Cycle 7
How Do We Know About Earth’s History?
OVERVIEW
In previous cycles, we investigated Earth systems and processes of change. We have not
established an age to the earth or the relative time distances between these events. The ability to
put ages on events and ultimately create a timeline has helped all scientific disciplines further
their understanding of biotic and abiotic systems. To begin our investigation of the age of the
earth, we must first investigate our initial ideas regarding the topic.
INITIAL IDEAS
How old is the Earth? On your own, write you answer below. If you have specific evidence for
your answer, be sure to explain it in detail.
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With a partner, organize the following list of major events in Earth history in order from oldest to
most recent. You will have cards at your table with these events listed on each to organize and
reorganize as you discuss. Once you have a final order, record it below.
NOTE: There is some discussion about the value of having students come up with a couple of
events in Earth’s History to add in to the timeline. Is it worth the extra time to allow some
student-generated ideas in this activity?
Extinction of
Dinosaurs
First Homo
Sapiens fossils
formed
First oxygen in
the atmosphere
Beginning of
Earth
First fossilized
skeletons formed
Your instructor is
born
NOTE: Are these the events that we will be discussing throughout the unit? Will students be
able to correctly place these particular events on a timeline or are there other events that we
should choose?
Final order:
1.
2.
3.
4.
5.
6.
What evidence did you discuss to help you formulate your order?
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Thinking about the age of Earth as an hour, with 0 minutes as the beginning of the Earth and 60
minutes as now, write the time that each took place.
Event
1.
2.
3.
4.
5.
6.
Time
NOTE: Should we have students place the cards on a string to represent the time from the
beginning of the Earth to the present? This could then be revisited as students move through the
cycle and eventually students would see that most of what we think of as Earth History has
happened in the recent past.
Group
Discussion
Place your ordered events and time onto a whiteboard and be ready
to share out with your class.
ACTIVITY 1: Relative Dating& Biostratigraphy
PURPOSE
Q. How do we know that the Earth has changed over time?
Do we also need initial ideas about dating (e.g. relative, absolute, etc.).
PART 1: Relative Dating
PART 2: Using fossils for relative dating and correlation of rock units
PURPOSE:
In Part 1 of this Activity, we saw that stratigraphy allows us to determine the relative ages of
sedimentary rocks. However, we can also use fossils to help us with relative ages. Throughout
time, the remains of living organisms have been buried and included in sedimentary rocks. They
are then preserved as fossils. If we can observe and describe the fossil evidence, then we can
make inferences about relative ages of rock outcrops in different locations. In this activity, we
will use fossils to match ages of rock outcrops separated by large geographic distances.
Q. Need Question Here
INITIAL IDEAS
We can use the type of sediments in sedimentary rock to say something about the environment
where those sediments formed. What are some places where you have seen sediments being
deposited?
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Imagine sediments being deposited on the deep ocean floor. What kind of organisms or remains
of organisms might you find in these sediments?
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Imagine you are examining some sedimentary rock layers and you infer that they formed on the
ocean floor. You see skeletons of microscopic marine organisms (plankton) in some of these
layers. How do you think those fossils got there?
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(TEACHER NOTE: How did something so delicate as the skeleton of plankton get preserved in
this rock? Students might think that small fragile remains are unlikely to preserved)
We know how to compare one layer of rock to another from Part 1 of this activity. How do you
think scientists could use the fossil remains in the rock to compare layers in the rock?
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SCIENTISTS’ IDEAS
Marine sediments are deposited at the bottom of the ocean; while at the surface of the ocean live
small plankton that often have small, thin shells. Some species of plankton can be found over
vast areas of the ocean surface because the environment is uniform and ocean currents can
transport the plankton long distances. When plankton die, their shells settle to the seafloor to
become part of the sediment. Despite being thin and small, the shells of these plankton can be
preserved in the low energy environment of the seafloor.
COLLECTING AND INTERPRETING EVIDENCE
Let’s say we want to compare layers of marine sedimentary rock found in Colorado with marine
layers in Texas. Can we determine whether they are the same age? (assuming deposition has
been continuous)
=
Texas
?
Colorado
Do you think it is possible to connect the rock outcrops from Colorado to those in Texas?
Explain your thinking.
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For what reasons might a particular fossil species appear in the middle layers and not be found in
the lowest layers or the top-most layers? See if you can think of 2 or 3 reasons.
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Imagine that a particular fossil species is found in all the layers from both states. Can you infer
that the formations are the same age? Why or why not?
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What do you think would be required of an organism and its fossil remains so that it could be
used to compare the ages of two distant outcrops?
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Fossils that can be used to indicate a narrow band of time are called “index fossils”. The process
is called “biostratigraphy” because biological remains allow us to link rocks that are otherwise
very different in location or appearance.
Texas
Dinoflagellate “U”
Colorado
You are looking at the outcrops in Texas and Colorado, and you find dinoflagellate fossils in
sedimentary layers only above the white line in the Texas outcrop, but only below the white line
in the Colorado outcrop. How could the same tiny fossil be found in both outcrops separated by
such great distance?
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Can we infer that the rock outcrops are the same age? Explain.
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Sediments usually contain more than one fossil type and it is these fossil “assemblages” that give
confidence that the correlation is meaningful.
Consider another location (called Mesa) that has been well studied. It has many planktonic
marine fossils that have been thoroughly described. The “Mesa” location records a continuous
sequence (no pauses in sedimentation) of sedimentary rock with 7 fossils. Below is a
diagrammatic sketch of this outcropping. The bottom row corresponds to the oldest occurrence
and the letters will be used to refer to the fossils in the questions that follow.
T
U
V
W
X
The diagram above represents a beachside cliff. There are 18 layers each
with their own fossil types.
Y
Z
Use the presence or absence of fossils in each layer in the diagram above to answer the following
questions.
Based on the diagram, would you expect to find fossil type “V” in the same layer as
dinoflagellate fossil “U” in the Mesa outcrop? How do you know?
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Now, given what is observed at the Mesa outcrop, can you predict what fossils are likely to be
found with dinoflagellate “U” in the Texas outcrop (note that the white line is the uppermost
layer that the dinoflagellate is found)?
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Can you predict which fossils are likely to be found with dinoflagellate “U” in the Colorado
outcrop?
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Why might the fossils found in the Texas outcrop differ from those in the Colorado outcrop?
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By comparing marine rocks from many different locations scientists determine that both species
X and Z are very geographically widespread. Does X or Z make the better fossil to compare
ages of outcrops? Why?
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How many layers contain both T and U?_____, both V and W? _____, both T and Z?_____ .
Look back at the diagram of the MESA outcrop. If each layer represents the same amount of
time what combination(s) represents the least amount time?
