29. Geologic Time
It is not uncommon for people to think of time in
two conceptually different ways and yet consciously distinguish between them only rarely. When inquiring
about the time of mail delivery, we may be told that mail
comes at 1:00 p.m., or that it arrives just after lunch.
When we ask when a baby was born, the reply might be
that she was born November 21 of last year or that she
was born the week before Thanksgiving. In casual conversation, we would probably consider the two answers
to each question essentially equivalent, but there is a difference. The first response in each example establishes
the absolute timing of an event—a specific time or date.
The second response in each establishes only a sequence
of events; one identifiable event is followed by another,
but the exact time or date of neither event is specified.
Despite the immense span of time over which geologic events have been going on, they can be dated in
the same two ways. We can say that a particular granite body is 17.2 million years old, or we can observe that
it is younger than, say, some limestone layer in contact
with it. In the first case, we must have carried out some
experimental technique that has yielded a specific age
for the granite; in the second, we have made some
observations that confirm only that the limestone existed before the granite, although we do not know when
either was formed. The first kind of determination is
called absolute dating, and the second is called relative dating. Both are of great use in geology.
Absolute dating requires expensive and sophisticated
laboratory equipment and painstaking effort, but the techniques of relative dating can often be applied successfully
by simple observation in the field. We shall begin our discussion of geologic time with relative dating.
the erosional debris of older rocks, or of chemical precipitates or organic material; they have been formed as
this sediment has been deposited in layers and buried.
Metamorphic rocks have been significantly altered by
intense heat and pressure; thus, they are modified preexisting rocks that are in essentially the same locations
relative to their surroundings as before metamorphism.
Given these basic definitions, most of the principles upon which relative dating are based are almost
intuitive:
(1) The Principle of Original Horizontality.
Sediment is deposited in layers that are essentially horizontal (see Fig. 29.1), and thus the
sedimentary rocks that form from them originate as essentially horizontal strata. If sedimentary rocks are tilted, folded, or otherwise
deformed, we can assume with confidence that
the deformation occurred after the deposition
and lithification of the sediment.
The Principles of Relative Dating
A prerequisite to successful relative dating of rocks
and geologic events is that we understand, in a rudimentary way, how rocks originate. Recall from Chapter
28 that igneous rocks are formed when molten material
cools and crystallizes; thus igneous rocks are those that
have been deposited on the surface of the earth by volcanic activity or those that have crystallized beneath the
surface from molten magma that has been intruded into
surrounding rock. Sedimentary rocks consist either of
Figure 29.1. Sediment tends to accumulate in horizontal layers, even if initial layers cover irregularities in the
surface. Recognition of this leads naturally to the
Principle of Original Horizontality.
(2) The Principle of Superposition. Consider
Figure 29.2, which shows a sequence of sedimentary rocks in cross section (that is, a “cut-
279
away” view). If the rocks formed in the way
that has been indicated above, then it is obvious
that the layers at the bottom are older than the
ones at the top. It is not necessarily true that
deposition has been uninterrupted, however.
Most sediment is deposited in bodies of water,
so if this section of sedimentary rocks was at
some time raised above sea level (by geologic
forces that will be discussed in a later chapter),
some layers that had previously existed might
have been entirely stripped away by erosion
before conditions allowed deposition to
resume. We also assume in using this principle
that the rocks are right side up; the same forces
that can raise the earth’s crust can severely
deform it, and if a sequence of rocks has been
completely overturned, then failure to recognize that will lead to an erroneous result by
application of the Principle of Superposition.
dreds of kilometers with respect to the rocks on
the other side, to small cracks in mineral grains
as seen under a microscope.
B
A
C
Figure 29.3. In this cross section the rock sequence
labeled A is truncated by the sequence labeled B, and
fault C cuts through A but not B. The conclusion reached
by applying the Principle of Cross-Cutting Relations is
that the oldest feature is A because it is cut by both B and
C; the next oldest is fault C, because it cuts A but not B;
and sequence B cuts across both sequence A and fault C,
so it must be the youngest of the three.
(4) The Principle of Inclusion. One kind of rock
cannot contain inclusions (fragments) of another kind unless the inclusions existed first.
Inclusions are therefore always older that the
rocks that contain them. Suppose that erosion
exposed a sandstone that became weathered as
it was exposed to the elements and then, subsequently, limestone sediment was deposited
over that. Pieces of the sandstone that had
become included in the limestone sediment
would reveal that the sandstone was the older
rock. This is a particularly useful way to determine if a sequence of sedimentary layers is
right side up or overturned, as shown schematically in Figure 29.4.
