Geologic Time
9
Geologic Time opens with a discussion of the fundamental principles of relative dating, including the law of
superposition, principle of original horizontality, principle of cross-cutting relationships, and the uses of
inclusions and unconformities. How rock units in different localities can be correlated is also investigated.
The types of fossils and their significance to understanding geologic time precede a discussion of the
conditions favoring preservation. Also examined is the use of fossils in correlating and dating rock units.
Following an explanation of radioactivity, the fundamentals and importance of radiometric dating are
presented. The chapter concludes with an examination of the geologic time scale.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Explain the difference between relative and absolute dating of earth materials.
Discuss the Principle of Original Horizontality and how it relates to the Law of Superposition.
Briefly explain other principles used in relative age dating.
List and briefly explain the three types of unconformities.
Briefly discuss fossilization, including the origin and types of fossils.
Discuss the correlation of rock layers using physical criteria and fossils.
Briefly explain radioactivity and how it relates to absolute age dating.
Discuss the procedure of radiometric dating and explain how it is used to obtain absolute ages.
List the isotopes commonly used in the radiometric dating of earth materials.
List and briefly discuss the major subdivisions of the geologic time scale.
Briefly explain the significance of the Precambrian division of the geologic time scale.
Chapter Outline___________________________________________________________________
I. Two types of dates are used in determining
geological ages
A. Relative dates – placing rocks and events
in their proper sequence of formation
B. Numerical dates – which specify the
actual number of years that have passed
since an event occurred
igneous rocks), the oldest rocks are
on the bottom
B. Principle of original horizontality
1. Layers of sediment are generally
deposited in a horizontal position
2. Rock layers that are flat have not
been disturbed
C. Principle of cross-cutting relationships –
a younger feature cuts through an older
feature
D. Inclusions
1. One rock unit is enclosed within
another
II. Principles and rules of relative dating
A. Law of superposition
1. Nicolaus Steno – 1669
2. In an undeformed sequence of
sedimentary rocks (or layered
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2. Rock containing the inclusions is
younger
E. Unconformities
1. An unconformity is a break in the
rock record, a long period during
which deposition ceased, erosion
removed previously formed rocks,
and then deposition resumed
2. Types of unconformities
a. Angular unconformity–tilted
rocks are overlain by flat-lying
rocks
b. Disconformity – strata on either
side are parallel
c. Nonconformity – older
metamorphic or intrusive igneous
rocks in contact with younger
sedimentary strata
III.
IV.
Fossils: evidence of past life
A. Fossil – the remains or traces of
prehistoric life
B. Types of fossils
1. The remains of relatively recent
organisms – teeth, bones, etc.
2. Entire animals, flesh included
3. Given enough time, remains may be
petrified (literally “turned into
stone”)
4. Molds and casts
5. Carbonization
6. Others
a. Tracks
b. Burrows
c. Coprolites (fossil dung)
d. Gastroliths (polished stomach
stones)
C. Conditions favoring preservation
1. Rapid burial
2. Possession of hard parts
Correlation of rock layers
Matching rocks of similar age in different
regions
B. Often relies upon fossils
A.
1. William Smith (late1700s-early
1800s) noted that sedimentary strata in
widely separated areas could be
identified and correlated by their
distinctive fossil content
2. Principle of fossil succession – fossil
organisms succeed one another in a
definite and determinable order, and
therefore any time period can be
recognized by its fossil content
V.
Dating with radioactivity
A. Reviewing basic atomic structure
1. Structure of an atom
a. Nucleus
1. Protons – positively charged
2. Neutrons
a. Neutral charge
b. Protons and electrons
combined
b. Orbiting the nucleus are electrons
– negative electrical charges
2. Atomic number
a. An element's identifying number
b. Number of protons in the atom's
nucleus
3. Mass number
a. Number of protons plus (in
addition to) the number of
neutrons in an atom's nucleus
b. Isotope
1. Variant of the same parent
atom
2. Different number of neutrons
3. Different mass number than the
parent atom
B. Radioactivity
1. Spontaneous breaking apart (decay)
of atomic nuclei
2. Radioactive decay
a. Types of radioactive decay
1. Alpha emission
a. Emission of 2 protons and 2
neutrons (an alpha particle)
b. Mass number is reduced by
4 and the atomic number is
lowered by 2
Geologic Time
2.
