UNRAVELLING THE EARTH’S PAST
GEOLOGIC LAWS POGIL
KEY
NAME
HOUR
Standard 3 Module 6
BACKGROUND: Geologists are scientists that study the Earth. Part of what geologists study is the history of
the Earth and everything that has happened through time. In order to do this there are some assumptions that
are made and laws that are followed. As you navigate the activities below you will learn about these
assumptions and laws. Mr. Wright will take you on a quick visual field trip and a demo to get you started.
MODEL 1:
The Law of Superposition assumes that rock layers on the bottom are older than those on top. Each layer is
younger as you go up. There is an exception to this rule, but we will go over that later.
Watch the demo with the teacher.
Questions
1.) Draw a picture or write a description of the model/demo.
2.) Which layer is oldest?
3.) Which layer is youngest?
4.) What law does this demo illustrate?
Model 2: Below is model 2. It represents 4 rock layers. These are sediments that have now turned to solid
rock. The Law of Superposition states that the oldest layers are at the bottom and the youngest at the top. We
can use this law to help order geologic events. We call this relative dating. So, if we record the order in data table 2
you can see the order.
Model 2
Data Table 2
A
R
C
Youngest
Oldest
M
Order
A
R
C
M
Reason (Law)
Superposition
Superposition
Superposition
Superposition
Practice with Law of Superposition: Fill out data table 3 and 4 using the same ideas as in model 2.
Model 3
E
G
Data Table 3
Youngest
D
Oldest
Order
E
G
D
R
Reason (Law)
Superposition
Superposition
Superposition
Superposition
R
Model 4
Data Table 4
B
Youngest
M
A
Z
Oldest
Order
B
M
A
Z
Reason (Law)
Superposition
Superposition
Superposition
Superposition
GEOLOGIC STRUCTURE INTRODUCTION: As you already know the Earth’s surface is always shifting and moving as large
plates move around. These plates are driven by liquid or nearly liquid material moving, rising, and faling. Kind of like an
object getting moved along a conveyor belt. As a result the surface of the Earth is always changing. There are
earthquakes that make faults, erosion occurs and make things called unconformities.
Faults Faults tell us that there was once an earthquake in this part of the world in the past. Faluts form in a couple
steps. First, pressure or stress builds up. Then the rock snaps and releases energy in the form of an earthquake. Look at
Figure 1 to see a fault.
Unconformities Unconformities tell us there was lots of erosion going on in that part of the world in the past. Look at
Figure 2 for a visual and description of how they form. The presence o f unconformities tell us there was lots of erosion
in the past and possibley there were mountains here in the past.
Intrusions Sometimes big blobs of magma cools, crystallizes into features called intrusions. Then erosion exposes them
at the surface. Intrustions suggest that there may have been subduction happening in this part of the world. This is when
a plate dives under another plate and melts. The melted magma rises and sometimes cools into solid rock. Look at
Figure 3.
Folded and Tilted Rock Layers Our last feature are folds and tilted rock layers.These are created by tectonic plates
running into each other and bending the rock layers or tilting them up. Folds tell us that great pressures once existed
and there may have been an ancient mountain system in this area in the past. Take a look at Figure 4.
Figure 1
The fault is this crack that breaks
the rock. Here the rocks on the left
went down and those on the right
went up.
Questions
5.) Draw a picture of a fault. Use Figure 1 to help you.
6.) Explain how faults are formed.
Pressure or stress builds up in the rocks and then they snap and release energy in the form of an earthquake
7.) What do faults tell us about the past?
There was once an earthquake in this area in the past
Figure 2
Step 1: layers 1 through 5 are deposited.
Step 2: all the layers are folded or tilted.
Step 3: The top layers are eroded away.
Step 4: The top layers are eroded away.
Questions
Unconformity is the wavy
line representing eroded
away material
8.) Draw a picture of an unconformity. Use Figure 2 to help you.
9.) Explain how unconformities are formed.
Layers are deposited, then tilted up. When they are tilted up there will be increased erosion on the upper layers.
These are eroded away. Finally, layers are deposited on top of the layers.
10.)What do unconformities tell us about the past?
There was once lots of erosion going on and maybe mountains where here in the past.
Figure 3a
Any of the red underneath the
ground is an intrusion. However,
it is only an intrusion if it cools
underground.
Figure 3b
Intrusions form here. As the
subducting plate melts the magma
rises up. If it cools before it gets to
the surface it will turn to solid rock an
form an intrusion.
Questions
11.) Draw a picture of an intrustion. Use Figure 3 to help you.
B is the intrusion
12.) Explain how intrusions are formed.
Intrusions often form where there are subducting plates. One plate going under another. The plate melts and some of
the magma rises, but cools before it gets to the surface.
13.)Look at Figure 3b. What do intrusions tell us about the past?
There ma have been subducting plates here in the past.
Possibly volcanoes if some of the magma made it to the surface.
Figure 4
Questions
14.) Draw a picture of a fold. Use Figure 4 to help you.
15.) Explain how folds are formed.
Pressure builds up and bends rock layers
16.)Look at Figure 3b. What do intrusions tell us about the past?
There was once great pressure and possibly an ancient mountain system.