Figure 29.2. In this sequence of sedimentary strata, the
oldest layers are those on the bottom, and the layers are
progressively younger toward the top. This is the
Principle of Superposition.
(3) The Principle of Cross-Cutting Relations. If
one rock unit cuts across or through another,
then the one that is cut must have existed first
and must, therefore, be the older of the two.
The same is true of a fault, which must be
younger than the rocks it cuts through. Figure
29.3 illustrates the principle, and no specialized
geologic training is required to understand it.
Even so, cross-cutting relationships can be very
complex, and careful observation is often
required to unravel a sequence of events. The
principle can be applied at all scales, from
faults that have moved rocks on one side hun-
So far, we have determined that even quite complex
geologic sequences can be deciphered by applying a few
simple ideas. The remaining technique of relative dating
involves fossils, and its application does require specialized training. Fossils will be discussed in some detail in
Chapter 33; for now, it is necessary only to understand
that they are the remains of once-living organisms and
are found in many common sedimentary rocks.
For many years during the late 18th and early 19th
centuries, a British civil engineer named William Smith
(1769-1839) worked around quarries and canals that
had been excavated in the rocks of southern England.
280
Sandstone
Limestone
E
D+E
C+D+E
C+E
A+C
A+B+C
Figure 29.4. Even though these beds have been overturned by forces acting on the crust of the earth, the
inclusions of sandstone in the limestone show the limestone to be the younger rock layer.
A+B
A
He observed that the rock layers occurred in a consistent
vertical sequence, that they contained fossils, and that
the fossils in the various layers were different. His careful study enabled him to characterize the rock layers by
the fossils found in them, so that if he were shown fossils obtained at the surface, he could predict not only
what kinds of rocks would lie beneath the surface, but
also how deep they would be. Over the subsequent century and a half, similar observations throughout much of
the world have resulted in our fifth principle of relative
dating:
A
B
C
D
E
Figure 29.5. A schematic illustration of the Principle of
Faunal Succession. Arrows indicate the sedimentary
layers in which each type of fossil is found and, therefore, the lengths of time during which each type of animal existed on the earth in this hypothetical location.
By noting the types of fossils found at any given level,
a relative age is determined. Depending on the lengths
of time represented by various fossils, this age may be
quite precise or only approximate.
(5) The Principle of Faunal Succession. A fauna
(flora) is a group of animals (plants) that are
somehow associated, and the Principle of
Faunal Succession holds that the assemblages
of fossil animals (or plants) found in sedimentary rocks are diagnostic of the ages of the
rocks. The fossil assemblages typically vary
from the bottom to the top of a sequence of sedimentary rocks, because some of the organisms
represented by the fossils became extinct while
others evolved. Experience has shown that
assemblages that disappear never reappear in
younger (that is, more recent) rocks. Thus, for
one trained in the science of paleontology (the
study of fossils), it is possible to look at a group
of fossils from a rock and determine, from the
fossils alone, the age of the rock relative to
rocks that contain different fossils. Figure 29.5
illustrates this principle.
sils in a freshwater limestone does not suggest that those
fossils are absent from rocks of the time period represented, but only that they are not present in freshwater
limestones. Again, we shall consider fossils as a specialized topic in Chapter 33.
Examples of Relative Dating
With the exception of the Principle of Faunal
Succession, you can apply the principles of relative dating to successfully determine a sequence of geologic
events, despite the fact that you may never have had any
geologic training beyond this sketchy introduction. All
you must remember is that (1) sedimentary rock layers
are essentially horizontal to begin with, (2) the ones at
the bottom of an undisturbed sequence are oldest, (3)
rocks cut by other rocks are the older of the two, and (4)
rocks containing inclusions of other rocks are younger
than those from which the inclusions came.
Now consider Figure 29.6, which shows a hypothetical geologic cross section of a small part of the
earth’s crust. Formations A and B consist of sedimenta-
Fossils often reveal a great deal about the geologic
environment in which a rock formed, and the paleontologist must consider the kind of rock being examined
when searching for fossils. Failure to find marine fos-
281
1
D
B
2
3
4
A
5
6
7
C
Figure 29.6. Shown on the left is a hypothetical geologic cross section containing sedimentary formations A and B,
igneous body C, and the erosional surface D. On the right is a series of “snapshots” depicting the geologic story told
by the cross section.
ry rocks, C is an igneous body consisting of solidified
magma, and D is the present land surface, including the
canyon. The geologic story told by the cross section is
shown in the small illustrations on the right, each of
which may be thought of as a “snapshot” taken after
some significant event in the geologic history of this
region. The “snapshot” in frame 1 shows formation A
as originally deposited in essentially horizontal layers.
