reliable dates are
those obtained by using at least two different radioactive decay series in the same
rock.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Table 4.2
Figure 4.8
Figure 4.9
Five of the Principal Long-Lived Radioactive Isotope Pairs Used
in Radiometric Dating
Crystallization of Magma Containing Radioactive Parent and
Stable Daughter Atoms
Effects of Metamorphism on Radiometric Dating
Enrichment Topic 3. The Oldest Known Rocks
Rocks more than 4 billion years old have been found along the Acasta River in Canada’s
Northwest Territories. The rock is gneiss, a metamorphic rock that exhibits banding of
minerals. It was probably among the first crystal rocks to form on Earth and has been
dated radiometrically using the uranium-lead method. In Australia, scientists have dated a
4.4 billion year old zircon crystal inside a rock that is 3.1 billion years old. National
Geographic, Sept 2001 v200 no3 p78 (23).
14. Fission-track dating, tree-ring dating, and radiocarbon dating are other absolute age
dating methods.Carbon-14 dating is effective back to about 70,000 years ago and
can only be used on organic matter such as wood, bones, and shells. Unlike the
long-lived isotopic pairs, carbon-14 dating determines age by the ratio of
radioactive carbon 14 to stable carbon 12.
Figure 4.10 Fission-Track Dating
Figure 4.11 Carbon-14 Dating Method (Active Figure)
Figure 4.12 Tree-Ring Dating Method
15. Geologists have reconstructed past climates by analyzing stalagmites from caves.
Radiometric dating of stalagmite layers using uranium 234 to thorium 230 is reliable
to about 500,000 years. Precise and accurate geologic calendars allow geologist to
reconstruct past climate changes and link them to possible causes.
Figure 4.13 Stalagmites and Climate Change
Enrichment Topic 4. Climate Change Assessment in the Geologic Record
In order to assess future climate impacts, paleoclimatologists are investigating the
Pliocene as a possible analog for future warmer climate. Occurring approximately 3.3-3.0
million years ago, the middle Pliocene is the most recent period in which global
temperatures reached the warmer temperatures projected for the end of this century. The
Pliocene is more appropriate analog because the continental positions and CO2
concentrations more closely match modern levels, as opposed to other warmer periods,
such as the Late Cretaceous. However, global warmth of the Pliocene was distributed
differently than in modern times. Paleoclimatologists are working on accurate data
synthesis and model simulation to “unlock the secret to climate sensitivity” through
research into the mid-Pliocene. Robinson, Dowett, & Chandler (2008). “Pliocene Role in
Assessing Future Climate Impacts,” EOS, 89 (49), p. 501
39
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Enrichment Topic 5. Climate Change in the News
Students can investigate current newspaper and Internet postings on climate change. For
example, in December 2008, GermanWatch and the Clean Action Network (CAN)Europe released the 2009 Climate Change Performance Index at the United Nations
Framework Convention on Climate Change in Poland. The top three rankings were left
empty because “the lack of will to engage themselves more strongly to avoid dangerous
climate change, none of the countries achieved positions one through three.” However,
Sweden, Germany, France, India, Brazil, the United Kingdom, and Denmark were ranked
4 through 10. Of the 60 countries ranked, the countries scoring at the bottom were the
United States, Canada, and Saudi Arabia. The 60 ranked countries account for 90% of the
world’s emissions. Countries are ranked on the estimates from each country’s emissions
trends, current emission levels, and current policy.
LECTURE SUGGESTIONS
Catastrophism versus Uniformitarianism
Investigate the differences between these two models with students. Geologists do accept
certain catastrophic events from the rock record, including the bollide impact at the end
of the Cretaceous Period, thought to be partially responsible for the demise of the
dinosaurs.
1. Do events such as asteroid impacts, which suddenly and globally disrupt
ecosystems and the stratigraphic record of a large area, invalidate the principle of
uniformitarianism?
2. Is the principle of uniformitarianism different than our modern assumption that
future events will follow the same physical and chemical laws as past events? If
not, why is it necessary to stipulate such a principle as a fundamental principle of
geology?
3. Evaluate the assertion that uniformitarianism requires that any geologic or
evolutionary phenomenon that is not observable today could not have happened in
the past.
Radioactive Decay and Half-Lives
Some students are confused with the concept of half-lives, and often have the
misconception that after 2 half-lives, the original sample has completely decayed to
daughter product. A simple demonstration with a piece of paper can help to overcome
this misconception.
Start with the original sample that is 100% parent (a sheet of paper). After one half-life,
one half of the original sample will decay to daughter product. To illustrate this, fold the
paper in half, cut, and remove one half. Note that after two half-lives, one half of the
remaining parent will undergo decay.
40
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Again, fold the remaining parent in half, cut, and remove half of the sample. Repeat.