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We think the question below is unnecessary since students will have a grasp of relative dating
from Part 1 of this activity.
Notice that no absolute age (years before present) is indicated by the position of the fossils in the
“Mesa” table. How then, might index fossils be helpful for constructing a history of Earth?
We think that the portion below can be removed since the previous 2 pages already address the
ideas that organisms can be used to make inferences about relative ages of outcrops in 2
geographically different locations and how also how scientists use this to infer time.
Remember that the Texas and Colorado outcrops both contain fossil “U”. A third outcrop in
Nevada is found to contain fossils of T, V, W and X.
Fossils found in the Nevada outcrop are displayed in the table below.
T U V W X Y Z
Nevada
1
2
3
4
5
6
7
8
t
t
t
t
v
v
v
v
v
v
v
v
w
w
w
w
w
w
x
x
x
x
x
x
x
x
Is the Nevada outcrop the same age as outcrop A and B? Explain your thinking.
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How many years do you think elapsed between deposition of the bottom of layer 8 in the Nevada
outcrop and the top of layer 1? Explain your reasoning.
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Notice that fossil X appears in every layer of the Nevada outcrop, but does not appear in every
layer of the “Mesa” location. What do you know about plankton or fossilization that might
account for this difference?
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Now let’s consider only a few layers within an outcrop.
Below are the fossil assemblages for some of the layers in the Texas outcrop and the Colorado
outcrop. The tables show only fossils in the layers that also contain “U”. The presence of a
fossil in a layer is indicated by the letter in the layer.
Texas
Colorado
Dinoflagellate “U”
T U V W X Y Z
T U V W X Y Z
1
2
3
4
5
t
u
u
u
u
u
x
x
x
x
x
y
y
y
y
y
1
2
3
4
5
u
u
u
u
u
z
z
z
x
x
x
y
Can you infer that the two layers are exactly the same age? Why or why not?
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Which fossils are most helpful in determining relative age? Why?
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By comparison with the “Mesa” data, can you infer which fossils would you expect to find in the
exposed rocks below fossil U in the Texas outcrop? Explain your reasoning.
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Can you infer which fossils would you expect to find in the exposed rocks above fossil U in the
Colorado outcrop? Explain.
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SUMMARIZING QUESTIONS
Why might biostratigraphy have been important to geologists and paleontologists before the
discovery of absolute dating techniques (using the decay of radioactive isotopes for example)?
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Why might biostratigraphy still be useful today?
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List the important concepts supporting the process of biostratigraphy.
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Write a short explanation of each of the concepts you listed above.
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Scientists Ideas
Brief note on geologic time scale (names of periods w/o dates)
ACTIVITY 2: Early Attempt at Absolute Dating of Age of Earth
The intro to this activity needs to be fleshed out.
So far we have worked with relative dating where we know one layer is older than another
because of their position.To get an absolute date for the age of Earth you need to look as specific
datasets with known rates. In this activity you will look at three specific data sets;
Sedimentation, Salinity, and Heat Loss to try to determine an actual number of years for the age
of Earth.
Purpose
To understand historical methods used to determine the absolute age of the Earth.
Q. How can we use absolute dating to determine the age of the Earth?
Initial Ideas
How old do you think Earth is?
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How do you think scientists determine the absolute age of Earth? What evidence or examples do
you have to support your thinking?
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This is as far as we got in today’s session
Experiment 1: A Jigsaw Activity – Early Attempts at Absolute Dating of the Age of Earth
Directions
For this jigsaw activity, you will be placed into Home Groups of 3 students. You will then join
a Specialty Group that will explore one of the three early methods for determining the age of
the Earth (Sedimentation, Salinity or Heat Loss). Your role in the Specialty Group will be to
become an expert on one particular method by completing the appropriate section below. Once
all your group members have become experts on the method used to calculate the absolute age of
the Earth, you will return to your Home Group. You will then share your ideas in this
heterogeneous group so that everyone in class understands the different methods used as early
attempts to give an absolute age to the Earth. After all group members have had the opportunity
to share their expertise you will complete the final section of this experiment discussing possible
sources of error. Finally, we will end this activity with a classroom discussion.
PART 1: Calculating the age of the Earth from Sedimentation and Deposition Rates
Recall in cycle 2 that you learned sediments can become sedimentary rock by the processes of
compaction and cementation.
Do you think there is a relationship between the rate of deposition of sediments and the thickness
of a sedimentary rock layer?
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In the late 1800’s numerous geologists attempted to date the age of the earth using rates of
deposition of sediments that eventually formed sedimentary rock. At this time it was widely
accepted by most geologists that the surface of Earth was formed by the same gradual geologic
processes that we see in action today (uniformitarianism). This was opposed to the idea that the
surface of Earth was formed by a series of sudden, short-lived, violent events (catastrophism). In
order to make any calculations based on deposition of sedimentary rock, it must be assumed that
there is some average rate of formation and therefore the idea of uniformitarianism must be
accepted for these methods to be valid. Today, uniformitarianism is the foundational principle
that geologists use to tell Earth’s story.
While many different attempts were made with varying levels of complexity, one relatively easy
example that attempted to answer the question of the age of Earth using sedimentary rock was
carried out by William Sollas from 1895 to 1909. He began by estimating the thickness of the
global sedimentary Phanerozoic rock layers (table below). He further assumed that the perPhanerozoic time period would be at least as long as the Phanerozoic time period. Sollas also
made the assumption (based on best estimates of the day) that it took 328 years to form 1 meter
of sedimentary rock.
Table 1
Eon
Phanerozoic
pre -Phanerozoic
Era
Cenozoic
Mesozoic
Paleozoic
Thickness
19,450 meters
21,050 meters
36,900 meters
77,400 meters
Using the rate of sedimentary rock formation (328yrs/meter) and the thickness of sedimentary
rock layers from the table above, see if you can calculate the age of Earth.
Calculate the overall thickness of sedimentary rock that Sollas measured.
Total thickness (in meters) of sedimentary rock: _______________________
His calculation for the age of Earth was:
________________ meters X ____________ years/meter
= ________________ years
50,774,400 years
With this information, Sollas was able to estimate the age of the Earth to be around 51 million
years.
Can we determine a minimum age of the Earth from the sequence of rocks in the Grand Canyon?
The Grand Canyon is made up ofmany
layers of rock that are several
kilometersthick. Fossils in these rocks
and other correlational arguments
suggest that rocks at the bottom of the
Grand Canyon were formed in the prePhanerozoic and rocks at the top were
deposited in thePermianPeriod. This
means that the rocks were deposited
over a very long span of time. But can
we use the principles that Sollas used to
come up with a minimum of just how
much time it took for these layers to be
formed? Can this small portion of Earth
history give us a minimum age of
Earth?