This is followed in frames 2 and 3 by the folding and
erosion of this formation. In frame 4, formation B is
deposited on the erosional surface; and then, in frame 5,
the entire section is tilted and a new erosional surface is
carved. In frame 6 the magma is injected to form the
igneous body, and finally the present erosional surface
is developed in frame 7. The relative ages are clearly A,
B, C, and D from oldest to youngest.
Even sequences of events that have occurred on the
moon or other bodies in the solar system can be deci-
phered using the principles of relative dating. (Because
the moon has never supported life, our lack of expertise
with the Principle of Faunal Succession is no disadvantage to us there.) Figure 29.7 is a photograph of a lunar
terrain in the region of Mare Humorum, the darkish oval
just above center. Mare Humorum is a huge basin some
400 kilometers long made by meteor (planetesimal?)
impact in the lunar southern highlands, the densely
cratered lighter terrain surrounding the mare. Some of
the larger craters cut across the margin of the basin, for
instance at locations marked A. At locations marked B,
lava that later filled the basin invaded and partially
filled some of those craters. The fault marked C cuts
through the lava flows. Small craters marked D are in
the solidified lavas; close examination will reveal diffuse white patches on the lavas around some of them—
material ejected from the impact sites, like the rays seen
in Color Plate 13. From these observations, we can say
282
that the highlands must be oldest, followed by the
Humorum basin. After creation of the basin, impact
craters of moderate size continued to be formed in and
around it, and then the mare lavas were erupted onto the
floor of the basin and into some of these craters. Last,
the faulting occurred, along with impacts yielding small
craters. Because the fault and the small craters both cut
the second-oldest feature, but do not come in contact
with each other, we cannot tell which of the two came
Quaternary
Cenozoic
Cretaceous
Abundant grasslands
Horses, elephants
Abundant mammals
Early primates
Flowering plants
First snakes
Jurassic
First birds
Triassic
Early mammals
Dinosaurs
Permian
Mammal-like reptiles
Tertiary
Mesozoic
Pennsylvanian
Mississippian
Paleozoic
Devonian
Silurian
Ordovician
Cambrian
Early reptiles
Flying insects
Abundant amphibians
Widespread forests
First land vertebrates
Abundant fishes
Land plants
Starfish, sea urchins
Abundant marine
invertebrates (corals,
sponges, molluscs,
etc.)
Worms and
other softbodied animals
Precambrian
Simple plants
Figure 29.8. The Geologic Column, showing life forms
typical of each era, as determined from the fossil record.
first.
Figure 29.7. The region of Mare Humorum in the lunar
southern hemisphere. See the discussion in the text for
an explanation.
similarity (or dissimilarity) between modern life forms
and the fossils in rocks of those ages. The Precambrian
Era is characterized by very rare fossils—mostly
impressions of soft-bodied creatures, single-celled
organisms, and other very primitive life forms. The
Paleozoic Era is characterized by abundant marine
invertebrates; but fish, amphibians, and reptiles, as well
as land plants, also appear in the fossil record of that
era. Reptiles flourished during the Mesozoic Era (it was
the age of the dinosaurs), but it also saw the emergence
of birds, mammals, and flowering plants. We are now
in the Cenozoic Era, and the fossils of those rocks, as
well as presently living organisms, testify that it is the
age of the mammals.
Because the Geologic Column evolved conceptually over many years, before there were any techniques
for determining the absolute ages of rocks, none of its
developers knew how long any of the periods were or
even whether they were all of the same length. They are
shown in Figure 29.8 as if they were of equal duration,
but we shall see later that this is far from the case. Even
so, the use of fossils enabled geologists to identify rocks
as, say, Mississippian in age, and know that they were
therefore younger than Devonian rocks, even if there
were no cross-cutting relationships or other relative-dat-
The Geologic Column
As the science of geology developed in the early
19th century, it became possible to define various arbitrary divisions of rocks by the fossils they contained.