Beta emission
a. An electron (beta particle)
is given off from the
nucleus
b. Mass number remains
unchanged and the atomic
number increases by 1
3. Electron capture
a. An electron is captured by
the nucleus
b. Electron combines with a
proton to form a neutron
c. Mass number remains
unchanged and the atomic
number decreases by 1
b. Parent – an unstable radioactive
isotope
c. Daughter products – isotopes
resulting from the decay of a
parent
3. Half-life – the time for one-half of
the radioactive nuclei in a sample to
decay
C.
Radiometric dating
1. Principle of radioactive dating
a. The percentage of radioactive
atoms that decay during one halflife is always the same: 50 percent
b. However, the actual number of
atoms that decay continually
decreases
c. Comparing the ratio of parent to
daughter yields the age of the
sample
2. Useful radioactive isotopes for
providing radiometric ages
a. Rubidium-87
b. Thorium-232
c. Two isotopes of uranium
d. Potassium-40
3. Sources of error
a. A closed system is required
b. To avoid problems, one safeguard
is to use only fresh, unweathered
material
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D.
Dating with carbon-14 (radiocarbon
dating)
1. Half-life of only 5730 years
2. Used to date very recent events
3. Carbon-14 is produced in the upper
atmosphere
a. Absorbed by living matter
4. Useful tool for anthropologists,
archeologists, and geologists who
study recent Earth history
E. Importance of radiometric dating
1. Radiometric dating is a complex
procedure that requires precise
measurement
2. Rocks from several localities have
been dated at more than 3 billion
years
3. Confirms the idea that geologic time
is immense
VI.
Geologic time scale
A. Subdivides geologic history into units
B. Originally created using relative dates
C. Structure of the time scale
1. Eon
a. Greatest expanse of time
b. Names
1. Phanerozoic ("visible life") –
the most recent eon, begins
about 540 million years ago
2. Proterozoic
3. Archean
4. Hadean – the oldest eon
c. Collectively, the Hadean, Archean,
and Proterozoic eons are often
referred to as the Precambrian
2. Era
a. Subdivision of an eon
b. Eras of the Phanerozoic eon
1. Cenozoic ("recent life")
2. Mesozoic ("middle life")
3. Paleozoic ("ancient life")
3. Eras are subdivided into periods
4. Periods are subdivided into epochs
D. Precambrian time
1.
2.
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Nearly 4 billion years prior to the
Cambrian period
Not divided into smaller time units
because the events of Precambrian
history are not known in great
enough detail
CHAPTER 9
a. First abundant fossil evidence does
not appear until the beginning of
the Cambrian period
b. Precambrian rocks have been
subjected to a great many changes
E. Difficulties in dating the geologic time
scale
1. Not all rocks can be dated by
radiometric methods
a. The grains composing detrital
sedimentary rocks are not the same
age as the rock in which they occur
b. The age of a particular mineral in a
metamorphic rock may not
necessarily represent the time when
the rock formed
2. Datable materials (e.g., volcanic ash
beds and igneous intrusions) are
often used to bracket various
episodes in Earth history and arrive
at age
Answers to the Review Questions
1. Absolute dating involves a numerical age measurement in actual time units, like thousands or millions of
years. Relative dating involves placing sequences of rocks, geological features, and events in the correct
order in which they occurred, without necessarily knowing their absolute ages.
2. The law of superposition is the idea or notion that beds in a sequence of horizontal, sedimentary strata
become younger upward in the sequence. In other words, younger strata are deposited over older strata. A
feature that truncates or cuts across another geologic feature is the younger of the two. This is known as
the principle of cross-cutting relationships. For example, a dike of basalt injected into a crack in
sedimentary strata is younger than the strata.