The diagram and table below contains
information about the layers of rock in
the Grand Canyon. Use this
information to complete the activities
below.
Rock
Unit
6d
Formation
6a
Kaibab
Limestone
Toroweap
Coconino
Sandstone
Hermit Shale
5d
Esplanade
6c
6b
Thickness
(meters)
100
75
100
100
350
Using the method that Sollas followed, complete the table below
and calculate a minimum age of Earth just from the Grand
Canyon sequence in the diagram on the previous page. Assume
328 years per meter is a typical rate of sedimentary rock
formation.
5c
Wescogame
5b
Manakacha
5a
4c
Watahomigi
Surprise
Canyon
Redwall
Limestone
Temple Butte
Limestone
Muav
Limestone
4b
4a
3c
3b
Bright Angel
Shale
3a
Tapeats
Sandstone
25
150
15
150
100
60
2
1b
1a
Rock Unit
6
5 Supai Group
4
3 Tonto Group
Rock
Subunit
6d
Formation
6c
Toroweap
6b
Coconino Sandstone
6a
Hermit Shale
5d
5c
5b
5a
4c
Esplanade
Wescogame
Manakacha
Watahomigi
Surprise Canyon
4b
Redwall Limestone
4a
Temple Butte Limestone
3c
Muav Limestone
3b
Bright Angel Shale
3a
Tapeats Sandstone
Kaibab Limestone
3700
Zoroaster
Granite
Vishnu Schist
Thickness (meters)
Unknown
Calculated Age
2 Grand Canyon
Supergroup
1 Vishnu Group
1b
1a
Zoroaster Granite
Vishnu Schist
Unknown
Calculated Age of Earth
Unknown
Now remember back to Activity 1 Part 1 where you explored the principles of relative dating. In Table 7x you described the principle of correlation. In reading about this topic, you saw that the classic example
of correlation is the sedimentary rock layers of the southwestern United States. Based on fossil
assemblages and other evidence, the Grand Canyon rock layers at the top of the canyon are very old, and
so younger rock layers must have been eroded away. But younger layers are present at Zion and Bryce
Canyons. That means that a correlated sequence of rock layers would put rock layers from Zion National
Park and Canyonlands on top of Grand Canyon Rocks, and rock layers from Bryce Canyon and Mesa
Verde National Park would be even above those. See the figure below that illustrates this
(http://pubs.usgs.gov/gip/geotime/section.html)
If the total thickness of rock layers from all those other southwestern locations adds up to another 1500 m.
This sequence of rock layers is known as the “Grand Canyon Staircase”. Adding these to your
calculations above, what is new age estimate for the deposition of the entire sequence of rocks?
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Reflection
Q1: Are you surprised by the numbers your group has arrived at in estimating the age of Earth
using the thickness of sedimentary rock deposits? Why or why not?
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Q2: What assumptions were taken into account in using the thickness of sedimentary rock as a
method to date the age of Earth?
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PART 2: Calculating the age of the earth using salinity data
Edmond Halley (1656 – 1742), of Halley’s comet fame, hypothesized that seawater became salty
with the infusion of salts dissolved from the ground. He proposed the dissolved salts were
carried down rivers to the sea and gradually accumulated, raising the salinity of seawater over
time.
In 1899, using Halley’s hypothesis, an Irish geologist, John Joly (1857 – 1933), proposed a
method for calculating the age of the oceans and perhaps the earth as well. Joly reasoned that if
the salinity of the oceans was known, along with the rate at which salt was being added by rivers
each year, the calculation could be done.
Activity:
See the sodium data and sample calculation below. The calculation uses Joly’s line of reasoning
to calculate the age of the earth using the data. Discuss the calculation with your group. Later,
you will use the same method of calculation using magnesium rather than sodium.
1)
2)
3)
4)
The volume of the world’s oceans is 1,370,000,000 km3
Sodium makes up approximately 1% of seawater by volume
The world’s rivers empty a volume of 30,000 km3/yr into the oceans
Rivers contain between 3 to 11 ppm (parts per million) Sodium by volume
Sample calculation:
1) Calculate the total volume of sodium in the world’s oceans. This would be 1% of the
total volume of water.
So, (.01)(1,370,000,000 km3) = 13,700,000 km3 salt in oceans.
2) Calculate how much sodium enters the oceans from the rivers annually. Since there is a
range of concentrations, calculate both high and low.
3ppm: (.000003)(30,000km3) = .09km3 salt entering oceans from rivers
11ppm: (.000011)(30,000km3) = .33km3salt entering oceans from rivers
3) Divide total volume of sodium in ocean by annual volume input. Calculate for high and
low river input to get a range.
13,700,000 km3 salt/.09km3 salt/year = ~ 152 million years
13,700,000 km3 salt/.033km3 salt/year = ~ 41 million years
See the data for Magnesium below. Use the sample calculation as a guide and perform the
simplified version of Joly’s calculation using Magnesium.
5)
6)
7)
8)
The volume of the world’s oceans is 1,370,000,000 km3
Magnesium makes up approximately .14% of seawater by volume
The world’s rivers empty a volume of 30,000 km3/yr into the oceans
Rivers contain approximately 4 ppm (parts per million) Magnesium by volume
Calculation:
Age estimate using magnesium ___________________________
Age estimate using sodium ____________________________
Reflection
1. Compare the values you got in your calculations with Na and with Mg. How close were
they? What inferences can you make regarding which ion is removed from rivers and
oceans at a faster rate? Brainstorm reasons for any differences you noted.
2. How close are the calculated values to your prior understanding of the age of the earth?
3. Identify any assumptions you can think of that this method of calculating the age of the
earth relies on for validity. Also indicate whether that assumption is correct or incorrect,
and explain your thinking in the last column.
Assumption
Correct/Incorrect
Salt stays put once in the ocean
Incorrect
Scientist ideas:
Reasoning
Plate tectonics causes
seawater to pass through the
ocean crust causing salt to
be removed from the water
Joly’s attempt to calculate the age of the earth was innovative at the time, however his reasoning
is based on some faulty assumptions leading to a significant underestimate of the age of the
earth. A better way to describe the values obtained through this calculation is as “residence
time,” or the time that salt stays in oceans before being removed through one of many processes.
Sodium and Magnesium are removed through different processes and thus at different rates,
accounting for the varying values obtained using each ion.
The main assumption that Joly made that led to the underestimate of the earth’s age is his
assumption that salt stays in the oceans once deposited. There are several processes that remove
salt from the oceans including the process of plate tectonics that causes salt removal from
seawater as it passes through the ocean’s crust. Salt is also removed from deposition of salt in
sediments on the sea floor, and through transfer onto land from sea spray. The method also
assumes that the influx of salt in to the oceans has been constant over time. Weather rates have
been known to change drastically throughout geologic time, so this assumption is likely false.