Often these divisions were given names based on geographic or historical features of the areas in which they
were described. Thus Cambrian, Ordovician, Silurian,
and Devonian rocks were named after the Roman name
for Wales, two ancient British tribes, and the English
county called Devonshire, respectively. Cambrian rocks
were identified as those in which the earliest, very abundant fossils of marine invertebrates were found; the presence of certain key fossils identified other rocks as
Ordovician, or Silurian, or Devonian, and so forth. Thus
evolved the Geologic Column, shown in Figure 29.8.
Note that the Geologic Column is divided into
periods, which are grouped into four eras—the
Cenozoic, the Mesozoic, the Paleozoic, and the
Precambrian. The first three literally mean recent life,
middle life, and ancient life, the names referring to the
283
ing indicators visible in the area.
the common practice then of ascribing the earth’s surface
features largely to one catastrophic event after another.
Hutton’s contention that geologic processes were
predictable consequences of immutable natural laws
was popularized and spread by other authors as the
Principle of Uniformitarianism, and in a form not too
dissimilar to the one accepted by most geologists today:
Uniformitarianism postulates that (1) natural laws do
not change with time; (2) there is a cause-and-effect
relationship between the laws and the geologic processes that operate on the earth; and (3) the rates of processes need not be considered constant because they are
governed by physical conditions that may change, but
understanding of the natural laws and the processes that
result places realistic limits on those rates.
The Principle of Uniformitarianism is often summarized as “The present is the key to the past,” meaning
that geologic processes operating today have probably
always operated in the same way; therefore, the study of
those modern processes can lead to correct interpretations of ancient geologic features. While this conveys a
generally correct impression of what uniformitarianism
is all about, it is not entirely accurate. For instance,
there is good reason to believe that the early atmosphere
of the earth was quite unlike the atmosphere that
presently surrounds us, and so some chemical reactions
between atmosphere and rocks that occur today may not
have occurred then and vice versa.
Absolute Time
While the principles of relative dating yield impressive results, they fail to answer some of the questions
that are scientifically and philosophically the most
interesting: When was this sediment deposited? When
did that volcano erupt? How old is the earth? To
answer questions like these requires more than determining a sequence of events, and we now proceed to
consider absolute dates for geologic phenomena.
People of virtually all civilizations have considered
the earth to be ancient, but the precise meaning of that
imprecise word has proved elusive. In 1654 Archbishop
James Ussher proclaimed that the world had been created in 4004 B.C., a date calculated by analyzing genealogies from the Old Testament. (A short time later a
Biblical scholar at Cambridge, Dr. John Lightfoot, identified the exact time of creation as 9:00 a.m. on October
26 in that same year!) A 6000-year-old earth may sound
ancient, and in terms of human generations it is; but
even from a scriptural viewpoint Ussher’s method is
faulty because the measurement of Adam’s age presumably did not begin until after the creation was finished.
Moreover, there is compelling evidence that the earth is
very much older than that. The hardy bristlecone pines
of Wheeler Peak, Nevada, show patterns in their annual
growth rings that document over 9,000 years of continuous existence (although no single tree is that old). In
the sedimentary Green River Formation of Utah and
Wyoming, over six million years of annual lake deposits
can be counted. That is a thousand times longer than the
age Bishop Ussher advocated, but even six million
years appears to be a substantial underestimate because
relative dating techniques reveal that the Green River
Formation is near the top of the Geologic Column.
Early Estimates of the Age of the Earth
Based on the Principle of Uniformitarianism, some
interesting methods of estimating the age of the earth
were devised during the 19th century. One method
assumed the oceans were originally freshwater and the
salt now present was produced by erosion and carried in
by rivers. It further assumed that erosion rates had
always been about constant, resulting in a calculated
age of 90 million years for the earth. What was not
appreciated at the time was that there are very thick
beds of rock salt beneath the surface in many areas of
the world, and this is salt not accounted for by measuring the salinity of the oceans. Evidently, this method
produced estimates that were too small.
Another approach was to estimate the total thickness of sedimentary rock that is present in the crust of
the earth and then determine how long it would have
taken to deposit that much sediment. It was assumed
that rates of erosion and deposition had been relatively
constant throughout the history of the earth, but there is
no real basis for this assumption. Further, the estimates
of total thickness varied widely, and there was no way
to account for sedimentary rock that had been eroded
and redeposited. Ages ranging from 3 million to 1500
million years were calculated by this method.
