Cycle 7
How Do We Know About Earth’s History?
OVERVIEW
(Steve)
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
We have 2 initial ideas activities that need to be combined or worked on. We also want to make
sure our initial ideas reflect the pieces we are using for the ‘Earth’s Story’ and assessment. Do
we also need initial ideas about dating (e.g. relative, absolute, etc.).
(Shawn)
Materials: string, 4 colors of note cards, tape, markers.
Setup: Individuals each get 5 note cards of the same color. (In step 2 students will form groups
of 4, each with different color cards.)
Step 1. Individual: Write down 5 big events in the history of Earth in your journal. Then copy
these events on 3” x 5” note cards. (One event per note card.)
Step 2. Group: Each student combines with 3 others that have different colored cards so that
every student in a group has different colored cards.
Step 3. Stretch string across room to represent the entire span of Earth history. Each individual
student places their cards along the string with the order and spacing that makes sense to them.
Step 4. Students discuss the order and spacing so that they arrive at a consensus on the order and
spacing of events in Earth history. Students then agree on the age of the Earth and mark it on an
additional card. This card is then taped to the beginning end of the string.
Step 5. Students draw their timeline to scale in their journals. The information on the cards must
be included on the timeline.
Step 6. Students then clearly explain in their journals their rationale for positioning these events
where they did.
(Steve)
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.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
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.
Extinction of
Dinosaurs
First Homo
Sapiens fossils
formed
First oxygen in
the atmosphere
Beginning of
Earth
First fossilized
skeletons formed
Your instructor is
born
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.
Group
Discussion
Time
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. What can we infer about the relative ages of rock layers? How do we know that the
Earth has changed over time? Can Earth be “read” like an open book? Can a geologic
history of an area be inferred by looking at rocks?
Do we want all of these questions?
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, we will see in
this activity how we can make inferences about relative ages of rock outcrops in different
locations. We will use fossils to match ages of rock outcrops separated by large distances.
At the right is a cross section of sedimentary rock.
Rank the layers oldest to youngest:
Sandstone A
Mudstone
Siltstone
Clay
Sandstone B
organic-rich mudstone
Marl
Given what you learned in Part 1, describe the principles that led you to the ranking you chose.
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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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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.
Lets 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? (assume 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 and many planktonic marine
fossils described. The “Mesa” location records a continuous sequence of deposition in the ocean
and contains the fossils indicated in the table (Need to insert a picture of the outcrop with layers
1 through 18 so they can visualize top and bottom) below:
A continuous sequence (no pauses in sedimentation) of sedimentary rock is found at Mesa with
the following 7 fossils. (Bottom row corresponds to oldest occurrence, letter indicates presence
in layer.)
T
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
t
t
t
t
t
t
t
t
t
U
V
v
v
v
v
v
v
W
w
w
w
w
w
X
Y
Z
x
x
x
x
x
u
u
u
u
u
u
u
z
z
z
z
x
x
x
x
x
x
y
y
y
y
y
NOTE: Can we create a diagrammatic sketch where students place the fossils into the
appropriate sedimentary layers to get a visual representation of what this would look like?
Based on the table, 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 widespread. Does X or Z make the better fossil to compare ages of outcrops?
Why?
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Notice that no absolute age (years before present) is indicated by the position of the fossils in the
“Mesa” table. Why then, might index fossils be helpful for constructing a history of Earth?
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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
NOTE: Can we create a diagrammatic sketch where students place the fossils into the
appropriate sedimentary layers to get a visual representation of what this would look like?
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
NOTE: Can we create a diagrammatic sketch where students place the fossils into the
appropriate sedimentary layers to get a visual representation of what this would look like?
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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Why might biostratigraphy been important to geologists and paleontologists before the discovery
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 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.
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 – Focused on the big ideas for this piece (variety of methods used, highly
variable, increased estimations of age of earth from 1,000s to millions of year).
Experiment 1: A Jigsaw Activity – Early Attempts at Absolute Dating of the Age of the
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: Sedimentation and Deposition Rates
Initial Ideas
Historical Estimations for the Age of the Earth using 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
Era
Cenozoic
Mesozoic
Paleozoic
pre -Phanerozoic
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 of many layers of rock that are several kilometers thick. Fossils in these
rocks and other correlational arguments suggest that rocks at the bottom of the Grand Canyon were
formed in the pre-Phanerozoic and rocks at the top were deposited in the Permian Period. 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
Formation
6d
100
6a
Kaibab
Limestone
Toroweap
Coconino
Sandstone
Hermit Shale
5d
Esplanade
350
5c
Wescogame
5b
Manakacha
5a
4c
Watahomigi
Surprise
Canyon
Redwall
Limestone
Temple Butte
Limestone
Muav
Limestone
6c
6b
4b
4a
3c
3b
Bright Angel
Shale
3a
Tapeats
Sandstone
2
1b
1a
http://en.wikipedia.org/wiki/File:Grand_Canyon_geologic_column.jpg
Thickness
(meters)
75
100
100
25
150
15
150
100
60
3700
Zoroaster
Granite
Vishnu Schist
Unknown
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.
Rock Unit
6
5 Supai Group
4
3 Tonto Group
Rock
Subunit
Formation
Thickness
(meters)
6d
Kaibab Limestone
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
1b
1a
Zoroaster Granite
Vishnu Schist
Calculated Age
2 Grand Canyon
Supergroup
1 Vishnu Group
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: Salinity of the Ocean
Allison is working on this activity.
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
John is working on completing this activity. Online simulation practice (PHET) may be done as
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 corrode by 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 formationsare rarein 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?
____________________________________________________
____________________________________________________
____________________________________________________
____________________________________________________
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?
____________________________________________________
____________________________________________________
____________________________________________________
____________________________________________________
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?
____________________________________________________
____________________________________________________
____________________________________________________
____________________________________________________
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