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Time and Geology
Chapter 8
Where would you hike to find the oldest rocks in this area?
(hint : you would use the principle of superposition)
Tasks
1. Read about relative ages on pages 179-190 (skip the sections titled “Correlation
by fossils” on page 189). Note that we will be learning about numerical ages
from the special text that I have added to this lecture.
2. Go to the discussion board to ask and answer a question.
3. In preparation for quiz 8, go to the content section of D2L for this chapter and
click on “Preparation for quiz…” to answer questions related to chapter 8. You
will find the answers to these questions in material presented in this file and in
the text book.
4. Prepare yourself a series of note cards with sketches to study with. Use these to
help you get to know all the material without looking at your notes.
5. Take the quiz. You may prepare for the quiz by doing items
1-4. You should know the material without looking at your notes. See class
calendar for quiz date.
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How do we assess the ages
of rocks?
In this chapter we will be concerned with how geologists assess the ages
of rocks. There are two types of ages, relative and numerical, and they
are assessed in different ways:
Relative age – A relative age is an age relative to something else. A
relative age assessment involves the ordering of events or objects, from oldest to
youngest. This requires knowledge of several geologic concepts and principles.
Numerical age – A numerical age is an age that is expressed as a number
or numbers (e.g., 10 million years old or 3.6 billion years old). A numerical age
is assessed by isotopic dating (determining how much radioactive decay of a
specific element has occurred since a rock formed or an event occurred)
Relative Age Determination
Because only a few rock types are amenable to isotopic dating
methods, geologists often assess the relative ages of rocks.
Assessment is relatively simple once you gain knowledge and
understanding of the principles and concepts listed below. Much
of this is geologic common sense, as you will see.
Principles Used to Determine Relative Age
•
•
•
•
•
original horizontality
superposition
lateral continuity
cross-cutting relationships
Other time relationships
• inclusions
• Metamorphosed (baked) contacts
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My guess is that, even before we discuss the concepts and principles mentioned on
the previous slide, you have already learned enough in this class to begin to assess
relative ages. For example, the photo below shows three rock bodies: plutonic
rock (marked pluton) and two dikes (dike “x” and dike “y”). Can you list the three
rock bodies in terms of their relative ages? Let 1 = oldest and 3 = youngest. A
quarter is shown for scale.
pluton
3=
Dike “Y”
pluton
2=
1=
pluton
Dike “Y”
pluton
Principles Used to Determine Relative
Ages
• Contacts - surfaces separating successive rock layers
(beds)
• Formations - bodies of rock of considerable thickness
with recognizable characteristics allowing them to be
distinguished from adjacent rock units
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Contacts are surfaces separating two different rock types or
ages of rocks. We will consider two types of contacts: depositional
and intrusive.
•A depositional contact is a contact between sedimentary formations (which are
groups of beds), beds, or extrusive volcanic rocks. The “bottom line” is that a
depositional contact is a contact between rocks that have been deposited on
other rocks.
•An intrusive contact is a contact between an intrusive igneous body (pluton,
dike, sill) and the country rock.
This sketch shows strata
(group of beds) overlain by a
basalt flow. All of the contacts
between the different rock types are
depositional contacts. The red
arrows highlight all the contacts.
Original horizontality - beds of sediment
deposited in water are initially formed as horizontal or
nearly horizontal layers
Superposition - within an undisturbed sequence
of sedimentary or volcanic rocks, layers or beds get
younger from bottom to top
Lateral continuity – an originally horizontal
bed extends laterally until it tapers or thins at its edges
Question
List the sedimentary formations shown in the diagram above from oldest (#1) to youngest
(#4).
4=
3=
2=
1=
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Cross-cutting relationships - a disrupted pattern is
older than the cause of the disruption
—Intrusions and faults are younger than the rocks they cut through
Questions
On the basis of the principles and concepts
we have discussed, list the rocks in terms of
their relative ages from oldest (#1) to
youngest (#5). (you may assume the
sedimentary rocks predate the pluton)
5=
4=
3=
2=
1=
What principle did you use to determine the
relative ages of #’s 1-4 on your list?
What principle did you use to determine the
relative age of #5 on your list?
Exercise
Study the block diagram and then list the rock formations (including the pluton) from
oldest (#1) to youngest (#10). Hint: there are two basic groups of rocks: a pluton and
strata. The sedimentary rocks are divided into formations that are labeled with red
letters. To figure out the relative ages of the sedimentary formations you will need to
visualize them un-tilted and then use the principle of superposition to assess their ages.