Magnesium is removed from seawater by various processes that differ from sodium’s removal.
For example, magnesium reacts with calcium carbonate in coral islands, through this process, the
magnesium becomes incorporated into the coral and thus is removed from the ocean water. The
rate at which magnesium is removed is higher, than the removal of sodium, resulting in a larger
underestimate of the age of the earth using this calculation method with magnesium as the ion
under consideration.
Discuss your responses to #1-3 and the scientists’ ideas above with your group. Would you
change any of your answers? Explain your thinking.
Part 3: Loss of the Earth’s Heat – Lord Kelvin
Scott is working on this part.
Reflection
After completing the work in your Specialty Group, return to your Home Group. Each group
member will take a turn and share the method for dating the age of Earth that they explored in
their Specialty Group. Complete the table below to record the different early methods for
establishing an absolute date for the age of Earth.
Method
Description
Assumptions
Estimated Age
of Earth (Range)
Sedimentary
Rock
Deposits
Salinity of
the Oceans
Earth’s Heat
Loss
Group Discussion
Once each member of your group has shared their method for dating the age of Earth, discuss the
possible sources of error for each method that may have led to the vastly different early estimates
for the age of Earth. Complete the table below.
Method
Sedimentary
Rock
Deposits
Salinity of
the Oceans
Earth’s Heat
Loss
Possible Sources of Error
ACTIVITY 3: Absolute Dating – Radiometric Dating
Purpose:
We have seen in previous activities that there are numerous techniques to compare the age of one
rock (or layer) to another rock (or layer). However, none of these techniques enable scientists to
observe the actual age. This activity will help you appreciate the observations that scientists
make in order to establish the age of a rock. Furthermore, in this activity you will engage in the
process scientists undergo to infer the age of the rock.
Q. How do scientists know that a rock is 1.2 billion years old (or any age for that matter)?
Introduction:
Initial ideas.
1. You go to a museum, where several rocks are on display. Underneath each rock is a
placard that states the type of rock, and its age. How do you think the age of the rock is
determined?
2. In the life cycle of a human, age is defined as time since birth. How should the age of a
rock be defined? Think back to ideas of the rock cycle.
3. Of the three rock types, rank them from the type that would have the “most clearly
defined age” to the “least clearly defined age.” Explain your thinking.
4. Use figure (__ from previous activity?) to answer the following questions:
a. Order the fossils from youngest to oldest. How do you know?
b. Can you tell how much older one fossil is than another? Explain.
c. Can you design a method to determine the difference in ages between one fossil
and another?
Experiment 1.
Background:
Modern Atomic Theory.
In 1803, John Dalton proposed the foundations of modern atomic theory: that all matter is
composed of indivisible and unchangeable atoms. Subsequent studies furthered Dalton’s original
ideas, helping scientists create a model for the inner structures of the atom. However, until the
late 1890s, there was no evidence that atoms ever changed. This observation, that all matter is
made of unchanging, atoms, is according to Nobel Laureate Richard Feynman, the most
significant scientific finding humanity has yet discovered.
In other words, there was no reason to believe that a Carbon atom was ever, or will ever be,
anything other than a Carbon atom.1 Take a moment to think about that. The matter in your
skin, the desk in front of you, the air you breathe, is composed of atoms, the very same atoms
that have existed since Earth formed.
Quick write and share about atomic theory. Take a Carbon atom in your body. Write a quick
biography to invent the life of that Carbon atom. Share your work with the class.
Transmutation of elements.
Between 1896 and 1905, several scientists such as Ernest
Rutherford and Marie Curie, observed that some atoms do not
remain constant, but can spontaneously change into other types of
atoms by releasing other smaller particles and energy. Both
Rutherford and Curie received Nobel prizes for their work. Marie
Curie discovered that this change was not due to an outside
source, but was a process inherent to the atom itself. In other
words, this change would occur regardless of the environment the
atom was in or other atoms which it interacted with. Even if the
atom were completely isolated from any other atom or
environment, it changed in the same way. This process, where an
atom spontaneously changed from one type of atom into another,
is called transmutation, or radioactive decay.
Figure 1. Marie Curie, one of the
scientists credited for the
discovery of the transmutation of
elements.
Ernest Rutherford studied particular decays, and found evidence
that Radium (Ra) was a product of Uranium (U) radioactive
decay. However, this took a long time, so he focused on the decay of Radium. Below is shown
Ernest Rutherford’s picture of the transmutation of an atom of Radium into a stable atom of
1
For more information about atomic theory, see
http://en.wikibooks.org/wiki/General_Chemistry/Atomic_Structure/History_of_Atomic_Structu
re
Lead. Note that this picture uses older terminology than what we use today, (for instance,
“EMAN.” Is now called “Radon gas.”)
Figure 2. Ernest Rutherford’s picture of the decay of Radium to stable Lead,
Philosophical Transactions of the Royal Society of London, 1905.
Question. Look at the diagram above, and note the descriptions of time. Do you think it was
likely that Rutherford observed the transmutation of an individual Radium atom? Explain your
thinking.
It was soon discovered that numerous atoms are unstable (radioactive), and can transmute. In a
transmutation, the original atoms are called “parent atoms” (i.e. Radium) while the resulting
atoms are called “daughter atoms” (Radio-Lead in figure above). Rutherford suggested that
observations about the proportions of daughter atoms to parent atoms could be used as a tool to
find the age of rocks. His discoveries paved the way for the current method of determining the
age of a rock, a method called Radiometric Dating, or Absolute Dating. The following activity
will help you understand how this process works.
Question. What would a lower proportion of parent atoms to daughter atoms indicate about the
age of a certain sample of atoms?
Experiment 1.
In this activity, we will model the radioactive decay of Uranium (U) to Lead (Pb). The
following reaction glosses over the details, but shows the full decay process of U-235 to
Pb-207.
U-235
Pb-237
Question. In the figure above, identify the parent and daughter atom for the entire process
(ignoring the intermediate steps). The number alongside the chemical symbol refers to the mass,
or heft, of the atom.
YOUR GROUP WILL NEED
Materials:
50 pennies to represent U atoms
50 marbles to represent Pb atoms
Metal tray
Bowl (or shaking vessel)
Procedure.
1. Place 50 pennies into a bowl or tray; shake the bowl to mix them up.
2. Pour pennies onto a tray. Replace all “tails up” pennies with marbles. Count the
number of pennies and the number of marbles. Record data under “round#1.”