Perhaps the most influential early estimate was
Uniformitarianism
Aside from the catastrophic events associated with
volcanoes, earthquakes, or floods, geologic processes
are almost imperceptibly slow, and you have probably
taken little notice of the changes they bring. Yet the
changes come as even the most sluggish processes
relentlessly modify the earth. As James Hutton (17261797), a Scottish gentleman farmer with a geologic bent,
contemplated erosion, deposition of sediment, and other
natural processes, he realized that given sufficient time
and the assumption that they also operated in the past
much as he then saw them, these processes could
account for all of the geology he saw about him. (You
may recognize in this concept something reminiscent of
the postulate of time symmetry introduced in Chapter 1,
and, indeed, time symmetry is the basis for Hutton’s conclusions.) Hutton’s conclusion was in stark contrast to
284
made by Lord Kelvin (1824-1907), a British physicist.
After assuming that the earth had originated in a molten
condition, he did a rather straightforward calculation to
determine how long would be required for it to cool to
its present state. He had to estimate the thermal properties of the rocks in the interior of the earth (about which
virtually nothing was known in those days), so his calculations were rough; but he concluded that the earth
could not be much older than 100 million years.
Further, his estimates of the temperature of the sun limited the duration of habitable conditions on earth to
between 20 million and 40 million years. This baffled
and worried geologists, who intuitively sensed that this
estimate had to be far too short; but in the face of sound
mathematics produced by a renowned scientist, their
intuition was not convincing.
In fact, there were two serious errors in Kelvin’s calculations, but he was not responsible for either of them.
In the first place, radioactivity had not yet been discovered, so he was unaware that there are radioactive elements inside the earth that produce heat as they decay;
thus, heat had not been lost as rapidly as he had supposed. Second, he assumed that the sun burned in a conventional way (that is, by combustion), because nuclear
fusion had not yet been discovered either. Had he
known about fusion, he would not have determined such
a limited life span for the sun and the resulting short habitable time span on earth. The geologists were right
about his results, but only later did they understand why.
Number of half-lives elapsed
1
2
3
4
Percent of parent remaining
100
80
60
50%
40
25%
20
12.5%
6.25%
0
1.35
4.5
9.0
13.5
18.0
Number of billions of years elapsed
Figure 29.9. The decay curve for 238U to 206Pb.
Actually, by changing the number of years per half-life,
the same curve could be used for the decay of any
radioactive isotope. If we could determine that the fraction of parent remaining in a rock was 0.80, then the age
of the rock would be 0.30 half-lives, or 1.35 billion
years for the 206Pb clock.
tope by changing only the time period corresponding to
a half-life.
The vertical axis of the decay curve shows the fraction of original parent isotope remaining. At “time
zero” there is no daughter product yet, so the fraction is
1. After one half-life, the fraction is 1/2 because half of
the original parent is left. Likewise, after two half-lives,
the fraction is 1/4, and so on. If we had some way of
determining how much uranium-238 there was in a rock
originally and how much is left at present, we could
divide the latter by the former to obtain the fraction. We
could then draw a horizontal line from that value on the
vertical axis, intersect the decay curve, and draw a vertical line from that intersection to the horizontal axis,
where we would read the age of the rock.
All of the information needed to perform this task
can be obtained by experiment. The various isotopes of
an element all behave identically in chemical reactions,
so a chemical analysis for uranium and for lead would
be insufficient. But we could analyze for each isotope
by using a mass spectrometer (see Chapter 15), and that
is exactly what is done. The total amount of parent isotope (uranium-238 in this example) originally present is
determined by adding the amount of uranium-238 represented by its daughter product (lead-206) to the
amount of uranium-238 currently in the rock.
You may have recognized that this procedure
requires the assumption that the rock, as originally
formed, had no lead-206, so that all of that isotope now
present can be ascribed to uranium decay. This assumption is not always valid, but there are ways (which are
beyond the scope of this book) to determine how much
Radiometric Dating
In 1896, Antoine-Henri Becquerel accidentally
exposed a photographic plate by placing a piece of uranium ore on top of it and thus introduced the scientific
world to radioactivity. Radioactivity, you will recall
from Chapter 24, is the spontaneous decay of unstable
nuclei by any of several processes. It takes place at a
rate that is unique but constant for each radioactive isotope. As experiment and observation revealed the
nature of radioactive decay, it became apparent that it
could be used as a geologic clock—a means of determining the age of a rock containing radioactive isotopes. Here is how.