10 =
9=
l
m
n a
b
c
8=
z
y
x
7=
6=
5=
4=
This block diagram shows tilted strata
intruded by a granite pluton and associated
dike and sills. The contact between the
intrusive rock and the country rock is an
intrusive contact.
3=
2=
1=
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Other time relationships
Inclusions are rock fragments embedded in host rock that
are older than the host rock
d
v
a
k
Inclusions are another thing that we can
use to determine relative ages of rocks. Two
important types of inclusions are xenoliths
and pebbles of rock derived by erosion of an
underlying formation. Study the adjacent
cross section and then answer, solely on the
basis of inclusions, the following questions.
What do the xenoliths tell you about the
relative ages of the tilted strata vs. the
granite?
What do the pebbles of the granite in
formation “k” tell you about whether the
pluton intruded before or after the deposition
of formation “k”?
Metamorphosed (baked )contacts – refers to a contact
between an igneous intrusion and surrounding rocks, wherein the
surrounding rocks have experienced contact metamorphism
If we see an intrusive igneous body, how do we
know whether the contact between the intrusion and
the surrounding rock is intrusive or depositional?
One important way is to see if the surrounding rocks
are metamorphosed in a contact aureole; if they are
then you are looking at an intrusive contact. If the
rocks are not metamorphosed then you are looking
at a depositional contact. The adjacent cross section
shows a pluton where all contacts are intrusive
except for the part that I have highlighted with a red
dashed line. We know this contact is not intrusive
because the rocks above it are not metamorphosed.
Please remember (for the remainder of this class), and understand ,the following
definition of a cross section. A cross section shows what the rocks would look like beneath
the Earth’s surface (as if you made a vertical cut through the crust and removed the rock on
one side of the cut so that you could see what is below the surface).
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Exercise
Determine the relative ages of rocks shown in the block diagram below.
You will need to use your knowledge of the principles and concepts that we
have just covered. There are ten different rock bodies. You may check your
answer in the book.
Unconformities
An unconformity is a contact that represents a gap in the
geologic record. Generally, there is a substantial amount of time
not represented by rocks across an unconformity.
•Unconformities are a special type of depositional contact.
•Unconformities are useful for assessing the geologic history
of an area
•There are three types of unconformities: disconformity,
angular unconformity, and a non-conformity
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Disconformity - an unconformity in which the contact representing
missing rock layers separates beds that are parallel to each other. The
sketches below illustrate how a disconformity commonly forms.
g
f
e
d
c
b
a
c
b
a
Deposition of a-g
Erosion of d-g
i
h
c
b
a
Deposition of h and then i
Angular unconformity - an unconformity in which the contact
separates overlying younger layers from eroded tilted or folded layers
Exercise
The adjacent diagram shows an example of how an
angular unconformity may form. Describe, in your
own words, the sequence of events necessary to form
an angular unconformity.
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Nonconformity - an unconformity in which an erosional surface on
plutonic or metamorphic rock has been covered by younger sedimentary or
volcanic rock
• Plutonic and metamorphic rocks are exposed by large amounts of erosion
• Typically represents a large gap in the geologic record
Exercise
The dashed red line denotes the approximate
location of a famous nonconformity in the Grand
Canyon. The layering you see above the
nonconformity represents bedding. The
metamorphic rocks below the nonconformity do
not appear layered. Describe, in your own words,
the sequence of events necessary to form this
nonconformity.
Question
R
T
X
A
G
This is a photo of the
entrance to Dunbar Cave.
The layers of rock that you
see are limestone beds. Please
determine which of the beds
that I have labeled with white
letters is the oldest and which
is the youngest?
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Exercise
y
x
Determine the relative
ages of the beds at points
x, y, and z.
z
Exercise
A cross section of the Grand Canyon area is shown below. List the Navajo
Sandstone, Vishnu Schist, Coconino Sandstone and Bright Angel Shale in
chronologic order from oldest to youngest. Also, indicate the type of unconformity
that the black arrow is pointing to.
4=
3=
2=
Bright
Angel
Shale
1=
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The Standard Geologic Time Scale
•Geologic time is divided into the Precambrian and the
three eras: Paleozoic, Mesozoic and Cenozoic. The three
eras are further subdivided into periods. Precambrian is a
generalized term that denotes the vast amount of time
preceding the Paleozoic era and it is not divided into eras
or periods.
