Module 3 Assignment
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School
University of Texas, Permian Basin *
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Course
1101
Subject
Geology
Date
Feb 20, 2024
Type
docx
Pages
6
Uploaded by AdmiralRoseTiger55
Module Assignment– 3: Geologic Time
(40 pts)
In this module assignment you will identify and use rock and fossil relationships to reconstruct parts of geologic history through both relative and absolute dating methods. In this laboratory exercise you will walk you through the vastness of the geologic timescale using our modern calendar and apply the principles of relative dating and absolute dating to calculate the age of rock units. You will then put it all together to unravel the geologic history of a rock formation.
Deep Time
Geologic Time
is vast. It is hard to wrap our minds around 4.5 billion years of Earth history. Humans have only been living on earth for about 0.0003% of geologic time, which makes our existence on Earth rather unimportant in terms of “deep time”. To aid in the enormity of the geologic time scale, geologists use metaphors to illustrate the history of our dynamic planet. Before understanding geologic time, remember geologists
measure events on the planet in years before present, and use “Ka” to represent 1,000 years and “Ma” to represent one million years, and “Ga”, for one billion years. Imagine this calendar below is the framework of the entirety of earth’s history.
Origin of the solar system and Earth: January 1
st
Formation of the moon: January 4
th
Oldest mineral grain (zircon, 4.4 Ga) : January 12th
Oldest rock (Acasta Gneiss, 4.0 Ga): February 13th
Oldest evidence (chemical) for life (3.8 Ga) : March 1st
Oldest known fossil evidence (bacteria, 3.5 Ga): March 25
th
Oldest animal fossils (600 Ma): November 13
th
Oldest terrestrial fungi (440 Ma): November 26th
Worst mass extinction (there have been at least five large mass extinctions):
December 11
th
Oldest known mammal fossils (180 Ma): December 17th
Extinction of the dinosaurs (65 Ma): December 26th
Humans: December 31
st
o
12:25 Humans and chimpanzees lineage diverge (6 Ma)
o
11:25 Origin of Homo sapiens (300,000 years ago)
o
11:58 Domestication of the dog (14,700 years ago)
o December 31
st
at 11:59 PM
22 seconds the oldest writing (5,518 years ago)
29 seconds the Great Pyramid of Giza was built (4,578 years ago)
57 seconds the Roman Empire
59 seconds the last 150 years or post-Industrial Revolution
Absolute Time (15 pts)
Geologists calculate the absolute time of a rock unit using radiometric dating techniques. Some radioactive elements decay on a predicable pathway called a half-life (i.e. the amount of time it takes for ½ of the element to decay to another element). We can calculate the number of half-lives of a radioactive decay series (see text for more reading on decay series) by measuring the amount of the original
element, or parent, and the amount of the decay product, or daughter, present in a rock using a mass spectrometer. The next page has a blank half-life graph and instructions for creating a complete half-life decay curve. This curve illustrates exponential decay.
Complete the table below to calculate the number of parent atoms and daughter atoms through each half-life step. This will allow us to complete the ideal half-life decay curve. Put in the number of parent atoms at half-life 1 (hint: half of them will have decayed to daughter atoms). Write in the number of daughter atoms formed during the decay step. Note the cumulative number of parent atoms (which should be decreasing at each step) and the cumulative number of daughter atoms (which should be increasing at half-life).
(5 pts)
Half-life
0
1
2
3
4
5
# Parent
100
50
25
12.5
6.25
3.125
# Daughter
0
50
75
87.5
93.75
96.875
P:D Ratio
100:0
1:1
1:3
1:8
1:16
1:32
This section is worth 10 points
to label the graph for illustrating a decay curve. See the Absolute Time: Half Life Graph on the next page. Note the red line this is the parent percentage curve called “Half-Life Decay Curve”. Note: This curve is exponential, as it decreases by a half, every interval, what is called a half-life. There are only four intervals on this particular graph on the next page. 1)
Label the four intervals, T1, T2, T3, and T4 on the x-axis. These represent the half-
lives.
