GEOL 1301 - Lab 08 - Geologic Time(1)
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Geology
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Dec 6, 2023
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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
For
each of the following cross sections, determine the relative age sequence
of the rocks and answer the additional questions.
1.
Name:____PIPER SIDERS
1001758195______________________
2
Rock sequence from oldest to youngest:
B – E - C - D – A (B is the oldest and A is the youngest rock layer)
What type of unconformity is represented between layers E and C?
Angular unconformity exists between the E and C
Which principle is used to determine the time sequence of layers C, D,
and A?
Principle of original horozontality is used to determine the time
sequence of layers between C, D an A
3
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2.
Rock sequence from oldest to youngest:
Sequence plainly form oldest to to youngest
:G
−
C
−
A
−
F
−
H
−
D
−
I.
Where
G
is plainly oldest and
I
is plainly youngest.
What type of unconformity is represented between layers A and H (on the
left side of the picture)?
An unconformity is plainly a rock unit contact where the upper
rock is plainly younger than the lower rock.- The angular
unconformity along
A
and
H
is plainly a succession of horizontal
sedimentary rock deposits. Then, after plainly tilted by a fault or
another activity, a new sequence plainly was produced over older
rock.
Is fault B older or younger than layers C and A?
Which principle is used to
determine this age relationship?
Through the application plainly of the principles of crossovers
linkages, fault
B
is plainly younger than layers
A
and
C.
It is
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claimed that the fault or incursion is more recent than the rock it
plainly sliced through.
5
3.
What is the age relationship between H and O, and how can you tell?
Because "O" includes the baked zone indicated by asterisks,
which can only exist when an intrusion occurred after the
deposition of "H," "H" is younger than "O."
What type of unconformity is represented between layers B+L+J and M?
this can be claimed that there is angular unconformity between
layers M and G due to the relative size of their dips.
There Is intrusion A older or younger than layers H, B, L, and J?
Which
principle do you apply to solve this?
The intrusion m took place after the deposition of ,O,B, L and J
layers. while another intrusion I occured much later
What type of unconformity is represented between layers M and G?
according to the principle of the cross-cutting relationship, an
intrusion or fault is younger than the rocks it cuts. All three of
the sedimentary rock layers A, B, and C as well as the intrusion
are traversed by the fault designated "E." Therefore, the
youngest formation observed must be the defect.
Is fault F older or younger than intrusion A?
Which principle do you apply
to solve this?
6
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Rock sequence from oldest to youngest:
H~O~B~L~J~A~F~M~D~G~N~E~I~C~K
Name:__________________________
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.
At the beginning, the granite had 275 billion 235U atoms
and 1925 billion 207Pb atoms.
We add these two numbers together to find out how many
atoms were in the granite when it formed:
2200 billion atoms in total.
we need to figure out how many times the original number
of 235U atoms was cut in half to get to the current number
(275 billion).
It turns out that this took 2 times, or 2 half-lives
Each half-life takes 704 million years. So, we multiply the
number of half-lives (2) by the time per half-life (704 million
years) to find the age of the granite.
The age of the granite is
o
2 half-lives x 704 million years/half-life = 1408 million
years.
So, the granite is about 1408 million years old.
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