Geologic Time 18
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.
After reading, studying, and discussing the chapter, students should be able to:
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.
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.
Briefly explain the significance and divisions of the geologic time scale.
The two types of dates used by geologists to interpret Earth history are (1) relative dates, which put events
in their proper sequence of formation, and (2) numerical dates, which pinpoint the time in years when an
Relative dates can be established using the law of superposition (in an underformed sequence of
sedimentary rocks or surface-deposited igneous rocks, each bed is older than the one above, and younger than
the one below), principle of original horizontality (most layers are deposited in a horizontal position),
principle of cross-cutting relationships (when a fault or intrusion cuts through another rock, the fault or
intrusion is younger than the rocks cut through), and inclusions (the rock mass containing the inclusion is
younger than the rock that provided the inclusion).
Unconformities are gaps in the rock record. Each represents a long period during which deposition ceased,
erosion removed previously formed rocks, and then deposition resumed. The three basic types of
unconformities are angular unconformities (tilted or folded sedimentary rocks that are overlain by younger,
more flat-lying strata), disconformities (the strata on either side of the unconformity are essentially parallel),
and nonconformities (where a break separates older metamorphic or intrusive igneous rocks from younger
148 CHAPTER 18
Correlation, the matching up of two or more geologic phenomena in different areas, is used to develop a
geologic time scale that applies to the whole Earth.
Fossils are the remains or traces of prehistoric life. The special conditions that favor preservation are rapid
burial and the possession of hard parts such as shells, bones, or teeth.
Fossils are used to correlate sedimentary rocks that are from different regions by using the rocks’
distinctive fossil content and applying the principle of fossil succession. It is based on the work of William
Smith in the late 1700s, and states that fossil organisms succeed one another in a definite and determinable
order, and therefore any time period can be recognized by its fossil content. The use of index fossils, those
that are widespread geographically and are limited to a short span of geologic time, provide an important
method for matching rocks of the same age.
Each atom has a nucleus containing protons (positively charged particles) and neutrons (neutral particles).
Orbiting the nucleus are negatively charged electrons. The atomic number of an atom is the number of
protons in the nucleus. The mass number is the number of protons plus the number of neutrons in an atom’s
nucleus. Isotopes are variants of the same atom, but with a different number of neutrons, and hence a different
Radioactivity is the spontaneous breaking apart (decay) of certain unstable atomic nuclei. Three common
types of radioactive decay are (1) emission of alpha particles from the nucleus, (2) emission of beta particles
from the nucleus, and (3) capture of electrons by the nucleus.
An unstable radioactive isotope, called the parent, will decay and form stable daughter products. The
length of time for one-half of the nuclei of a radioactive isotope to decay is called the half-life of the isotope.
If the half-life of the isotope is known, and the parent/daughter ratio can be measured, the age of a sample can
be calculated. An accurate radiometric date can only be obtained if the mineral containing the radioactive
isotope remained in a closed system during the entire period since its formation.
The geologic time scale divides Earth’s history into units of varying magnitude. It is commonly presented
in chart form, with the oldest time and event at the bottom and the youngest at the top. The principle
subdivisions of the geologic time scale, called eons, include the Hadean, Archean, Proterozoic (together,
these three eons are commonly referred to as the Precambrian), and, beginning about 540 million years ago,
the Phanerozoic. The Phanerozoic (meaning “visible life”) eon is divided into the following eras: Paleozoic
(“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”).
A significant problem in assigning numerical dates is that not all rocks can be radiometrically dated. A
sedimentary rock may contain particles of many ages that have been weathered from different rocks that
formed at various times. One way geologists assign numerical dates to sedimentary rocks is to relate them to
datable igneous masses, such as volcanic ash beds.
Geologic Time 149
I. Two types of dates are used in determining III. Fossils: evidence of past life
geological ages A. Fossil – the remains or traces of
A. Relative dates - placing rocks and events prehistoric life
in their proper sequence of formation B. Types of fossils
B. Numerical dates – which specify the 1. The remains of relatively recent
actual number of years that have passed organisms – teeth, bones, etc.