Consider the uranium isotope 238U, often written as
uranium-238. It decays to a daughter product that is
also radioactive, and which then decays to yet another
radioactive isotope, and so on through a complex series
of alpha and beta decays that finally arrive at lead-206,
which is a stable isotope that undergoes no further
decay. The entire 238U to 206Pb decay sequence has a
half-life of 4.5 billion years. Given that a half-life is the
time required for one-half of the amount of parent isotope present to decay to the daughter product, it is a
simple exercise to construct the decay curve of Figure
29.9. This graph would work for any radioactive iso-
285
Table 29.1. Radioactive isotopes used in absolute dating of rocks.
Parent
isotope
Daughter
isotope
Half-life
(years)
rubidium-87
thorium-232
uranium-238
potassium-40
uranium-235
carbon-14
strontium-87
lead-208
lead-206
argon-40
lead-207
nitrogen-14
47.0 billion
14.1 billion
4.5 billion
1.3 billion
713 million
5730
of the lead-206 is “original” and to correct for that.
In order to be useful as a geologic clock, a radioactive isotope must meet certain requirements. First, it
must have a sufficiently long half-life that the rocks still
have a measurable amount of parent isotope in them.
Second, the parent isotope must be relatively abundant
in common minerals so that the rocks will contain
enough of it to be useful. Several qualifying isotopes
exist, and the most commonly used parents and daughters are listed in Table 29.1, along with their half-lives.
The experimental procedures required to measure
minute quantities of isotopes are demanding and difficult, and whenever possible, two or more “clocks” are
used on the same rock to verify the results.
You will observe that carbon-14 has a very short
half-life compared to the other parent isotopes. It is different from the other clocks in other ways, too. It only
works for dating objects that have once been alive.
Carbon-14 is produced naturally in the atmosphere, and
a living organism continuously replaces what decays.
When the organism dies, however, the carbon-14 is no
longer replaced, and the amount remaining reveals how
long ago the organism died. After a few half-lives, there
is not enough 14C left to measure accurately, so the
method is limited to organisms that died within the last
70,000 years or so. It is thus not universally useful in
geology but is well suited to archaeology and recent
Cenozoic geology.
Cenozoic
Era
Quaternary Period
0
2
Tertiary Period
66
Cretaceous Period
Jurassic Period
208
Triassic Period
Permian Period
Pennsylvanian Period
Mississippian Period
Paleozoic
Era
Devonian Period
Silurian Period
245
286
320
360
408
438
Millions of years ago
144
Mesozoic
Era
Ordovician Period
505
The Geologic Column Revisited
Cambrian Period
570
With the ability to determine absolute ages of
rocks, we are now almost ready to place ages on the
Geologic Column and turn it into a quantitative tool.
But first we have to ask ourselves the question, “What
are we actually dating when we determine a radiometric
age?” For igneous rocks, we are ideally determining
time since crystallization from the magma or lava, and
this corresponds to what we would likely have meant
intuitively by “the age of the rock.” Some radiometric
clocks are more sensitive to heat than others, though, so
some will give us the time since the igneous body
cooled below a certain temperature. This can be very
Precambrian
Figure 29.10. The modern geologic timescale.
Absolute ages are obtained from radiometric dating, and
the periods are shown with correct relative durations.
286
useful information, but we do have to understand the
clocks well enough to properly interpret the “age.”
Thus, the “age” of a metamorphic rock may be the time
since the metamorphic event, not the time since the origin of the premetamorphic rock. A sedimentary rock
generally consists of sediment from several sources, and
so an “age” determined from such a rock would not
mean very much.
For these reasons, the types of rocks on which most
of the Geologic Column is based—sedimentary rocks—
are not the sort that can usually be used to find absolute
dates for the time scale. However, by dating igneous
rocks and noting their relative-dating relations to fossilbearing sedimentary rocks, it is possible to bracket the
absolute ages within which the fossils lived. By continually refining the measurements, we can “zero in” on
the absolute ages of key fossils and establish dates for
the beginning and end of each of the geologic periods.
Figure 29.10 shows the modern geologic timescale,
with absolute dates and the lengths of the periods shown
to scale.
Considered together, all of this suggests that the
solar system came into being about 4.6 billion years
ago, and that is the current best estimate of the age of
the earth.
Summary
Geologic time is measured in both relative and
absolute terms. Absolute dating is more informative,
but also more expensive, time consuming, and technically demanding. Relative dating is accomplished in
essentially two ways: Either the principles of original
horizontality, superposition, cross-cutting relationships,
and inclusions are applied to determine a sequence of
events, or the law of faunal succession is used by
experts in paleontology. Absolute dating techniques
have provided estimates for the ages of subdivisions of
the Geologic Column, so that fossils may now be used
to yield approximate absolute dates for the rocks in
which they occur.
