GEOL 1301 - Lab 06 - Geologic Time
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University of Texas, Arlington *
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1301
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Geology
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Feb 20, 2024
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GEOL1301 Name:____________________
Geologic Time Lab
Learning and Lab objectives:
In this lab, we will review the concepts of relative and
numerical geologic time. This follows on very nicely from both the rock cycle, and from
sedimentary rocks in both lecture and lab, as the concepts we are exploring rely on how these
rocks are situated in relation to each other. For this lab, you will first examine a few examples of relative ages. These are ages that are
based on the age of one rock relative to another rock. In other words, the sedimentary rock
that is deposited first in a lake is older than the sedimentary rock that is formed from material
deposited on top of the first rock. In this case, we are only interested in what is older or
younger in relative terms. Absolute ages rely on the use of radiometric dating, which gives you the actual age of a rock
relative to the present day. In our previous example, the sedimentary rock that is formed of
material that is deposited first might be six million years old. The younger sedimentary rock
that is formed of material that is deposited on top of the first one might be three million years
old. The difference here is there is a year assigned in absolute ages.
Both absolute and relative age relationships can be used to establish the order of geologic
events; most of the time you would want to use them in conjunction with each other.
Some helpful websites to view about these terms are here:
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
Here is a key to rocks and symbols for the following activity:
Your instructor will also introduce these concepts. 1
Please submit all the worksheets here to your instructor via Canvas.
For each of the following cross sections, determine the relative age sequence of the rocks and
answer the additional questions.
QUESTION 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?
Principle of superposition
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QUESTION 2.
Rock sequence from oldest to youngest:
G, C, A,F,H,D,I,
What type of unconformity is represented between layers A and H (on the left side of the picture)?
Angular unconformity & Disconformity Is fault B older or younger than layers C and A? Which principle is used to determine this age relationship?
Fault B is younger because the principle of Cross Cutting
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QUESTION 3.
What is the age relationship between H and O, and how can you tell?
O Is older you can tell because H cross-cutting O
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?
A is younger than HBLJ because of the principle of cross-cutting.
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?
F is younger than A due to the principle of cross-cutting
Rock sequence from oldest to youngest:
O, H, B, L, J, A, M, D, G, N, E, I, C, K
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QUESTION 4.
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.
275+1925=2200 Billion
1
st
half life =22/2=1100
2
nd
half life=2700/2=1100
3
rd
half life= 540/2=270
Age of granite=3x704=2112 mil.years
2,112 million years or 2.112 billion years
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