since an event occurred 2. Entire animals, flesh include
3. Given enough time, remains may be
II. Principles and rules of relative dating petrified (literally “turned into
A. Law of superposition stone”)
1. Nicolaus Steno – 1669 4. Molds and casts
2. In an undeformed sequence of 5. Carbonization
sedimentary rocks (or layered 6. Others
igneous rocks), the oldest rocks are a. Tracks
on the bottom b. Burrows
B. Principle of original horizontality c. Coprolites (fossil dung)
1. Layers of sediment are generally d. Gastroliths (polished stomach
deposited in a horizontal position stones)
2. Rock layers that are flat have not C. Conditions favoring preservation
been disturbed 1. Rapid burial
C. Principle of cross-cutting relationships – 2. Possession of hard parts
a younger feature cuts through an older D. Correlation of rock layers
feature 1. Matching rocks of similar age in
D. Inclusions different regions
1. One rock unit is enclosed within 2. Often relies upon fossils
another a. William Smith (late1700s-early
2. Rock containing the inclusions is 1800s) noted that sedimentary
younger strata in widely separated areas
E. Unconformities could be identified and
1. An unconformity is a break in the rock correlated by their distinctive
record, a long period during which fossil content
deposition ceased, erosion removed b. Principle of fossil succession –
previously formed rocks, and then fossil organisms succeed one
deposition resumed another in a definite and
2. Types of unconformities determinable order, and therefore
a. Angular unconformity - tilted any time period can be
rocks are overlain by flat-lying recognized by its fossil content
rocks 3. Index fossils
b. Disconformity - strata on either a. Widespread geographically
side are parallel b. Limited to short span of geologic
c. Nonconformity – older time
metamorphic or intrusive igneous E. Fossils are also important environmental
rocks in contact with younger indicators
150 CHAPTER 18
IV. Dating with radioactivity b. Electron combines with a
A. Reviewing basic atomic structure proton to form a neutron
1. Structure of an atom c. Mass number remains
a. Nucleus unchanged and the atomic
1. Protons - positively charged number decreases by 1
2. Neutrons b. Parent - an unstable radioactive
a. Neutral charge isotope
b. Protons and electrons c. Daughter products - isotopes
combined resulting from the decay of a
b. Orbiting the nucleus are electrons parent
- negative electrical charges C. Radiometric dating
2. Atomic number 1. Principle of radioactive dating
a. An element's identifying number a. The percentage of radioactive
b. Number of protons in the atom's atoms that decay during one half-
nucleus life is always the same: 50 percent
3. Mass number b. However, the actual number of
a. Number of protons plus (in atoms that decay continually
addition to) the number of decreases
neutrons in an atom's nucleus c. Comparing the ratio of parent to
b. Isotope daughter yields the age of the
1. Variant of the same parent sample
atom 2. Useful radioactive isotopes for
2. Different number of neutrons providing radiometric ages
3. Different mass number than the a. Rubidium-87
parent atom b. Thorium-232
B. Radioactivity c. Two isotopes of uranium
1. Spontaneous breaking apart (decay) d. Potassium-40
of atomic nuclei 3. Sources of error
2. Radioactive decay a. A closed system is required
a. Types of radioactive decay b. To avoid problems, one safeguard
1. Alpha emission is to use only fresh, unweathered
a. Emission of 2 protons and 2 material
neutrons (an alpha particle) D. Dating with carbon-14 (radiocarbon
b. Mass number is reduced by dating)
4 and the atomic number is 1. Half-life of only 5730 years
lowered by 2 2. Used to date very recent events
2. Beta emission 3. Carbon-14 is produced in the upper
a. An electron (beta particle) atmosphere
is given off from the a. Incorporated into carbon dioxide
nucleus b. Absorbed by living matter
b. Mass number remains 4. Useful tool for anthropologists,
unchanged and the atomic archeologists, historians, and
number increases by 1 geologists who study very recent
3. Electron capture Earth history
a. An electron is captured by E. Importance of radiometric dating
the nucleus 1. Radiometric dating is a complex
procedure that requires precise
Geologic Time 151
2. Rocks from several localities have 4. Periods are subdivided into epochs
been dated at more than 3 billion D. Precambrian time
years 1. Nearly 4 billion years prior to the
3. Confirms the idea that geologic time Cambrian period
is immense 2. Not divided into smaller time units
because the events of Precambrian
V. Geologic time scale history are not known in great
A. Subdivides geologic history into units enough detail
B. Originally created using relative dates a. First abundant fossil evidence does
C. Structure of the time scale not appear until the beginning of
1. Eon the Cambrian period
a. Greatest expanse of time b. Precambrian rocks have been
b. Names subjected to a great many changes
1. Phanerozoic ("visible life") - E. Difficulties in dating the geologic time
the most recent eon, begins scale
about 540 million years ago 1. Not all rocks can be dated by
2. Proterozoic radiometric methods
a. The grains composing detrital
4. Hadean - the oldest eon
sedimentary rocks are not the same
c. Collectively, the Hadean, Archean,
age as the rock in which they occur
and Proterozoic eons are often
b. The age of a particular mineral in a
referred to as the Precambrian
2. Era metamorphic rock may not
a. Subdivision of an eon necessarily represent the time when
b. Eras of the Phanerozoic eon the rock formed
1. Cenozoic ("recent life") 2. Datable materials (e.g., volcanic ash
2. Mesozoic ("middle life") beds and igneous intrusions) are often
3. Paleozoic ("ancient life") used to bracket various episodes in
3. Eras are subdivided into periods 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. The principle of original horizontality states that, in general, stratification in sedimentary beds was
horizontal when the beds were deposited.
152 CHAPTER 18
4. (a) Is fault A older or younger than the sandstone layer? Fault A cuts the sandstone layer so the fault is
(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.
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. 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
Geologic Time 153
9. 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).
10. 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.
11. Fossils are very useful environmental indicators.
12. 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.
13. 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.
14. 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).
15. Tree rings are the concentric rings visible at the end of a log or a tree stump. They represent the layer of
wood that is added each year to the tree in a temperate region. The size and density of rings reflect the
environmental conditions (primarily climate) that existed during the year in which the tree formed.
Because rings are added each year, the age of the tree when it was cut down can be determined. This
procedure could be used to help date recent geologic events if such events, like a landslide or flood,
created a new land surface. Fallen trees or stumps could then be used to determine the minimum number
of years since the geologic event occurred.
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
154 CHAPTER 18
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.
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 lO9 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,
20. 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.
21. 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
I is a nonconformity.
(10) alluvial fan E; dike, cinder cone, and lava flow, F (5) sedimentary beds I
(9) fault G (4) intrusive igneous rock D (a
(8) igneous rock, dike and sill, C batholith)
(7) igneous intrusion K (3) dike of igneous rock B
(6) sedimentary beds J (2) sedimentary strata A
(1) metamorphic rock mass H
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