Chapter 8
Geological Time
Notes
(3 classes notes + 1 class Xword & review + 1 class test)
Objectives:
1. Construct/Interpret cross-sectional diagrams of the earth
using the concepts of superposition, uniformitarianism,
correlation, horizontality, contact metamorphism, cross
cutting relationships, unconformities, folding and faulting.
pp. 218-224
Uniformitarianism - the same external and internal processes
happening on the earth today are similar to those that have
happened in the past. This idea was first published by James Hutton
and was covered earlier.
Superposition - in an undeformed sequence of sedimentary rocks,
each bed is older than the one above it and younger than the one
below it. This idea was credited to Nicolaus Steno and was covered
earlier. See example here.
Original Horizontality - The principle of horizontality states that
most layers of sediment are deposited in the horizontal position.
This idea was also credited to Nicolaus Steno. This principle means
that layers in the horizontal position have not been disturbed but
that layers that folded or inclined have been moved from their
horizontal position (Compare the two pictures in Figure 8.2 p. 219
and Figure 8.3 p. 220 text).
Cross-cutting Relationships - The principle of cross-cutting
relationships states that when a fault cuts through rocks or when
magma intrudes between or across rock layers, the fault or magma
is younger than the rocks it affected. In Figure 8.4, Fault A occurs
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across all the layers of rock up to the conglomerate so this fault
must be younger than those layers. Also in Figure 8.4, Dike A cuts
across all the layers of rock up to the shale and so must be younger
than all those layers.
Inclusions - these are pieces of one rock that are contained within
another rock. This means that the rock layer containing the inclusions
must be younger than the rock layer that supplied the inclusions. In
Figure 8.5 p. 221 text, the sedimentary layer containing the igneous
rock inclusions was deposited on top the weathered igneous rock
rather than being intruded from below. Therefore the sedimentary
rock is younger.
Unconformities - these represent a long period in which deposition
ceased, erosion and uplifting occurred, and then deposition began
again. There are 3 main types of unconformities.
i. Angular Unconformity - this consists of tilted or folded
sedimentary rock layers that are then covered by flat layers of rock
(see Figure 8.6 p. 222 and Figure 8.7 p. 223 text). Angular
unconformities indicate that deposition stopped, the layers were
tilted or folded, and then deposition began again. See here and
here, and here.
ii. Disconformity - this occurs when deposition stops but the rock
layers are not folded or tilted. Erosion occurs and then deposition
begins again. Disconformities can be difficult to identify but can be
recognized by an uneven surface between rock layers (see Figure 8.6
p. 222 text). See here and animation here
iii. Nonconformity - this is a break which separates older
metamorphic or igneous rock from younger sedimentary rock.
Nonconformities occur when igneous or metamorphic rock which
formed far below the surface becomes uplifted and eroded and then
subsides. The rock is then covered with sediments that eventually
become sedimentary rock (see Figure 8.5 p. 221 and Figure 8.6 p.
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222 text). See here. See animations of sequence of geological
formations and here.
See how all three unconformities are made here.
See formation of geological structures here.
Correlation - this is the process of matching up rocks of similar
age at different locations on the earth. If locations are close
together, rocks can be matched up by following a single rock layer
from one location to the other or by identifying a distinctive (by color
or mineral content) layer at both locations. If locations are far apart,
geologists must rely on fossils for correlation. Animation here
Sample Exam Questions
1. What feature is indicated by "T"?
(A) fault
(B) fold
(C) ripple mark
(D) unconformity
2. Use the diagram below to answer the following questions.
(i) Is layer "B" extrusive or intrusive?
Explain your answer.
(ii) Arrange the letters in the order they occur beginning with the
oldest and ending with the youngest.
Activities:
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 Relative Time Principles Worksheet, Edukit
 Relative Time Principles & Newfoundland & Labrador
Worksheet, Edukit
 Core Lab #1
 Identification & Creation of Sequence of Rock Layers &
Other Features
Do #'s 2-6, 10, p. 241 text.
Read pp. 225-235 text for next day.