Discuss with the class the ratios of remaining parent material relative to the total, original
sample.
Half-lives
1
2
3
4
n
Ratio of Parent/(Parent + Daughter)
1/2
1/4
1/8
1/16
1/ 2n
Also discuss the decreasing amount of original parent material. Do the students see why
absolute age methods can be limited by the number of half-lives that passed, and the
amount of remaining parent material?
Radiometric Dating
The concept of radioactive decay and its application to dating geologic materials is best
illustrated by a specific example. In this activity, students participate in “age dating” a
container of pennies.
Any unstable isotope can be used, but it is best to use one such as 10C, which has a half life
of about 20 seconds, so that the example runs in real time. Begin the lecture by
explaining that 10C is an unstable isotope that undergoes a spontaneous B+ decay to 10B, a
stable isotope. Every 20 seconds on average one half of the 10C atoms in a system will
decay to 10B. Produce a plastic container (the sample we are trying to date), containing
100 pennies with heads up (the 10C atoms). (Pennies with tails up will be considered 10B
atoms; see below.) Four members of the class are designated as follows: two Counters, a
Timer, and a Geologist.
Begin the demonstration by drawing a graph of “amount of parent” versus “time” on a
board or overhead slide. Put the lid on the container and explain the significance of
“closing” the system. The demonstration proceeds with the Counters' shaking the
container for the first half-life called off by the 'timer'. At the end of this period, time is
stopped, the pennies are poured out of the container, and Counters count the number of
pennies with heads up. Plot the number on the graph, and the pennies with tails up are
returned to the container. This sequence is repeated for several “half lives.” To close,
explain how the Geologist makes an age determination for the container by knowing
where he/she is on the graph.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Geologic Time and Human Induced Climate Change
Review how time is measured in the human life (seconds, minutes, hours, days, years),
and how time is measured in the geologic past (hundreds of thousands, millions, billions
of years).
1. How long have scientists been measuring data that are used to document climate
change? If we consider the Industrial Revolution to be the catalyst for global
warming, how does this time interval compare with any of the geologic time
intervals (age, epoch, period, era, eon)?
2. If the historical human scale is miniscule on the geologic time scale, does this imply
that humans cannot alter Earth systems? Why or why not?
3. What data do scientists currently consider most important when evaluating climate
change? Are scientists looking at overall trends, or anomalies within a data set?
CONSIDER THIS
1. What component of the rock record was most important in establishing the fact that
Earth has a history that extends beyond that of recorded human history?
2. Using a geologic map of a state or the United States, have students consider the
relative proportions of rocks of different ages. Where are rocks of Precambrian age
preserved? Why do we see rocks as old as the Archean at all? How have geologists
determined the ages of rocks shown on these maps?
IMPORTANT TERMS
absolute dating
carbon-14 dating
catastrophism
fission track dating
half-life
neptunism
principle of cross-cutting
relationships
principle of lateral
continuity
principle of original
horizontality
principle of superposition
principle of
uniformitarianism
radioactive decay
relative dating
tree-ring dating
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
SUGGESTED MEDIA
Videos
1. Geologic Time, Earth Revealed #10, Annenberg/CPB
2. The Earth Has a History, Geological Society of America
3. Earth Time: Evolution and Human Memory, BBC
4. Dating the Earth, BBC
5. Classroom Encounters® with Global Change Scientists, Climate Change and Our
Future
6. Classroom Encounters® with Global Change Scientists, Thin Ice: Earth in the
Time of Climate Change;
7. Classroom Encounters® with Global Change Scientists,Freeze, Freeze, Fry:
Climate Change Past, Present, and Future
8. Physical Geography II: Fossils, Rocks, and Time, Quantum Leap
9. Geologic Time, Tell ME Why Sales Co.
10. Global Warming?, The History Channel
Software and Demonstration Aids
1. Explore Cross Sections CD-ROM, Geological Society of America
2. Explore Deep Time, Geological Time and Beyond, Geological Society of
America
CHAPTER 4 – ANSWERS TO QUESTIONS IN TEXT
Multiple Choice Review Questions
1.
2.
3.
4.
e
b
e
a
5.
6.
7.
8.
e
d
a
a
9. b
10. b
Short Answer Essay Review Questions
11.
In relative age dating, a rock or a geologic event is put into an order in which it
occurred. Therefore, geologists can discuss the “oldest” and “youngest” events, but
they do not have dates for the events. In absolute dating, geologists can assign a
date to a rock or geologic event, usually expressed as years before the present.
12. Metamorphism may allow the daughter isotope of a radioactive pair to leak out,
causing the calculated age of the mineral to be too young. This is especially a
problem with potassium 40–argon 40 dating, in which argon gas must be trapped
within a mineral’s structure until it is released into a mass spectrometer for the
technique to work. Rubidium 87–strontium 87 dating is more forgiving since
strontium is not a gas. Metamorphism can also erase fission tracks, causing the
calculated age to be too young in fission track dating.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
13.