Absolute (radiometric) dating, most often applied
to igneous rocks, depends on the radioactivity of certain
natural isotopes that occur in rocks. In order to be useful as a geologic clock, an unstable isotope must have a
half-life sufficiently long that after substantial periods
of time there is still enough left to measure accurately,
and it must be abundant enough in rocks to allow accurate analysis of both parent and daughter product. A
decay curve, in which the fraction of remaining parent
isotope is plotted against the number of half-lives
elapsed, can be constructed without reference to any
experimental data at all. Once this is done and the fraction of parent material remaining in a given specimen is
determined by experiment, the curve will yield the number of half-lives elapsed. Multiplying by the number of
years per half-life for the given isotope (also determined
by experiment) yields the age of the rock in years.
There are various corrections that must be applied in
practice, depending on which isotopes are being used,
and usually agreement between dates obtained by two
different methods is sought.
Radiometric dating of terrestrial materials yields 3.8
billion years as the age of the oldest rocks and 4.2 billion
years as the age of the oldest minerals (which now happen to be part of a younger sedimentary formation).
Meteorites of the oldest kind consistently turn out to be
4.6 billion years old, as do the oldest rocks from the
moon. Because it is likely that erosion and other processes that operate on earth have destroyed the earliest rocks
of our planet, we take the age of the earth to be 4.6 billion years, the same as the moon and the meteorites.
The Age of the Earth
We return now to a consideration of the age of the
earth. The oldest terrestrial minerals that have so far
been radiometrically dated come from Australia and
yield an age of 4.2 billion years. These minerals are
part of younger sedimentary rocks now, but their presence testifies of rocks that existed on the earth at least
4.2 billion years ago. This establishes a lower limit on
the age of the earth, but the planet could be much older
than that because (1) erosion or other processes that
modify the surface of the earth could have obliterated
rocks older than that, and (2) there might be older rocks
somewhere that have simply not yet been radiometrically analyzed.
Meteors, you will recall from the last chapter, are
pieces of rock that orbit the sun and frequently strike the
earth. If they survive their fiery trip through the atmosphere and land on the surface, we call them “meteorites.” They are thought to be remnants from the formation of the solar system and to be of essentially the
same age as the sun and planets. There are several types
of meteorites, but the oldest ones turn out to be uniformly about 4.6 billion years old, as determined by
radiometric dating.
During the Apollo space program of the late 1960s
and 1970s, men traveled to the moon and returned with
samples of rock from its surface. Like the meteorites,
there are differences between one lunar rock and another, and all are not the same age. As implied by Figure
29.7, the moon, like the earth, has a long and complex
history; and we are not surprised that lunar rocks vary in
age. The oldest ones, however, also yield radiometric
dates of about 4.6 billion years.
287
STUDY GUIDE
Chapter 29: Geologic Time
7.
A. FUNDAMENTAL PRINCIPLES
1. Time Symmetry and Causality: See Chapter 1.
Time symmetry and causality become the essential
features of Uniformitarianism in this chapter.
2. Uniformitarianism: The idea that (1) natural laws
do not change with time; (2) there is a cause-andeffect relationship between the laws and the geologic precesses that operate on the Earth; and (3)
the rates of processes need not be considered constant because they are governed by physical conditions which may change, but understanding of the
natural laws and the processes that result places
realistic limits on those rates.
8.
9.
10.
11.
B. MODELS, IDEAS, QUESTIONS, OR APPLICATIONS
1. How do many geologists view the importance of
catastrophism and uniformitarianism in understanding the history of the earth?
2. What principles make it possible to determine the
relative ages of geologic events?
3. What principle makes it possible to estimate the
absolute age of a geological event?
4. What are the main conclusions reached about the
geological and biological history of the earth after
applying the principles of both relative and
absolute dating?
5. When did the Precambrian Era begin and end?
Paleozoic Era? Mesozoic Era? Cenozoic Era?
What events mark these boundaries?
6. Are fossils rare? How are fossils preserved? What
do fossils teach us?
12.
13.
14.
15.
16.
17.
18.
19.
C. GLOSSARY
1. Absolute Dating: The determination of a specific
time, age, or date in geologic history.
2. Cambrian Period: The beginning subdivision
(period) of the Paleozoic Era. During the Cambrian
period, life-forms began to flourish. Trilobites
appear.
3. Catastrophism: The idea that geologic features
are caused by unique cataclysmic events that are
noncyclical and of relatively short duration in time.