Calculate the % U remaining by doing the math:
# ofUatoms
%U =
´ 100
Totalatoms
3. Pour the mixture of pennies and marbles back into the bowl. Mix them up, and
pour back onto the tray.
4. Replace all “tails up” pennies with marbles. Count the number of pennies and
the number of marbles. Record data under the next round. Calculate the % U
remaining.
5. Repeat steps 3 and 4 six more times, or until all the pennies have “transmuted.”
Data.
Round #
0
1
2
3
4
5
6
7
Number of U
50
Number of Pb
0
% U remaining
100
Graph your data on the graph below. Use a line-graph to connect the data points.
% U remaining
100.0%
90.0%
80.0%
70.0%
60.0%
50.0%
40.0%
30.0%
20.0%
10.0%
0.0%
0
2
4
6
8
10
Round #
Reflection Questions.
1. In this activity, what does each penny represent?
2. In this activity, what process in nature does the changing of a penny into a marble
represent?
3. In this activity, what process in nature does the step of “shaking, then pouring” represent?
4. In the activity, approximately how often does 50% of the Uranium change from one type
of atom into another? How does that relate to a coin?
5. There are actually different forms of Uranium, known as isotopes, which differ slightly in
their original mass. These isotopes decay into different daughter sequences. Here are
two examples:


U-238 decays into Pb-206
U-235 decays into Pb-207
However, the rate at which they transmute is different. Scientists measure this rate by a
term called the “half-life” of an element. For instance, the half-life of the decay from U238 to Pb-206 is approximately 4.5 billion years. The half-life of a different form of
Uranium to a different form of lead, known as the U-235  Pb-207 decay has a half-life
of ~700 million years. What do you think a “half-life of 700 million years” means?
Discuss your answers with an instructor.
Experiment 2. Application of radioactive decay.
You will need:
 One set of “rock samples” per group (Instructor note: each “set” is a bag of 4 items with
a different proportion of U to Pb in them. 2:9, two that are the same ratio (just different
multiples)
Geologists often use the mineral Zircon (a mineral found in igneous rocks) for radiometric
dating., Zircon has Uranium in it. In this activity, we will use the decay series of U-235 to Pb207, with a half-life of ~700 million years.
Suppose a sample of Zircon is found. Scientists are able to isolate and measure the number of U235 atoms and Pb-207 atom that exist in the rock today. They find:
- 150 trillion atoms of U-235, and 450 trillion atoms of Pb-207.
Using that observation and what you learned from the activity above about radioactive decay,
answer the questions below to make inferences about the history of the rock.
Assuming that all of the Pb-207 atoms came from U-235, how many U-235 atoms were there to
begin with?
How many times has one-half (or 50%) of the sample of original U-235 atoms transmuted to
Lead? Explain.
Using the ideas of radioactive decay, how old would scientists conclude that this rock was?
Explain your reasoning.
Obtain a set of 6 “rock samples” from the supply table. For each sample, you will determine the
age of the “rock” by first recording observations in the table below, and then inferring ages in the
following tables. Record your results in the table below.
- Count the number of “U-235” atoms in the rock.
- Count the number of “Pb-207” atoms in the rock.
- Determine the total number of U-235 atoms that would have existed initially.
- Determine the % of parent element remaining. Use the formula:
# ofUatoms
%U =
´ 100
Totalatoms
Data table 1. Observations of “rock samples.”
Sample Current
Current
Beginning
number of
number of
number of UU-235 atoms Pb-207 atoms 235 atoms
A
2
9
11
B
C
D
E
F
% U-235 remaining
2/11*100 = 18%
The figure below shows a statistically accurate decay rate for all radioactive atoms. You will use
this decay-rate curve to infer the ages of each of your rock samples.
% U remaining
100.0%
90.0%
80.0%
70.0%
60.0%
50.0%
40.0%
30.0%
20.0%
10.0%
0.0%
0
2
4
6
Half-life #
8
10
Data table 2. Inferences about rock ages.
Sample % U-235
# Half-lives which
remaining have occurred
A
18%
Approximately 2.3
B
C
D
E
F
Age of rock
2.3 * 700 = 1,600 million years
Check your answers with your instructor.
Summarizing Questions.
1. Which experiment (#1 or #2) represents how geologists determine the actual age of a rock?
2. In each “rock” sample, what do you think the other stuff (beans, rice kernels, etc.)
represents? Is the other stuff necessary? Explain your thinking.
3. Look at your answers to the initial ideas, questions # 1-3, at the beginning of this activity.
Reflect upon your ideas from the beginning of this cycle. Would you make any changes
to your answers and why?
4. You’ve now learned about relative dating and absolute dating. Which dating method
may be appropriate to answer the following questions?
a. Which fossil species roamed the planet before another?
b. How old is the Earth?
c. How much time elapsed between two adjacent layers of rock where there is a
discontinuity?
References and links:
Figure 1, Marie Curie.
http://www.radiochemistry.org/nuclearmedicine/pioneers/curie_m.shtml
Figure 2, Rutherford’s Diagram of transmutation of Radium.
http://www.aip.org/history/curie/unstable.htm
Figure XXX, Decay series of U-235 to Pb-207.
http://uni-leipzig.de/~energy/ef/18.htm
John is working on homework
ACTIVITY 4:The Earth’s Story
The introduction to this activity needs to be fleshed out.
Purpose
Q. What are the major events in the Earth’s History and When Did they Happen?
Introduction
Scenario 1: Formation of the Earth
NOTE TO EDITOR: this scenario needs formatting, lines for answers, etc.
Inferring some BIG Events in Earth’s Story
Data Point (??) Is this necessary for the scenario or will this come later when we add more
points to the overall timeline?
3.465 billion years
The first microfossils are preserved in crustal rocks that are 3.465 billion years old in Australia
(see picture below). They are a kind of cyanobacteria, bacteria that produce oxygen from
photosynthesis. They were found in rocks that were sediment on the floor of an ancient lake or
ocean. Cyanobacteria still exist today, as blue-green algae.
http://www.lpi.usra.edu/publications/slidesets/marslife/slide_31.html
These cyanobacteria can trap, bind, and cement sedimentary material and grow into large,
layered structures called stromatolites. Stromatolites are present throughout the Precambrian,
and show that cyanobacteria are the first and oldest life forms on Earth.
PreCambrian Fossil stromatolites
Modern stromatolites are still formed today
(Fig xx)
Figure xxx
Modern Stromatolites in Shark Bay
http://en.wikipedia.org/wiki/Stromatolite
Scenario #1: What do we know about the beginning?
The oldest radiometric dates that humans have measured for rock samples collected on Earth are
summarized in Table 1. Some really tough minerals called zircons in these old rocks have yield
even older ages (Table 2). Some dates from samples returned from the Moon are in Table 3.