•This timescale subdivides geologic time based on fossil
assemblages. Sedimentary rocks from each period are
defined by unique fossils of organisms that evolved and
went extinct in that particular period. Consequently, if you
find the unique fossils in the rocks you can assign a
relative age to the rocks.
Precambrian
•This timescale expresses relative (not numerical) geologic
time
Questions
Disconformities are generally found by recognizing that strata of a certain
period are missing. Can you find the disconformities in the cross sections
showing sedimentary formations below?
Tertiary
Quaternary
Cretaceous
Triassic
Jurassic
Permian
Mississippian
Pennsylvanian
Devonian
Mississippian
Silurian
Devonian
Cambrian
Precambrian
What geologic era are we living in today?
What geologic period are we living in today?
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Isotopic Dating
Isotopic dating utilizes radioactive isotopes to place numerical ages on rocks. It is mostly used
for dating igneous and metamorphic rocks that contain minerals that are amenable to isotopic dating.
The methods of isotopic dating are somewhat complicated, but our goal here is to show you how it
works in principle. Your task is to read the following discussion about isotopic dating and then answer
the questions at the end. However, before we talk about dating, you need to review some chemistry that
we discussed in chapter two on minerals. Please retrieve your notes from Chapter two and answer the
following questions:
What defines an element? What is an isotope of an element? Isotopes are defined by mass number. For example, there is an isotope called U238 where 238 is the mass number. What is the mass number of an isotope? Minerals that contain radioactive (unstable) isotopes may be used to date the time a rock formed.
Most isotopes are stable, but radioactive isotopes are unstable because they lose or gain protons
overtime and will ultimately decay into a stable but different element. A radioactive isotope is often
called the “parent” and the stable isotope that it decays into is called the “daughter product”.
How can we use minerals that contain a radioactive isotope to date a rock? The best way to
explain this is to proceed by example using the uranium (U238)-lead (Pb206) method, which is
commonly used for dating igneous rocks. The radioactive isotope U238 will decay into a stable isotope
of lead (Pb) called Pb 206. First, we will explore some of the basics about the conversion of U238 to
Pb206 and then we will discuss how this phenomenon can be used to date a rock.
The isotope U238 has 92 protons and
146 neutrons (92 + 146 = 238 = mass
number) and is radioactive (Figure 1).
Overtime an atom of U238 will lose 10
protons and 22 neutrons and will turn into Pb
206, which has 82 protons and 24 neutrons
(82 + 124 = 206 = mass number). Because
this decay process involves changing the
Figure 1. Diagram illustrating the decay of U238 to Pb 206.
atomic number from 92 to 82, the element
changes from U to Pb. The isotope Pb206 is unique because it only forms
from the decay of U238.
For igneous rocks, the U-Pb method is typically used to date a
mineral called zircon that grew at the time the rock crystallized. Zircon
occurs in trace amounts in igneous rocks (Figure 2) and it contains the
radioactive isotope U238. When zircon forms it contains some U238 but no
Pb 206. Zircon does not incorporate the element lead during growth because
Figure 2. Example of tiny zircon grains that a geologist has mechanically separated from granite (the penny is for scale). 12
it is not part of its chemical structure. However, after the mineral is formed Pb206 will be produced by
decay of U238, and this Pb will be locked inside the zircon. Consequently, as time goes on the amount
of U238 decreases and the amount of Pb206 correspondingly increases. The decay of U to Pb can be
used to date the rock because we can measure the amount of U remaining and the amount of Pb
produced (this is done by a device called a mass spectrometer), and we know the rate of decay, i.e., the
rate at which U turns into lead. In summary, these basic assumptions and techniques are used in dating:
(1) No Pb is assumed to exist in the mineral when it formed
(2) The amount of U238 decreases over time and the amount of Pb206 increases
(3) We know the rate of decay and we can measure the amount of U and Pb in the mineral using
a device called a mass spectrometer
We will now discuss the basic idea behind producing an isotopic age. The most important thing
to understand is the rate of decay, which is measured in terms of half-life. A half life is the time it takes
for a given amount of radioactive isotope to be reduced in half. The graph in Figure 3 illustrates the
concept of half life. The percent of original
radioactive isotope is on the vertical axis and time
in terms of half life is on the horizontal axis. The
black line shows the amount of radioactive isotope
remaining and the red dashed line shows the
growing percentage of stable daughter product.
