Geologic Time Lab 2
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Durham Technical Community College *
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230
Subject
Geology
Date
Dec 6, 2023
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6
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Geologic Time Laboratory Assignment
Part 1 – Relative Age Dating Techniques
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TRODUCTIO
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Cliffs and road cuts
allow us glimpses into geology, which is often hidden from view.
This produces ‘side
views’ or ‘geologic cross-sections’ showing us the relative positions of various rock layers and structures for a
particular area.
Applying principles of relative age dating to these rock exposures (also called ‘outcrops’), we can
reconstruct
the sequence of events
that created the geologic features which we see.
Examples of events include deposition
of a sedimentary rock layer, the eruption of a lava flow, the intrusion of magma to form a batholith, dike or sill,
a fault in the rock that shifts one block of rock relative to another block of rock, a fold that bends and distorts
rock layers, erosion and formation of an unconformity, and any number of other geologic processes.
Working
out the sequence of events using cross-sections is much like solving a puzzle, determining ‘which came first’,
and so on.
We aren’t determining absolute ages for these events (i.e. this rock is 90 Ma old), rather we are
determining their ages ‘relative’ to one another (what came first, second, and so on).
In your Course Content
, you learned that there are several Principles that are used to determine relative ages.
These include the Principle of Superposition, the Principle of Original Horizontality, the Principle of Faunal
(Fossil) Succession, the Principle of Crosscutting Relationships, and the Principle of Inclusions.
If these are
not familiar to you, go back and re-visit your Course Content material for Unit 7.
As an example
, let’s look at the cross-section below
.
Notice that there are layers of sedimentary rock in this
example (a key for the symbols follows) and a dike that have all been offset by a fault (E).
To put a series of
geologic events in order, I always start with the oldest event first.
Using the
Principle of Superposition
, I
know that the oldest is on the bottom – this would be deposition of C.
The next event in the sequence is
deposition of B, followed by deposition of A.
Since the dike D cuts across all of these layers (
Principle of
Cross-Cutting Relationships
), I know that layers C, B, and A had to be in place to be intruded by D.
So, my
next event would be intrusion of D (note that it is an igneous intrusion).
That leaves the fault – notice that it
cuts across everything, including the dike D.
This would be my youngest event.
I would record my solution as
a list:
Youngest – Fault E
Intrusion of igneous rock D
Deposition of Sandstone A
Deposition of Limestone B
Oldest – Deposition of Shale C
In addition to the materials in the ‘Content’ section of this Unit, there are numerous learning tools available on
the internet that will be helpful in learning how to interpret geologic cross-sections.
One of those is available
through “
Athro Limited
”, a private company providing education modules for geology students.
Try solving
the cross-sections available and you will get immediate feedback!
QUESTIO
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S FOR PART 1
Cross-Section 1.
a.
For the following cross-section, determine the order of events (A, B, C, D, E, F, H, and G) from oldest to
youngest, and record them in your
Lab document.
Layer-E, Layer-D, Layer-H, Layer-C, Layer-B, Layer-A, Layer-F, Layer-G
b.
Identify and describe which relative age dating principles helped you to determine this sequence.
Started with the youngest which was layer E since it’s all the way at the bottom, then d, h, c and b because
they’re layered over each other then f because it goes through all of the previous layers meaning they had to be
there before it intersected and finally g because it’s at the very top and doesn’t intersect anything
it’s just the
upper most layer making it the youngest.
Cross-Section 2.
a.
For the following cross-section, determine the order of events (A, B, G, H, K, M, and X) from oldest to
youngest, and record them in your
Lab document.
Layer-K, Layer-H, Layer-G, Layer-A, Layer-X, Layer-B, Layer-M
b.
Identify and describe which relative age dating principles helped you to determine this sequence.
This one was the easiest because they just layered over each other (original horizontality). Layer k is the oldest
since it’s all the way at the bottom, h layered over it, g over h, a over g, x over a, b over x and m over b.
Cross Section 3.
a.
For the following cross-section, determine the order of events (B, D, K, L, M, N, Q, S, T, X, and Y),
from oldest to youngest, and record them in your
Lab document.
Layer-T, Layer-S, Layer-X, Layer-Y, Layer-D, Layer-B, Layer-N, Layer-M, Layer-L, Layer-Q, Layer-K
b.
Identify and describe which relative age dating principles helped you to determine this sequence.
T is all the way at the bottom and s and x layered over it until y, d and b intersected (cross-cutting relationships)
but they dont intersect N which means it wasn’t there when they originally intersected. Then original
horizontality began again after N with m over n, l over m, q over l, and k over q.
Part 2 – Absolute Age Dating
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Some elements form isotopes
with unstable atomic nuclei that have a tendency to change, or decay.
For
example, U-235 (Uranium 235) is an unstable isotope (
parent
) of uranium that has 92 protons and 143 neutrons
in the nucleus of each atom.
Through a series of changes within the nucleus, it emits several particles, ending
up with 82 protons and 125 neutrons.
This is a stable condition (
daughter product
), and there are no more
changes in the atomic nucleus.
This stable atom is the element lead (chemical symbol Pb, 82 protons, 125
neutrons).
This particular form of lead is called Pb-207 (adding up the protons and neutrons remember gives us
the atomic mass).
In this case, U-235 is the parent isotope and Pb-207 is the stable daughter product.
Many rocks contain small amounts of unstable isotopes and the daughter isotopes into which they decay.
Where the amounts of parent and daughter isotopes can be accurately measured,
the ratio can be used to
determine the absolute age of the rock.
QUESTIO
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S FOR PART 2
# of half-lives elapsed
The figure above shows the relationship between the percentage of parent material and the number of half-lives
that have passed.
1.
What percentage of the parent material is present after…
a.
1 half-life=
50%
b.
2 half-lives=
25%
c.
3 half lives=
12.5%
d.
4 half -lives=
6.25%
2.
If you start with 80 grams of an isotope (parent material), how much would be left…
a.
after-one half-life=
40
b.
after three half-lives=
12.5%
3.
If an isotope has a half-life of 600 million years…
a.
How old is a rock that contains the isotope after 50% of the parent has decayed?
600 Million
b.
How old is the rock after four half-lives have passed?
2.4 billion because 4x600=2,400
4.
You discover the parent isotope in a lava flow has gone through 0.75 half-lives.
If a half-life is 800
million years, how old is that rock?
0.75x 800= 600
Percentage (%) of Parent Material
5.
In Cross Section 2 in Part 1 of this Laboratory Assignment, assume that Layer X was dated using
isotopic evidence at 260 million years old.
Layer B was determined to be 235 million years old.
When
did the fold occur?
Part 3 – Correlating Cross Sections
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Cross sections are graphical representations of vertical slices through the Earth.
They can be used to look for
geological relationships between layers of rock in different areas.
Below are three cross section containing
various fossils represented by symbols.
Assume that these three cross sections were constructed based on
observations made from three road cuts (vertical slices of the Earth).
Take a few moments and notice how each
cross-section relates to the others.
You may consider printing out a copy to color layers with the same fossils.
1.
If the 5-pointed star fossil is 325 million years old (Ma), and the heptagon (the 7-pointed star fossil) is
337 Ma, how old is the 15-pointed fossil in between?
The difference is 337- 325 = 22 , so
325 + 22/2 = 336 ma
2.
If the 5-pointed star fossil existed for 3 million years, from 324ma-327ma, how old must the arched
arrow in Section 3 be?
if the star existed from 324-327 ma, then the arched arrow must have existed for 336 -327 = 8 years
Section 1
Section 2
Section 3
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