GEOL 1301 - Lab 08 - Geologic Time
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Geology
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Apr 3, 2024
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GEOL 1301
Lab 08
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 upload your completed worksheets on Canvas.
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
Key to rocks and symbols for the following activity:
1
Name:
Malik Brown
For each of the following cross sections, determine the relative age sequence of the rocks and
answer the additional questions.
2
1.
Rock sequence from oldest to youngest:
E, B, C, D, A
What type of unconformity is represented between layers E and C?
Non-conformity
Which principle is used to determine the time sequence of layers C, D, and A?
The law of superposition
3
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2.
Rock sequence from oldest to youngest:
G, C, A, F, B, H, D, I, E
What type of unconformity is represented between layers A and H (on the left side of the picture)?
Angular unconformity
Is fault B older or younger than layers C and A? Which principle is used to determine this age relationship?
Fault B is younger than layers C and A, the cross-cutting principle is used to determine this age relationship.
4
3. What is the age relationship between H and O, and how can you tell?
H is older than O and can be determined by its zone of contact.
What type of unconformity is represented between layers B+L+J and M?
Angular Unconformity.
Is intrusion A older or younger than layers H, B, L, and J? Which principle do you apply to solve this?
Intrusion A is younger than layers H, B, L, and J. The cross-cutting principle is used to solve this.
What type of unconformity is represented between layers M and G?
Angular Unconformity.
Is fault F older or younger than intrusion A? Which principle do you apply to solve this?
Fault F is younger than intrusion A, the cross-cutting principle is applied to solve this.
Rock sequence from oldest to youngest:
H, O, B, L, A, F, J, M, D, G, N, E, I, C, K
5
Name: Malik Brown
Numerical Geologic Time – Calculation
A granite contains zircon crystals with 275 billion 235
U atoms and 1925 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.
age of granite = n × T 1/2
age of granite =
3 × 704 million years
age of granite =
2112 million years
6
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