Compare these ‘terrestrial’ and ‘lunar’ ages to the radiometric dates for meteorites that have
fallen to Earth (Table 4). How does the distribution of dates (e.g. range, mean) from Earth rocks
compare to that from meteorites? How might the Moon fit into the story?
Grand Canyon National Park, Arizona. Contorted gneiss in Hance Canyon. Photo by G.
Billingsley, April 19, 1969. Source: http://libraryphoto.cr.usgs.gov/
Banded gneiss, folded contorted, and injected with granite, one mile southeast of Gibbs
Crossing, Mass. Palmer quadrangle. Hampshire County, Massachusetts. May 30, 1907. Source:
http://libraryphoto.cr.usgs.gov/
Table 1: Dated Rock Samples
#
Rock type and location
1
Qorqut granites from Godthaab, Greenland
2
Amitsoq gneisses from Godthaab, Greenland
3
Amitsoq gneisses from Isua, Greenland
4
Uivak gneisses from Saglek and Hebron, Labrador
5
Sacred Heart Granite from Morton, Minnesota
6
Puritan Quartz Monzonite from Watersmeet, Mich
7
Older Granitoid gneisses from Morton Minn
8
Older Granitoid gneisses from Watersmeet, Mich
9
North Star Basalt, Marble Bar, Western Australia
Method
Rb-Sr
Rb-Sr
Pb-Pb
U-Pb
U-Pb
Pb-Pb
Pb-Pb
Rb-Sr
U-Pb
Rb-Sr
Rb-Sr
Rb-Sr
Sm-Nd
Pb-Pb
Rb-Sr
Rb-Sr
Rb-Sr
U-Pb
U-Pb
U-Pb
Rb-Sr
U-Pb
Rb-Sr
Sm-Nd
Sm-Nd
Age (Gyr)
2.53
2.52
2.58
2.53
3.60
3.56
3.74
3.64
3.76
3.55
3.66
3.61
3.56
2.6
2.64
2.32
2.65
3.59
3.54
3.66
3.48
3.62
3.57
3.56
3.56
Age range of oldest terrestrial rocks:
Zircons are igneous minerals formed on Earth that are extremely durable. The core of these
zircons can survive repeated erosion and metamorphism. The zircons preserved in the oldest
rock formations are fragments of even older rocks. Table 2 shows some of the oldest zircon ages
yet found.
The following figures are of the fascinating mineral zircon. One of the most amazing properties
of zircon is its durability. At hardness 7.5 it is difficult to mechanically weather. Furthermore, it
resists chemical weathering, including melting (!) because of a very high melting temperature.
Further-furthermore, zircons can add rinds (or rims) of new zircon during crystallization events
(metamorphic and igneous).
Image Source???
The photo on the left shows a collection of zircons separated from a Mesozoic granitic rock
(magnified about 100 times). The diagram on the right shows one such zircon as a regular
photomicrograph (top), when bombarded with electrons it reveals a layered internal structure
(middle) and a map of the internal zones (bottom). By using a beam of high-energy oxygen ions
to vaporize small spots, the zones can be dated individually with the uranium-lead (motherdaughter element) system. Such analysis gives the following ages for the zones: 1 = 3,900
million years; 2 = 3,100 million years; 3 = 1,900 million years, 4 = 210 million years.
Table 2. Radiometric Ages of Zircon Crystals from Old Terrestrial Rocks
1
2
3
4
Rock type and location
Acasta Gneiss (metamorphic igneous rocks), Northwest Territories, Canada.
Geochemistry indicates metamorphosed granitoid rocks.
Narryer Gneiss (metasedimentary rocks), Jack Hills, Western Australia.
Rocks preserve graded bedding and cross bedding of original sediments.
Isua Greenstone Belt, Southwest Greenland. Geochemistry and texture indicate
metamorphosed mafic volcanic and sedimentary rocks.
Nuvvuagittuq greenstone belt, Northern Quebec, Canada.
This age has been questioned.
Method
U-Pb
Age (Gyr)
4.03
U-Pb
4.1- 4.2
U-Pb
3.9
142Nd
4.28
Table 3. Radiometric Ages of Moon rocks
1
Moon Sample 10062 basalt, Apollo 11 landing site
2
Moon Sample 76535, troctolite, Apollo 17
3
Moon Sample 78236, norite, Apollo 17
Range of moon rock ages:
Ar-Ar
Ar-Ar
Sm-Nd
Rb-Sr
Sm-Nd
Rb-Sr
K-Ar
Ar-Ar
Ar-Ar
Ar-Ar
Ar-Ar
Sm-Nd
Sm-Nd
Rb-Sr
Ar-Ar
3.78
3.79
3.88
3.92
4.26
4.51
4.27
4.16
4.19
4.20
4.19
4.34
4.43
4.29
4.36
Allende meteorite fragment that landed in Chihuahua, Northern Mexico, February 8, 1969 (source: Wikipedia)
Table 4. Radiometric Ages of Meteorites
Dated
Method
Age (billions
of years)
whole rock
Ar-Ar
4.52 +/- 0.02
2
whole rock
Ar-Ar
4.53 +/- 0.02
3
whole rock
Ar-Ar
4.48 +/- 0.02
4
whole rock
Ar-Ar
4.55 +/- 0.03
5
whole rock
Ar-Ar
4.55 +/- 0.03
6
whole rock
Ar-Ar
4.57 +/- 0.03
Meteorite
1
Allende
7
whole rock
Ar-Ar
4.50 +/- 0.02
8
whole rock
Ar-Ar
4.56 +/- 0.05
whole rock
Ar-Ar
4.44 +/- 0.06
13 samples
Rb-Sr
4.46 +/- 0.08
whole rock
Ar-Ar
4.43 +/- 0.06
12
whole rock
Ar-Ar
4.40 +/- 0.06
13
whole rock
Ar-Ar
4.29 +/- 0.06
18 samples
Rb-Sr
4.53 +/- 0.16
whole rock
Ar-Ar
4.49 +/- 0.06
4 samples
Sm-Nd
4.55 +/- 0.33
17
10 samples
Rb-Sr
4.51 +/- 0.15
18
whole rock
Ar-Ar
4.43 +/- 0.04
19
whole rock
Ar-Ar
4.38 +/- 0.04
20
whole rock
Ar-Ar
4.42 +/- 0.04
9 samples
Rb-Sr
4.46 +/- 0.08
12 samples
Rb-Sr
4.39 +/- 0.04
5 samples
Sm-Nd
4.56 +/- 0.08
5 samples
Rb-Sr
4.50 +/- 0.07
3 samples
Sm-Nd
4.46 +/- 0.03
4 samples
Sm-Nd
4.52 +/- 0.05
9 samples
Rb-Sr
4.50 +/- 0.05
28
7 samples
Sm-Nd
4.52 +/- 0.16
29
5 samples
Rb-Sr
4.46 +/- 0.06
30
4 samples
Sm-Nd
4.52 +/- 0.33
9
Guarena
10
11
14
Shaw
Olivenza
15
16
21
Saint Severin
Indarch
22
23
Juvinas
24
25
Moama
26
27
Y-75011
31
Angra dos Reis
7 samples
Sm-Nd
4.55 +/- 0.04
3 samples
Sm-Nd
4.56 +/- 0.04
silicates
Ar-Ar
4.50 +/- 0.06
34
silicates
Ar-Ar
4.57 +/- 0.06
35
olivine
Ar-Ar
4.54 +/- 0.04
36
plagioclase
Ar-Ar
4.50 +/- 0.04
4 samples
Rb-Sr
4.39 +/- 0.07
silicates
Ar-Ar
4.54 +/- 0.03
32
33
37
Mundrabrilla
Weekeroo Station
38
Range of meteorite ages:
Mean of meteorite ages:
Why do you think there are no meteorites that are 5 billion years? 7 billion years?