Objectives:
1. Describe how fossils are the key to the interpretation of past
events. pp. 226-228
2. Use the geological time scale to compare the ages and lengths of
various segments of geological time. pp. 10, 237
3. Explain how half-lives of radioactive elements are used in
estimating ages of materials. p. 230
4. Determine the age of a sample using radiometric dating. pp. 229231
5. Evaluate sources of error in estimating radiometric age. p. 231
6. Sequence the major events in the earth's history such as: p. 237
i. Beginning of each geological era.
ii. Formation of oldest rocks.
iii. Formation of the earth.
iv. Pleistocene glaciation
v. East Coast orogeny
vi. Appalachian orogeny
Fossils are the remains or traces of prehistoric life and are found in
sediments or sedimentary rocks. They are important for interpreting
the geological past in that:
i. knowing the nature of life forms that existed at a particular
time helps researchers understand past environmental
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conditions.
ii. they play a key role in correlating rocks of similar ages from
different places.
Fossils and Correlation
Fossils became an important geological tool after William Smith
discovered that each rock layer contained fossils unlike those in the
layers above or below it and that sedimentary rock layers in areas
far apart could be correlated using this fact. The findings of Smith
and other geologists helped formulate the Principle of Fossil
Succession. This principle 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. This lead to the
dividing of geological time into the Age of Trilobites, Age of Fishes,
Age of Coal Swamps, Age of Reptiles, and Age of Mammals based
on when their fossils were most plentiful.
When correlating rocks, geologists pay particular attention to index
fossils. These are fossils that are widespread and existed for only a
short time on the geological time scale. They allow geologists to
date rocks more precisely than those rock layers not containing
index fossils. However, rock formations do not always contain
index fossils. In this situation, groups of fossils are used to
determine the age of the rock. Geologists use fossils that have an
overlapping range to help determine the age of the rock (see Figure
8.10 p. 226 text).
Fossils and the Environment
Fossils can indicate the nature of the environment during
fossilization. For example, when clam shells are found in limestone,
the clams lived in a shallow sea because that is where clams are
found today (Law of Uniformitarianism). As well, fossils of
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animals with thick shells capable of withstanding the pounding surf
inhabited the shoreline whereas the fossils of animals with thin,
delicate shells inhabited the calmer offshore waters. Therefore the
type of shells found in an area can indicate the position of an
ancient shoreline. Finally, since corals today exist in warm and
shallow tropical seas, the fossils of corals at a certain location
indicate that these conditions must have once existed in that
location.
Sample Exam Questions
1. Which fossil is most useful to geologists in correlating
widely separated sedimentary rock outcrops?
(A) One that has worldwide distribution and confined
to sediments deposited during the Ordovician.
(B) One that has worldwide distribution and ranges
from Cambrian to Triassic.
(C) One that is found in one region of the world and is
confined to sediments of Cambrian age.
(D) One that is found in one region of the world and is
different in appearance from other species.
Radiometric Dating
The radioactivity of certain substances (e.g.: carbon 14) has
provided a reliable means of calculating the absolute ages of rocks
and minerals that contain certain radioactive isotopes. The
procedure of actually finding the absolute age of a substance is
called radiometric dating. To understand radiometric dating, we
must understand the meaning of the half life of an isotope. This is
the time it takes for one-half of the parent isotope to decay into a
more stable daughter isotope.
For example, the half-life of potassium-40 is 1.3 billion years. This
means that if a rock originally contained l0g of potassium-40 (parent
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isotope), after 1.3 billion years, one-half of the potassium-40 (5g in
this case) would have decayed into argon-40 (the stable daughter
isotope). In another 1.3 billon years, one-half of the remaining 5g of
potassium-40 (or another 2.5 grams) would have decayed to argon40. By now there would be only 2.5g of potassium-40 left and 7.5g
of argon-40 would be present in the rock sample.
Using another isotope commonly used in radiometric dating, when a
plant or animal dies, the carbon-14 in it decays to the more stable
carbon-12 isotope. The half life of carbon-14 is 5730 years.
Therefore, if an animal had 68g of carbon-14 in it when it died, after
5730 years, 34g would still be carbon-14 and 34g would be carbon-12.
After another 5730 years, only 17g of carbon-14 would be present in
the remains but 51 g of carbon-12 would be present.
The half-lives of other radioactive parent isotopes are given in
Table 8.1 on p. 231 of the text.
Let's set up a table showing the fraction of the parent isotope
remaining after each half-life.
1st
Half-Life
Fraction of parent 1
isotope remaining 2
2nd
3rd
4th
1
4
1
8
1
16
The data in this table has been graphed in Figure 8.13 on p. 230 of
the text. Note that the graph is exponential.