Lyell postulated a “steady-state” Earth system in which the same geologic
processes operating today have prevailed at the same rate over geologic time. By
accepting this, if we understand Earth’s processes today we can use them to
reconstruct Earth’s history.
14.
To calculate the absolute age of an intrusive body, you will need to separate
minerals that are rich in the isotope pair you will use to measure the amount of time
that has passed since the crystallization of the rock. You should ensure that the
system has been closed since the rock’s formation, and that daughter product has
not escaped the system. Use the ratio of the amounts of parent and daughter
isotopes and the half life for the isotope system to calculate the absolute age of the
rock.
15.
The most likely explanation for the discrepancy between the calculated ages is that
the potassium 40-argon 40 involves a gas as a daughter product. It is more likely
that a gas escapes from a system than a solid mineral. In order to confirm this
hypothesis, the calculated dates using the two isotope systems should be compared.
If argon gas has escaped the system since the deposition of the ash, the date for the
ash using the potassium 40-argon 40 method should be younger than the date
calculated using the rubidium 87-strontium 87 isotope pair.
16. Students may produce a variety of answers for this question; their individual
methods should be accompanied by the identified drawbacks. Many of the other
absolute dating methods that students may uncover are more useful for younger
objects instead of our ancient Earth. For example, other absolute methods include
thermoluminescence (for archaeological dating). Chemical methods such as aminoacid racemization and obsidian hydration are more useful for
archaeological/younger sites. However, paleomagnetic dating—which uses the
changes of the Earth’s magnetic field—can now be extended back for rocks as old
as 4 billion years. A drawback of the paleomagnetic method is the oceanic crust is
more easily dated through paleomagnetism. Older rocks require refined methods.
17.
Kelvin assumed that Earth had begun as a molten mass, and then he calculated the
temperature under those conditions. By estimating the Earth’s current temperature
and rate of heat loss, Kelvin determined how long it would have taken for Earth to
cool to its present temperature. His calculations appeared scientific and convincing,
but the calculations produced a date that was far too young. Kelvin’s basic flaw
was his failure to account for a heat source within the Earth since its formation. He
was unaware that radioactive decay within the Earth generated heat. However, even
if radioactivity had not been discovered, there is other scientific evidence for the
ancient age of the Earth. Scientists would have accumulated more data, which
would lead to eventual acceptance of the ancient age of Earth.
18. Many of the same principles of relative dating can be used on Mars to reconstruct
its geologic history. For example, if a dike cuts across a rock feature on Mars, we
know that the dike is younger than the rocks it intruded.
44
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
The principle of superposition can be applied, in which the oldest rocks are at the
bottom of a sequence. However, we do not have fossil evidence from Mars, so we
could not apply a principle of fossil succession. The organic evolutionary history of
Earth is unique to our planet.
19. Igneous rocks yield the most accurate radiometric dates because as magma cools and
begins to crystallize, radioactive parent atoms are separated from previously
formed daughter atoms. Because they are the right size, some radioactive parent
atoms are incorporated into the crystal structure of certain minerals. The stable
daughter atoms are a different size and cannot fit into the crystal structure of the
same mineral as the parent atoms. Therefore, a mineral crystallizing in a cooling
magma will contain radioactive parent atoms but no stable daughter atoms. This
allows us to measure the time of crystallization of the mineral. Sedimentary rocks
cannot be dated because the minerals within them are not heated to temperatures
above their Curie points. (The exception is glauconite.) Minerals within some
sedimentary rocks can be dated, but the age determined will be that of the
formation of the mineral within the sedimentary rock, and not that of the
lithification (formation) of the sedimentary rock.
20.
Superposition, cross-cutting relationships, original horizontality, and lateral
continuity are some of the fundamental principles used in relative dating. They are
important because they are intuitive and can be easily applied in the field by
observation. Without these principles, it would be impossible to piece together a
geologic history for the Earth.
Apply Your Knowledge
1. The fraction of rubidium to the total system is 1,250,000,000/ (1,250,000,000 +
38,750,000,000) = 1/32, or 3.125%. Five half-lives have passed since the
crystallization of the mineral.
2. There are a variety of answers for this question. Using the principles of
uniformitarianism, we can attempt to extrapolate conditions 10,000 years into the
future. Many researchers attempt to do this in an attempt to show human effects
on the Earth system. However, the conclusions are not yet definitive, so students
may produce a variety of answers based upon their reading and understanding of
current science news articles. It would be much more difficult to extrapolate one
million years into the future, since student models will probably not include
possible catastrophic events that we can not accurately predict.
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