GEOL 1302 - Lab 03 - GeolTime (1)
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University of Texas, Arlington *
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1302
Subject
Geology
Date
Feb 20, 2024
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docx
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7
Uploaded by ConstableStarMule34
Jiovanni Martinez
GEOL 1302
Lab 03
Relative and Numerical Geologic Time
In this lab, you are going to review the concepts of relative and numerical geologic time and work on examples to establish the order of geologic events and to use radiometric dating. Please submit your worksheets to your instructor.
Supplies needed:
1 sheet of graph paper, simple calculator
Helpful Websites:
http://en.wikipedia.org/wiki/Law_of_superposition
http://en.wikipedia.org/wiki/Principle_of_original_horizontality
http://en.wikipedia.org/wiki/Principle_of_cross-cutting_relationships
http://en.wikipedia.org/wiki/Unconformity
http://en.wikipedia.org/wiki/Radiometric_dating
You also find very useful information in your textbook.
Key to rocks and symbols for the following activity:
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2
Relative Geologic Time
What type of unconformity is represented between layers B+I+F and E?
Angular unconformity
Is intrusion C older or younger than layers D, H, and A? Which principle do you apply to solve this?
C is younger than them. I used the principle of cross-cutting relationships to find this answer.
C cuts through layers D, H, A
Is fault G older or younger than intrusion C? Which principle do you apply to solve this?
Younger, I used the principle of cross-cutting relationships.
Rock sequence from oldest to youngest:
(oldest) D, H, A, C, G, F, I, B, E (youngest)
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Numerical Geologic Time “Dice Game” – Deriving the “Half Life”
Pick a number from 1 to 6. Record the number you picked below.
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Roll 50 dice in a carton. Count all the dice that came up with the number you chose and remove them from the carton. In the table below, record the number of dice that remain and the number of dice removed per toss as well as their total. Roll the remaining dice again and remove those with the number that you chose at the beginning. Record the remaining dice as
well as the total number of dice removed and repeat this procedure until the 15
th
toss or until only 2 dice remain, whichever comes first.
Toss Number
Dice Remaining
Dice Removed
per Toss
Total Number of
Dice Removed
1
44
6
6
2
41
3
9
3
33
8
17
4
28
5
22
5
25
3
25
6
24
1
26
7
20
4
30
8
17
3
33
9
14
3
36
10
12
2
38
11
10
2
40
12
7
3
43
13
5
2
45
14
5
0
45
15
5
0
45
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Graph the number of dice remaining (symbolizing parent isotopes) and the total number of dice removed (symbolizing daughter isotopes) for the experiment with different colors/symbols and complete the legend and draw curves through the corresponding points. Describe the curves. What is the “half life” that you can infer (in numbers of tosses)? How did you figure it out?
In the first 5(1-5) tosses the average half-life was about 5 removed dice per toss.
In the next 5 (6-10) tosses the average half-life was about 2-3 dice removed per toss.
In the last 5 (11-15) tosses the average half-life was about 1 dice removed per toss.
I figured this out by adding up the number of dice lost per 5 tosses and averaged it out.
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Numerical Geologic Time Graphic Demonstration of Radiometric Decay
Count the number of squares on your piece of graph paper that you brought to the lab. The number is:
1,170
Now start a stopwatch on your phone, or count the seconds in your head. After 10 seconds, fold the paper in half and write down the number of squares on the paper half. Continue folding the paper in half and noting the numbers of squares every 10 seconds. Fill out the table below.
Time (in seconds)
Number of squares
0
1280
10
640
20
320
30
160
40
80
50
40
60
20
If you regard this exercise as an equivalent of radiometric decay, what does the number of squares on the graph paper represent?
An isotope
What is the half-life of the imaginary isotope in this experiment?
10 seconds
What fraction of paper remains after 1, 2, 3, 4, 5, and 6 time intervals?
After 1; 50%
After 2; 25%
After 3; 12.5%
After 4; 6.25%
After 5; 3.125%
After 6; 1.5625%
After each fold it decayed in half
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Numerical Geologic Time – Calculation
A granite contains zircon crystals with 175 billion 235
U atoms and 1225 billion 207
Pb atoms. The half-life for 235
U-to-
207
Pb decay is 704 million years. How old is the granite?
Hint:
In order to solve this problem, you have to calculate the total number of parent and daughter atoms (equaling the original number of parent atoms when the rock formed) and then figure out how many half-lives have gone by until the current number of parent atoms was reached (by repeatedly dividing the original number by 2). The final step is to multiply this number of half-lives with the years that correspond to one half-life. – List all the steps in your answer.
(175+1225) / 2 = 1400/2 = 700; 1 half-life
700/2=350; 2 half-lives
350/2=175; 3 half-lives
3 * 704 million = 2112 million
The granite is 2,112,000,000 years old.
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