How did we get these fractions? Let's take a specific example.
Suppose a plant contains 100g of carbon-14 in it when it dies. After
5730 years (1 half-life), 50g of this 100g will have decayed to carbon12. Since 50g of the original 100g of carbon-14 is now present, the
50 1

100 2 .
fraction of parent isotope remaining is
After 11460 years (2
half-lives), 25g of the 50g of carbon-14 will have decayed to carbon7
12. Since 25g of the original l 00g of carbon-14 is now present, the
25 1

100 4 .
fraction of parent isotope remaining is
After 17190 years (3
half-lives), 12.5g of the 25g of carbon-14 will have decayed to
carbon-12. Since 12.5g of the original 100g of carbon-14 is now
12.5 125 1


100 1000 8 .
present, the fraction of parent isotope remaining is
After
22920 years (4 half-lives), 6.25g of the 12.5g of carbon-14 will have
decayed to carbon-12. Since 6.25g of the original 100g of carbon-14
is now present, the fraction of parent isotope remaining is
6.25
625
1


100 10000 16
Sample Test Question
How many half-lives have passed when 1/64 of the parent isotope
remains?
(A) 3
(B) 4
(C) 5
(D) 6
Let's set up a table comparing the ratios of parent to daughter isotopes
after each half-life.
Half-Life
Parent:Daughter
1st
2nd
3rd
4th
1:1
1:3
1:7
1:15
How did we get these ratios? Let's take a specific example. Suppose
plant contains 24g of carbon-14 in it when it dies. After 5730 years
1 half-life), 12g of this 24g will have decayed to carbon-12. Since 12g
of carbon-14 and 12g of carbon-12 are present, the ratio of parent
isotope to daughter isotope is 12:12 or 1:1. After 11460 years (2 halflives), 6g of the 12g of carbon-14 will have decayed to carbon- 12. We
now have a total of 6g of carbon-14 and 18g (12+6) of carbon-12, so
the ratio is 6:18 or 1:3. After 17190 years (3 half-lives), 3 g of the 6g
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of carbon-14 will have decayed to carbon-12. We now have a total
of 3g of carbon-14 and 21 g (12+6+3) of carbon-12, so the ratio is
3:21 or 1:7. After 22920 years (4 half-lives), 1.5g of the 3 g of
carbon-14 will have decayed to carbon-12. We now have a total of 1.
5 g of carbon-14 and 22.5g (12+6+3+1.5) of carbon-12, so the ratio is
1.5:22.5 or 15:225 or 1:15.
Sample Test Question
1. Calculate the age of a rock using K - 40 = Ar - 40 (half - life
1.3 billion years) if you know that 12.5% of the parent
material now remains in the sample. (Show your workings.)
2. How much radioactive parent isotope will be left after 3 halflives? Express your answer as a percentage.
3. A radioactive parent isotope has a half-life of 1000 years. If the
initial quantity was 16 grams, what quantity of parent isotope
will be left after 4000 years?
4. A mineral contains 60g of daughter isotope and 20g of
parent isotope. The half-life of the isotope is 870000 years.
Estimate the age of the mineral sample.
5. Which material is the best choice to test for a rock thought to
be about 400 million years old (the half-lives are in
parentheses)?
a) Carbon-14 (5730 y)
b) Thorium-232 (14 billion y)
c) Uranium-238 (4.5 billion years)
d) Uranium-235 (704 million y)
Sources of Error in Radiometric Dating
 Rocks could undergo metamorphism which would destroy
the parent material and reset the radioactive clock.
 Material could be added to the rocks by hydrothermal fluids.
If this material is selected and dated, the age of the rock
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



would be incorrect.
Material could be removed to the rocks by leaching. This
would change the ratio of parent to daughter material and
result in the incorrect age for the rock.
Sedimentary rock can be made up of many different types of
sedimentary of different ages. Dating this type of rock
correctly would be very difficult.
Carbon-14 can only be used to date living or once living
things. Trying to date rocks using the wrong radioactive
material could result in the incorrect age.
Using a radioactive material with a half-life that is too long
or two short may result in the incorrect age.