2)
Label the y-axis. This is the half-life ratio; label 50%, 25%, 12.5%, 6.25%. 3)
At the point on the graph where the two indices meet to create a point and write:
a.
write ½ where 50% and T1 meet
b.
write ¼ where 25% and T2 meet
c.
write 1/8 where 12.5% and T3 meet
d.
write 1/16 where 6.25 and 4T meet
These intercepts indicate how much material has decayed. At one half life, half the material decayed. 4)
Draw in blue the approximate curve of daughter percentage atoms on the half-
life graph (hint: this should mirror the parent’s curve). Now that you’ve created an ideal half-life curve, you’re ready to calculate some radiometric ages for different radioactive decay series. You will need resources from the
online access point from Kendall Hunt to do calculations.
Absolute Time: Half-Life Graph
1/16
1/2
1/4 1/8 Half-Life Ratio
12.5%
6.25%
25%
50%
Half-Lives
5T 4T 3T 2T 1T
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Calculating Half Lives (15 pts)
You are ready to calculate some radiometric ages for different radioactive decay series. You will need resources from the online access point from Kendall Hunt to do calculations.
The access page has half lifetimes for each decay species (e.g.: Uranium-Lead). Have handy the Index Fossil Chart
that accompanies this exercise and the final page of this assignment which has a diagram (Fig.3.1
) illustrating each layer associated with the index
fossil. A mineral sample from rock layer B in the fossil chart has 150,000 atoms of uranium-235
and 50,000 atoms of lead-207. 5pts
The absolute age of the rock is: 352,500,000 years
=(150,000/(150,000+50,000)) = 75% = ½ half-life in half-life graph
= ½ half-life * 705 million = 352,500,000
A mineral sample from rock unit C in the fossil chart has 50,000 atoms of uranium-235 and 150,000 atoms of lead-207.
5pts
The absolute age of the rock is: 1,408,000,000 years
=(50,000/(50,000+150,000)) = 25% = 2 half-lives in half-life graph
= 2 half-lives * 704 million = 1,408,000,000
A mineral sample from rock unit G in the fossil chart has 100,000 atoms of potassium-40
and 100,000 atoms of argon-40. 5pts
The absolute age of the rock is: 1,250,000,000 years
=(100,000/(100,000+100,000)) = 50% = 1 half-life in half-life graph
= 1 half-life * 1.25 billion = 1,250,000,000
Example to calculate:
I.
Get the percentage of parent isotope remaining (c) by taking parent atoms divided by the total of the parent plus the daughter atoms. (
a/a+b=c
)
a.
parent atoms
b.
daughter atoms
c.
percent of parent isotope remaining
II.
Go to Half-life Graph and use the Y axis (% of parent isotope remaining which is variable c above) to then move across and down using the figure to collect the correct number of half-
lives at the X axis which corresponds to the % remaining. (e.g., 2 half-lives)
III.
To calculate the time or absolute date for each of the igneous or metamorphic rock layers:
a.
Multiply the half-life value from X axis (e.g., 2 half-lives) with the number of Half- Life
in Years (
Common isotope elemental pairs chart
) for those isotope series (e.g.,
uranium-235 decays to Lead-207 in 704,000,000 years)
b.
Time=half-lives * isotope half-life in years
c.
Time = 2*704,000,000 million years ago
d.
=1,408,000,000 or 140.8 million years ago
Index Fossils (10 pts)
Refer to the Fossil Chart from the KHP site. Note the age of the rock units should be words not numerical values, e.g.: Permian Period, Jurassic Period. (2 pts each)
Note: mya = millions of years ago
This fossil was found in Layer A:
Layer A was formed
_23.03_ mya during
the Neogene Period.
This fossil was found in Layer B
: Layer B was formed
251.9 – 298.9 mya during the
Triassic or Permian Period.
This fossil was found in Layer D
: Layer D was formed
358.9 – 485.4 mya during the Carboniferous or Devonian Period.
This fossil was found in Layer E
: Layer E was formed 485.4 – 443.8 mya during the
Devonian or Silurian Period.
The following fossils were found in Layer F:
Layer F was formed _541.0 – 485.4
mya during the Cambrian or Ordovician Period.
Figure 3.1.:
The Block Diagram 3.1 shows various layers these index fossils were produced from in the Index Fossil Chart.
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