What do you think the narrow range of meteorite ages means for the formation of the solar
system?
Given all this age data, tell a story about Earth’s early history. Things to think about:
1. Earth’s interior is presently layered (core, mantle, crust). Was it always?
2. Earth’s surface hosts a giant ocean. Was there always liquid water?
3. Earth’s crust is divided into continents and ocean basins. Was this distinction always
present?
Scenario 2: Oxygenation of the Earth’s Atmosphere (Sue, Randy, Allison)
Scenario #1
Pyrite (fool’s gold) is an iron sulfide mineral that has reduced iron in it. If it is exposed to
oxygen in the atmosphere, pyrite will corrodeby oxidation. Pyrite is found in sedimentary rocks
on Earth that are older than 2.5 billion years. In contrast, sedimentary pyrite is rare in
sedimentary rocks that are younger than 2.3 billion years. What inference can you make about
Earth’s atmosphere(Is this a useful addition? Or too leading?)from this information?
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Banded iron formations are a type of rock that contains iron-bearing minerals. These are the
rocks that we get all of our iron from to make steel. The iron in these minerals is partially
oxidized. They cannot form when the atmosphere is very reducing, and they cannot form when
the atmosphere is very oxidizing.
Banded iron formationsarerarein rocks that are older than 2.5 billion years; they are really
common in rocks that are 2.5-1.9 billion years old, and then they become rare again after 1.9
billion years.
Figure xxx. A banded
iron formation outcrop in
the Midwestern US from
rocks that are 2.3 billion
years old.
http://www.networlddirec
tory.com/blogs/permalink
s/10-2009/banded-rocksreveal-earth-secrets.html
What inference can you make about Earth’s atmosphere from this information?
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
“Red beds” are sedimentary rocks that contain minerals that have very oxidized iron (hematite or
goethite). You can think of them as rusty minerals. Interestingly, red beds are never found in
rocks that are older than 2 billion years.
Figure xxx. Red beds in a rock
wall in the southwestern United
States.
http://keyecommentary.blogspot.
com/2010/05/meaning-of-rockwall.html
What inference can you make about Earth’s atmosphere from this information? How does this
tie in to your earlier inferences?
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Part 2
Purpose:
To analyze observations from the fossil record to construct a timeline of atmospheric oxygen
levels over time.
Biodiversity, mass extinctions, rate of appearance of new species, land colonization by plants,
insect size, and occurrence of fires have changed over time. In the following exercise we will
examine the table of observations to identify inferences about atmospheric oxygen over time.
Examine the last column, fire occurrences, would an era of high fire frequency correlate with
high atmospheric oxygen, or low?
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
Continue to similarly analyze each observation set and create inferences about atmospheric
oxygen during different eras.
Table 1: A summary of observations from the fossil record.
Mass
extinction
Increased
land
colonization
by plants
Evolution
of
flapping
flight
Insect
gigantism
Rate of
speciation
Biodiversity
Fire
occurrences
Yes
No
No
Absent
High
Low
Lower
190
million
years
ago
Yes
No
Yes, birds,
bats
Present
High
Low
Lower
275
million
years
ago
No
Yes
Yes,
insects
Pterosaurs
(?)
Present
Low
High
Higher
410
million
years
ago
No
Yes
No
None
Low
High
Higher
475
million
years
ago
Yes
No
No
None
High
Low
Lower
550
million
years
ago?
Yes
No
No
None
High
Low
Lower
Present?
75
million
years
ago
should
be time
spans
Inferences:
~75 million years ago:_______________________________________________________
~190 million years ago:______________________________________________________
~275 million years ago:______________________________________________________
~410 million years ago:_____________________________________________________
~475 million years ago:_______________________________________________________
~550 million years ago:_______________________________________________________
higher than present day
Plot a line on the following graph of relative oxygen levels over time. It is not necessary to have
the absolute oxygen % data points correct, rather focus on relative oxygen levels over time.
Atmospheric Oxygen Levels Over Time
Atmospheric oxygen (% of total atmosphere)
35
30
25
20
lower than present day
Present day oxygen %
15
10
5
0
600
500
400
300
200
100
0
Scientist ideas:
Atmospheric oxygen fluctuations throughout the earth’s history have coincided with many largescale changes in biota. In general, during times of increased atmospheric oxygen, species are
more diverse, larger, and more complex. Below are explanations of how each phenomenon in
the data table are affected by atmospheric oxygen levels.
Insect gigantism: The limiting factor controlling insect body size is oxygen perfusion to the
tissues throughout the body. The bigger the insect, the more oxygen is necessary to adequately
perfuse all tissues Therefore, high oxygen environments better support larger insect bodies.
Very large insects such as a dragonfly with a 30-inch wingspan are known from fossils of the
Carboniferous time.
Biodiversity: Throughout time, all mass extinctions have coincided with sharp drops in
atmospheric oxygen levels. Species that evolve to be adapted to higher atmospheric oxygen
levels are unable to adequately perfuse tissues with oxygen in lower oxygen environments,
leading to high rates of extinction, and corresponding lower biodiversity. Closely tied to this is
the trend that low oxygen environments coincide with higher rates of speciation. Lower
biodiversity opens up niches allowing for the evolution of new low oxygen environment tolerant
species, to fill those niches.
Evolution of flight: flying insects and birds have some of the highest metabolic rates of any
animals in order to support the energy demands of flight. Higher metabolic rates necessitate
higher rates of oxygen intake. Periods of relatively high atmospheric oxygen facilitate increased
oxygen intake, allowing for increased evolution of flight. Also, the addition of more oxygen to
the atmosphere increases the overall gas pressure of the atmosphere resulting in increased lift
during flight.