3. (a) Is Fault A older or younger than the sandstone layer? Fault A cuts the sandstone layer so the fault is
younger.
(b) Is Dike A older or younger than the sandstone layer? Dike A also crosscuts the sandstone layer so the
dike is younger.
(c) Was the conglomerate deposited before or after Fault A? Fault A stops at the base of the
conglomerate; thus the conglomerate layer truncates the fault and is younger than the fault.
(d) Was the conglomerate deposited before or after Fault B? The conglomerate is cut and displaced by
Fault B; thus Fault B is younger.
(e) Which fault is older, A or B? The faults do not cross, but the relationship between the faults and the
conglomerate proves that Fault A is older than Fault B.
(f) Is Dike A older or younger than the batholith? Dike A does not cut the batholith so other relationships
must be used. Dike B clearly cuts the batholith; the sill fed by Dike B is crosscut by Dike A, proving
that Dike A is younger than Dike B and younger than the batholith.
4. The principle of original horizontality states that, in general, stratification in sedimentary beds was
horizontal when the beds were deposited.
Geologic Time
77
5. A depositional contact or unconformity would be proven if detrital rock and mineral grains from the
granite were found in the sandstone. Also the granite just below the contact might show reddish
discoloration or other evidences of having been weathered before the sandstone was deposited. Bedding
in the sandstone will be parallel or nearly parallel to the contact; there will be no evidence for contact
metamorphism in the sandstone; and the sandstone will not be cut by the granitic dikes.
If the contact is intrusive, the sandstone may be cut by granitic dikes and may show contact
metamorphism. Rock and mineral grains in the sandstone will not show any direct correlation to the
granite, and bedding in the sandstone will probably not be parallel to the contact.
6. These are all erosion surfaces buried beneath younger strata. The older strata below an angular
unconformity were tilted before the younger strata were deposited; thus the older and younger strata
exhibit a sharp, angular, erosional discordance. Strata above and below a disconformity exhibit parallel
stratification or bedding orientations, indicating that the underlying, older strata were not tilted or
deformed before the younger strata were deposited. Younger, sedimentary beds deposited on an eroded
mass of older, igneous or metamorphic rock comprise a nonconformity.
7. Correlation is the process of establishing equivalency of rock units, ages, depositional environments, and
events in geologic history (faults, tectonic events, unconformities, etc.) in different areas. Correlation can
be local (between rocks intersected in neighboring drill holes) or world-wide (continent to continent).
8. Smith was an English naturalist who first convinced other geologic thinkers of his day that strata
containing the same assemblages of fossils were correlative from place to place. Thus Smith can be
thought of as the founder of the study of stratigraphy and as a leading advocate of using fossil
assemblages to correlate equivalent-aged strata (the principle of faunal succession).
9. Different types of fossilization include:
(a) actual remains: usually hard parts from organisms of the recent geologic past
(b) petrified: the original substance has been replaced by mineral matter or pore spaces have been filled
with a mineral
(c) mold: when a shell or other structure is buried in sediment and then dissolved by underground water
(d) cast: the hollow space of a mold is subsequently filled with mineral matter
(e) tracks: animal footprints made in soft sediment that was later lithified
10. Two conditions that favor preservation of an organism as a fossil are rapid burial and the possession of
hard parts, such as a shell or skeleton.
11. Fossil organisms have great diversity, and certain individual organisms and/or assemblages of organisms
are characteristic of beds deposited during specific periods of geologic time. Thus fossils are useful for
correlating the same bed or same sequence of beds among different localities and for determining the
geologic ages of the beds.
12. The contact between sedimentary beds I (younger and horizontal) and sedimentary beds A (older and
tilted) is an angular unconformity. The contact between igneous rock D (older) and the sedimentary beds
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CHAPTER 9
I is a nonconformity.