4. Cenozoic Era: The era of the Geologic Column
we are now in, characterized by mammals and
flowering plants. The era began 66 million years
ago.
5. Cross-Cutting Relations (Principle of ): A principle of relative dating which observes that if one
rock unit cuts across or through another, then the
one that is cut must have existed first and must
therefore be the older of the two rock units.
6. Daughter Product: The isotope to which the
20.
21.
22.
23.
24.
288
radioactive parent isotope decays.
Era: The Geologic Column is subdivided into
eras; eras are subdivided into periods; periods are
subdivided into epochs.
Faunal Succession (Principle of): A principle of
relative dating which observes that assemblages of
fossil animals (fauna) or plants (flora) found in
sedimentary rocks are diagnostic of the ages of the
rocks. Extinct assemblages never reappear in
younger, more recent rocks.
Geologic Column: A division of rocks in the
earth’s crust by age according to the fossils they
contain.
Igneous Rock: See Chapter 28.
Inclusion (Principle of): A principle of relative
dating which observes that inclusions (rock fragments) are always older than the rocks that contain
them.
Isotope: See Chapter 24.
Mesozoic Era: The second-youngest era of the
Geologic Column, characterized by reptiles and
dinosaurs. This era lasted from 245 million years
ago to 66 million years ago.
Metamorphic Rock: See Chapter 28.
Original Horizontality (Principle of): A principle
of relative dating which observes that sediment is
deposited in layers that are essentially horizontal
when originally formed.
Paleontology: The study of fossils.
Paleozoic Era: The second-oldest era of the four
eras in the Geologic Column, characterized by
abundant marine invertebrates and fish. This era
lasted from 570 million years ago to 245 million
years ago.
Parent Isotope: The original, undecayed radioactive isotope used in radiometric dating.
Precambrian Era: The oldest era of the Geologic
Column, characterized by very rare fossils, mostly
impressions of soft-bodied creatures, single-celled
organisms, and other very primitive life. This era
began with the formation of the earth and ended
570 million years ago.
Radioactive Half-Life: See Chapter 24.
Radiometric Dating: Use of radioactive isotopes
to determine the age of a rock by determining the
ratio of parent isotope to daughter product and calculating, based on the parent isotope half-life, the
time needed to obtain this ratio.
Relative Dating: The establishment of a sequence
of events without specifying an exact time or span
of time in geologic history.
Sedimentary Rock: See Chapter 28.
Superposition (Principle of): A principle of relative dating which observes that, for rocks formed in
layers, the bottom layers are older than the top layers, unless the sequence of rocks has been com-
pletely overturned.
25. Uniformitarianism:
Principles” above.
See
and lunar rocks affect our thinking about the age of the
earth?
“Fundamental
29.9. A fault that cuts through a sedimentary rock
layer is
(a) younger than that layer.
(b) older than that layer.
(c) either (a) or (b), depending on how the
absolute dating turns out.
D. FOCUS QUESTIONS
1. Consider the Geologic Column:
a. Name and state in your own words five principles of relative dating.
b. Sketch the Geologic Column showing the four
eras and representative life forms found in the
rocks of each era.
c. Describe the general process of radiometric
dating.
d. Indicate on your sketch of the Geologic
Column the absolute dates of the beginning of each
era as determined from radiometric dating.
E. EXERCISES
29.1. How are relative dating and absolute dating
different in concept?
29.2. Which of the following choices is not used in
relative dating?
(a) radioactive isotopes
(b) the Principle of Superposition
(c) the Principle of Cross-Cutting relations
(d) the Principle of Inclusion
29.3 If a sedimentary rock layer lies on top of a
lava flow dated radiometrically at 44 million years, but
is itself cut by an igneous body dated at 27 million
years, its age
(a) is less than 27 million years.
(b) is more than 44 million years.
(c) is the average of 27 and 44 million years, or
35.5 million years.
(d) is between 27 and 44 million years.
29.4. Why are the geologic time periods not of
equal length?
29.5. Explain how the same decay curve can be
used for any radioactive isotope, regardless of the
length of its half-life.
29.6. To be useful as geologic clocks, radioactive
isotopes must be
(a) extremely rare and have long half-lives.
(b) extremely rare and have short half-lives.
(c) fairly common and have long half-lives.
(d) fairly common and have short half-lives.
29.7. What two factors limit the usefulness of carbon-14 dating in geology?
29.8. How do the radiometric ages of meteorites
289
290