Fire frequency: Flammability is closely related to atmospheric oxygen levels. Various
atmospheric oxygen concentrations yield the following results: 15% oxygen is too low for fires
to spread, 25% - 30% oxygen is high enough for even wet plants to burn, and oxygen levels of
30-35% is enough for fires to be frequent and catastrophic.
Increased land colonization by plants. During this era, a greater proportion of the planet’s
biomass was terrestrially based (on the continents). Since plants photosynthesize, they tend to
raise atmospheric Oxygen. However, much of this oxygen reacts in the decaying of those plants,
resulting in no net change to the atmospheric concentration. This balance is disrupted if the
plants are being buried before decomposing. When more of the biomass is terrestrial, global
burial rates of dead plant tissue increase, reducing the amount of oxygen required to decay the
carbon based material. Overall, this results in a net increase of atmospheric oxygen levels.
Small group discussion. Share the most surprising thing you discovered about how the
connection between Oxygen and life over time on the Earth.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
The following is a plot of inferred oxygen levels in Earth’s atmosphere, based on some of the
data just presented in the preceding scenarios.
Figure x. O2 build-up in the earth's atmosphere. Red and green lines represent the range of the
estimates while time is measured in billions of years ago (Ga).
Stage 1 (3.85–2.45 Ga): No O2 produced.
Stage 2 (2.45–1.85 Ga): O2 produced, but absorbed in oceans & seabed rock.
Stage 3 (1.85–0.85 Ga): O2 starts to gas out of the oceans, but is absorbed by land surfaces and
formation of ozone layer.
Stages 4 & 5 (0.85–0.54 Ga) & (0.54 Ga–present): O2 sinks filled and the gas accumulates.
http://en.wikipedia.org/wiki/Great_Oxygenation_Event
Do you think this data has any implications for why there was an “explosion” in the diversity and
number of fossils in sedimentary rocks that are between 700 and 500 million years old?
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
Scenario 3: Colonization of Land
Fossils are remains of living things found preserved in sedimentary rock. Fossils are most
commonly bones or shells from animals and leaves or stems from plants. Occasionally, other
parts of living things (skin, flowers, seeds, etc...) are preserved as well. Fossils are found in rocks
that have been deposited over hundreds of millions of years, so they allow us to see how plants
and animals have changed over time. Using what you know about biostratigraphy, the layering
of the fossils will tell you the relative ages of the fossils.
Will earliest land plant fossils or earliest land animal fossils be found lower in the rock
outcroppings? Why?
____________________________________________________
____________________________________________________
____________________________________________________
This following picture shows a rock outcropping from the grand canyon. What do you notice
about the relative positions of the land plant and land animals? What does this tell you about
which came first? land plants or land animals?
____________________________________________________
____________________________________________________
____________________________________________________
Why do you think land plants developed before land animals?
____________________________________________________
____________________________________________________
____________________________________________________
In thinking where the colonization of land plants and animals fits into the order of things, what
had to happen in order for there to be plants on land?
____________________________________________________
____________________________________________________
____________________________________________________
Scenario 4: Tectonic Movement
Part 1: Historical Position of the Continents
Q1: Using the geochronology data, what can you infer about the position of the North American
and African continents180 million years ago? What evidence from the map leads you to this
inference?
____________________________________________________
____________________________________________________
____________________________________________________
____________________________________________________
Q2: Using the geochronology data, what can you infer about the position of the South American
and African continents 140 million years ago? What evidence from the map leads you to this
inference?
____________________________________________________
____________________________________________________
____________________________________________________
____________________________________________________
Q3: Using the geochronology data, what can you infer about the position of the Australian and
Antarctic continents 70 million years ago? What evidence from the map leads you to this
inference?
____________________________________________________
____________________________________________________
____________________________________________________
____________________________________________________
Q4: Using the geochronology data, what can you infer about the position of the North American,
African, South American, Antarctic and Australian Plates 250 million years ago?
____________________________________________________
____________________________________________________
____________________________________________________
____________________________________________________
Part 2: India versus Asia
Q1: Using the above figure, estimate how many years ago India collided with the Eurasian plate?
What evidence from the map leads you to this inference?
____________________________________________________
____________________________________________________
____________________________________________________
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Part 3: Fossil Evidence
Idea: Should we create a mini-activity asking students to cut out the continents and arrange them
to match up the fossil record?
Q1: If Cynognathus, a large, terrestrial reptile, fossils have been dated between 237 to 247
million years old, what conclusions can you draw about how the Earth looked during that time?
How do you know this?
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Q2: If Lystrosaurus fossils have been dated at 250 million years old, Glossopteridales at 299 to
250 million years, and Mesosaurus from 280 to 248 million years, what conclusions can you
draw about how the earth looked during that time? How do you know this?
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Projection into the future here or at end of Activity?
Scenario 5: Mass Extinctions
Scenario #5: Have there been sudden (dramatic) changes in life on Earth?
Focus on ammonites:
Need to document
 long successful history (400-65.5 my)
 diversity
 sudden disappearance at K-Pg boundary
Ask students to think about other animals that might have been affected.
Hypotheses for Cause:
 impact at Yucatan
 basalt eruptions at Indian
Discussion of mass extinctions and their impact on history of life (describe the big seven?)
Scenario 6: Hominid Migration – The Peopling of Earth
Homework (Randy)
Read the NPR article at (the link is clickable on my blackboard site)
http://www.npr.org/blogs/krulwich/2010/09/14/129858314/my-grandson-the-rock
After reading the article, listen to the audio by either clicking the speaker button to listen in your
web browser or download the file to listen away from the computer. The audio is approximately
7 minutes long.
Now answer the following questions about the reading and audio.
1. Why would a dry Mercury and a dry moon contain less minerals than Earth if they were
2.
3.
4.
5.
6.
formed around the same time?
Why would mars have more minerals than Mercury or our Moon, but less than Earth?
Mineral evolution began with the emergence of rocky planets. planets are engines of
mineral formation. Why? What features would accelerate mineral changes?
Geo-sphere and biosphere have continued to co-evolve, why are life and rocks so
intertwined with each other? What life event led to the proliferation of minerals on Earth?
How could we use minerals for identifying the possibility of life on other planets?
What did you find to be the most interesting piece of information from this article? What
surprised you the most?
Closure to Activity 4:
Summarizing Questions
Projection into the future?
Do students need to gain some awareness of number sense (how big is 4.6 billion?)?
Creating a timeline, adding additional events (data points), class discussion.
Assessment