Activity: Radiometric Dating Problems Worksheet, Edukit
STSE: Labrador Zircons and Their Link to Radiometeric Dating
and Absolute Time
Lab-Aids: Introduction to Radioactivity and Half-Life Experiment
Lab: Half-Life
Television Episode: Geologic Journey – Canadian Shield
Do #'s 1, 7, 8, 9, 12, 13, p. 241 text.
Read pp. 236-240 for next day
Objectives:
1. Describe the progression of life forms from Precambrian time
through the Paleozoic, Mesozoic, and Cenozoic eras. pp. 236-240
2. Explain the correlation between biological evolution and the
physical and chemical changes in the hydrosphere, atmosphere
and geosphere. Notes
3. Examine current theories that attempt to explain mass
extinctions. p. 238 and Internet.
4. Use mass extinctions as a context to trace the development of a
scientific theory using relevant examples of how major shifts
occur in the scientific world view as a result of testing, revising or
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replacing theories and how this leads to a better understanding of
the universe.
The Progression of Life Forms
Precambrian time: - (about 4 billion years) we know little about
Precambrian time because plate tectonics, erosion, and deposition
have destroyed much of the rock record. Few of the rocks contain
fossils and the rocks themselves are greatly metamorphosed and
deformed, eroded, and covered by other rock layers. This was a time
of primitive single-celled organisms initially inhabiting an oxygenfree environment with extreme temperatures and UV levels.
Transition from Precambrian to Paleozoic: - during this period,
oxygen levels were increasing. Early animals lacking skeletons
appeared.
Paleozoic era: - (about 325 million years) a time of abundant
invertebrates and the appearance of land plants, amphibians, and
reptiles. The early part of the Paleozoic era is known as the Age of
Invertebrates, the middle part as the Age of Fishes, and the late part as
the Age of Amphibians (see Figure 1.7 p. 10 text). Trilobites were
dominant in the early Paleozoic era. The first part of the late
Paleozoic era saw the formation of Gondwanaland and Laurasia. The
last part of the late Paleozoic era saw the formation of Pangaea.
Mesozoic era: - (about 180 million years) the time of continued
development of marine and land invertebrates. This era is known as
the Age of Reptiles (see Figure 1.7 p. 10 text). Dinosaurs are the
dominant land vertebrate. Mammals, birds, and flowers appear.
Cenozoic era: - (about 66 million years) This era is known as the Era
of Recent Life. It is characterized by the continued development of
invertebrates, domination of the land by mammals, birds, flowering
plants, and the emergence of hominoids over the past 4 million years.
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In the space below, construct a pie graph showing the various
amounts of time represented by each era.
Sample Exam Questions
1. Which era was dominated by dinosaurs?
(A) Cenozoic
(B) Mesozoic
(C) Paleozoic
(D) Precambrian
2. Rocks from which era would contain fossils of trilobites?
(A) Cenozoic
(B) Mesozoic
(C) Paleozoic
(D) Phanerozoic
Activity: Create a Geological Time Scale to scale
Activity: Evolution through Geological Time Student Worksheet,
Topic 1, Unit 4, Edukit
Examples of the Interdependence of the Biosphere,
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Hydrosphere, Atmosphere, and Geosphere.
1. The development of plants added oxygen to the atmosphere.
2. The distribution of marsupials is correlated to the movement of
the tectonic plates.
3. Glaciation and the variation of the seasons cause changes in life
forms.
4. Catastrophic events such as meteorites and volcanoes are at least
partly responsible for the extinction of the dinosaurs and the
evolution of mammals.
Theories of Mass Extinction
The divisions of the time periods in the geological time scale
represent times when the earth and the organisms living on/in it
changed significantly. For example, the division between the
Mesozoic and Cenozoic eras (Cretaceous – Tertiary Boundary)
about 65 million years ago coincides with the extinction of the
dinosaurs. The Paleozoic era (Permian – Triassic Boundary) ended
about 245 million years ago with the great Paleozoic extinction.
Organisms such as Trilobites became extinct. These changes are
sometimes indicated when fossils in the rock record suddenly
disappear. The causes of some mass extinctions are unknown and
some are the result of a single or a series of events. Some of these
theories are briefly described at
http://www.factmonster.com/ce6/sci/A0832139.html
See asteroid impact here and mass extinction movie here
Activity: A Geo TV Special – What Caused Mass Extinctions?,
Topic 3, Unit 4, Edukit
Do #'s 17-19 p. 242 text.
Read pp. 413-434 for next day.
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