(10)
(9)
(8)
(7)
(6)
(5)
(4)
(3)
(2)
(1)
Youngest
alluvial fan E; dike, cinder cone, and lava flow, F
fault G
igneous rock, dike and sill, C
igneous intrusion K
sedimentary beds J
sedimentary beds I
intrusive igneous rock D (a batholith)
dike of igneous rock B
sedimentary strata A
metamorphic rock mass H is
oldest
13. Each time beta decay occurs the atomic number raises by one and does not affect the mass number. Each
alpha decay decreases the atomic number by 2 and the mass number by 4. Thus, for 6 alpha decays and 4
betas, the atomic number of the daughter would be (90 - 6X2 +4) = 82, which is the atomic number of
lead. The mass number of the daughter would be (232 - 6X4) = 208. The stable daughter is lead-208.
14. With careful sample collection and laboratory procedures, the radiometric methods consistently give
accurate, reliable, absolute ages. No other method can be applied to all of geologic time. Fossils are
accurate and reliable for Phanerozoic sedimentary rocks but are not found in most igneous and
metamorphic rocks and are very rare in Precambrian rocks. The Phanerozoic time scale has been
accurately calibrated with radiometric ages, and Proterozoic and Archean chronologies are based entirely
on radiometric dates.
15. A ratio of 1: 1 would be produced in 10,000 years (one half-life). After two half-lives, 25 percent of the
original parent would be left and 75 percent of the daughter would have formed. The ratio (25 : 75) is 1:
3, so the sample is 20,000 years old (2 half-lives x 10,000 years in one half-life = 20,000 years).
16. If the abundances of the parent or daughter isotopes in a mineral or rock sample have been changed by
any process other than radioactive decay, the parent to daughter ratio will not be a true measure of the age
of the sample.
17. The work must be done carefully, and the laboratory environment must be free of materials that might
contaminate the sample and produce a change in the measured, parent to daughter isotopic ratio. Other
precautions include careful sample collection, good mineral separations, repeated analyses of the same
samples to establish precision limits, and age determinations by other methods to check for consistency
and accuracy. Finally, careful attention to geologic relationships will reduce the chances of
misinterpreting the results.
Geologic Time
79
18. To make calculations easier, let us round the age of Earth to 5 billion years.
(a) What fraction of geologic time is represented by recorded history (assume 5000 years for the length of
recorded history)? The percentage is 5 x 103 yrs divided by 5 x 109 yrs x 100% which equals 1 x 10~4% or
0.0001%.
(b) The first abundant fossil evidence does not appear until the beginning of the Cambrian period (570
million years ago). What percentage of geologic time is represented by abundant fossil evidence? The
percentage is 6 x 108 yrs divided by 5 x 109 yrs x 100% = 1.2 x 10% or 12%.
19. The following are the various divisions listed from longest to shortest time intervals: eons, eras, periods,
and epochs.
20. Three main factors are involved. Fossils are lacking or very difficult (single celled organisms) to use for
age determinations; many Precambrian rocks formed deep in the ancient crust and can be dated only by
radiometric methods; and Precambrian rocks are deeply eroded and/or buried by Phanerozoic rocks. Thus
compared to younger strata, Precambrian rocks are less accessible and their geologic record is much less
detailed.
21. In general, sedimentary rocks do not contain minerals that are both suitable for dating and that
crystallized when the bed was deposited. One exception would be feldspar or mica grains in volcanic ash
deposited at the time of the eruption. Minerals such as glauconite crystallize as sedimentary grains but
contain large quantities of non-radiogenic daughter element, making an age determination imprecise.
In recent years, advances in instrumentation and the application of new geochronological methods have
led to much more precision and accuracy in dating of sedimentary rocks. For example, extensive, detailed
micropaleontological data and very precise Sr-87/Sr-86 measurements in Mesozoic and Cenozoic marine
limestones have been correlated, resulting in very precise age assignments for many marine carbonate
strata. In addition, paleomagnetic measurements combined with the known geomagnetic time scale and
paleontological data can often result in very precise age assignments for some strata.
Lecture outline, art-only, and animation PowerPoint presentations for each chapter of Earth,
9e are available on the Instructor’s Resource Center CD (0131566911).
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