When the mineral first forms there is 100 % parent
and 0% daughter. After one half life there is 50%
parent remaining and 50 % has decayed to the
daughter product. After two half lives there is 25%
parent remaining and 75 % has decayed to the
Figure 3.
daughter product. From the graph you can see that
a sample whose age is equal to one half life would have a ratio of parent to daughter of 1:1. When this
sample is two half-lives old the ratio of parent to daughter would be 1:3. Because the half life of a
radioactive isotope is known, all we need to
Time when mineral Time after 1 half life Time after 2 half formed
has elapsed
lives have elapsed
do to place an isotopic age on a mineral or
Age
0 b.y.
4.5 b.y.
9.0 b.y.
rock is to measure the amount of parent and
Amount of daughter and look at their ratios. To better
1000 U
500 U
250 U
parent present
visualize this concept lets imagine that we
Amount of 0 Pb
500 Pb
750 Pb
daughter present
have a zircon crystal with 1000 U238 atoms
and see what happens to the amount of U238
Figure 4
over time (Figure 4). U238 has a half life of
~4.5 billion years (b.y.). When the mineral first forms there are 1000 U atoms and no Pb atoms. After
4.5 b.y. there are 500 U atoms present and 500 U atoms have converted to Pb, hence the ratio of U to Pb
is 1:1. After another half-life has elapsed and the rock is 9.0 b.y. old, there are 250 U atoms remaining
and 750 Pb atoms have been produced by decay of U. Thus after two half-lives the ratio of parent to
daughter is 1:3. Although we have only discussed a few ratios of parent to daughter, it is possible to
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mathematically assess an age for any ratio if you
have been given the half life and the ratio of parent
to daughter.
Although we have focused on the U-Pb
method of dating, there are many other radioactive
isotopes that can be used for dating. These other
methods are based on principles that are similar to
what we have described for U-Pb. One example of
a common method of dating is the K-Ar method. This involves the decay of a radioactive isotope of
potassium (K) to a stable isotope of argon (Ar).
Figure 5. Photographs of muscovite and orthoclase, which are two K‐bearing minerals used for K‐Ar dating. Do you know which Minerals containing K are used for this method.
picture is of muscovite and which is of orthoclase?
For example, it is commonly used for orthoclase
and muscovite (see Figure 5) in igneous rocks.
Questions
(please answer the following questions based on what you have just read)
(1) What is the difference between a stable and a radioactive isotope?
(2) What is the meaning of the terms parent and daughter product?
(3) Explain what happens to an atom of the radioactive isotope U238 over time.
(4) If a granite sample has 5000 atoms of U238, how many are remaining after 1 half life? How many are
remaining after 2 half lives?
(5) If a granite sample has 400 atoms of U238, how many Pb206 atoms have been produced after 1 half life?
How many have been produced after 2 half lives?
(6) Explain in your own words the reasons why zircon can be used for isotopic dating.
(7) Imagine you have a sample with 500 atoms of radioactive isotope X. Isotope X decays to stable isotope
Y with a half life of 2 million years. How much time will it take to produce 375 atoms of isotope Y?
Hint: to solve this problem you might want to construct a diagram similar to that shown in Figure 4.
(8) Imagine you have a sample with 36 atoms of radioactive isotope W. Isotope W decays into stable isotope
Z with a half life of 1 million years how much time will it take for 27 atoms of W to decay to Z? Hint: to
solve this problem you might want to construct a diagram similar to that shown in Figure 4.
(9) Name an example of a mineral that is commonly used for the U-Pb method of dating and a mineral that is
commonly used for the K-Ar method of dating.
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Things we have covered
•Relative vs. Numerical ages
•Concepts and principles used for relative age determination: contacts, original
horizontality, superposition, lateral continuity, cross-cutting relationships, baked contacts,
inclusions, unconformities (angular unconformities, nonconformities, disconformities)
•How to view rocks and cross sections and determine the relative ages of rocks and to
delineate the various types of unconformities
•The eras and periods of the geologic timescale and what the geologic time scale is based
on
•Atomic number vs. mass number and stable vs. unstable isotopes
•The principles behind isotopic dating (decay, half-life etc.) utilizing the U-Pb method as
an example
Next up: Now that you have reviewed this lecture file and read the chapter, please
go to the content section of D2L for this chapter and click on “Preparation for quiz
…” to answer questions related to this chapter. You will find the answers to these
questions in material presented in this file and in the text book.
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