MORE ROCK LAWS
Law of Original Horizontality This law states that rock layers are deposited in horizontal layer IF they have NOT been
messed with. Remember the sediment jar? The layers were horizontal after the shaking. We can use this law to help
order geologic events. We call this relative dating which is simply putting events in order from oldest to youngest. You
can see in model 5 all the layers had to be there first. It kind of make sense because you can’t tilt something that isn’t
there yet. That is why the folding and faulting are always younger than the thing they fold or fault. Also, we know that
the bottom layers are the oldest because of the law of superposition. Look at model 5 as an example then try the next
two with your table team.
Model 5
Data Table 5
Youngest
A
R
C
Oldest
M
Order
Tilting and folding
A
B
C
M
Reason (Law)
Original Horizontality
Superposition
Superposition
Superposition
Superposition
Model 6
Data Table 6
B
G
Youngest
D
E
Oldest
Order
Tilting and folding
B
G
D
E
Reason (Law)
Original Horizontality
Superposition
Superposition
Superposition
Superposition
Model 7
Data Table 7
B
Youngest
M
A
Z
Oldest
Order
folding
B
M
A
Z
Reason (Law)
Original Horizontality
Superposition
Superposition
Superposition
Superposition
Law of Cross Cutting Relationships This law states if something gets cut across it had to be there first and therefore is
older than the thing cutting it. For instance what is older a rock layer or the fault that cuts across it? The layer of rock
had to be there first so it is older than the fault. In model 8 the fault is breaking or cutting across the rock layers. The
rock layers had to be there first. This is true for any geologic event. You follow the same procedure and complete Data
table 9 and 10. Try Model 11. In Model 11 you will run into a problem. There is more than one correct answer. See the
teacher if you are unsure.
Data Table 8
Model 8
Youngest
Oldest
Order
The fault
C
R
A
Reason (Law)
Cross Cutting Relationships
Superposition
Superposition
Superposition
Data Table 9
Model 9
Youngest
Unconformity
Oldest
Order
Unconformity
Tilting of
layers
A
R
D
M
B
Model 10
Reason (Law)
Cross Cutting
Original Horizontality
Superposition
Superposition
Superposition
Superposition
Superposition
Data Table 10
Youngest
Oldest
Order
Intrusion
T
B
Z
D
R
Data Table 11
Model 11: This is the tricky one.
Youngest
Oldest
Reason (Law)
Cross Cutting
Superposition
Superposition
Superposition
Superposition
Superposition
Order
B
W
W L
L
B
A
A
R
R
Z
Z
W
B
L
A
R
Z
Reason (Law)
Cross cutting
Superposition
Superposition
Superposition
Superposition
Superposition
Superposition
Superposition
Cross cutting
Superposition
Superposition
Superposition
Superposition
Cross cutting
Superposition
Superposition
Superposition
Superposition
Putting It All Together: Now we will put ALL our understanding together. For each model we will do the
relative dating (ordering) of events, state what law supports the placement, and write evidence and inference
about the past for each model. Model 12 will be the starting point. You should be able to do model 13 and 14
for extra credit (it is at the end).
Data Table 12
Model 12
Order/Event
F
J
B
Reason (law)
Superposition
Superposition
Superposition
Erosion A
(unconformity)
R
S
M
K
P
Superposition or
Cross cutting
Cross cutting
Superposition
Superposition
Superposition
Superposition
Evidence Inference about the past
Evidence From Model 9
Intrusion R
Erosion A/Unconformity
Inference we can make about the past
Subducting plates and possibly volcanoes if the magma reached
the surface.
Lots of erosion happened in the past and possibly there was an
ancient mountain system here.
Model 13
Data Table 13
Order/Event Reason (law)
K
Superposition
R
(B) Tilting of
folding layers Q,O,
N,M,L
Z
B AND z could be switched .
You canst tell which came
first with the given evidence.
13
12
R
11
10
9
8
7
6
5
4
3
2
1
J
I
H
Z
B
P
L
M
N
O
Q
Super position OR cross
cutting relationships
Superposition
Superposition
Superposition
Cross Cutting Relationships
Original Horzontality
Superposition
Superposition
Superposition
Superposition
Superposition
Evidence From Model 9
Intrusion P
Unconformity K and Z
No faults
Inference we can make about the past
Subducting plates and possibly volcanoes if the magma reached
the surface.
Lots of erosion happened in the past and possibly there was an
ancient mountain system here. Because there are two there may
have been two different ancient mountains.
No earthquakes in the past in this area.
“BREAKING THE LAW”
Background Information: As we have learned there are certain rules and laws that scientists use to determine
the order of events in the Earth’s past. The law of Superposition states that the oldest layer is on the bottom
and the youngest is on top. However, there are some instances that “break” this law. One instance is a fold
that is so folded that the bottom layers are on top of younger ones. See Figure 5 below. Another instance is
when a fault moves older rocks on top of younger rocks. See Figure 6. We don’t really break the Law of
Superposition because the law says it is true for undisturbed rock layers. Clearly these rocks have been
disturbed. They have been folded and faulted by great pressure.
Figure 5
You can see that layers are folded over on top
of each other. So an older layer could be above
a younger layer.
Questions
17.) What does the Law of Superposition assume?
Figure 6
You can see that layers are faulted and the left
side is pushed up over on top of the right side.
Notice that the older blue layer on the left is
now over the younger tan layer on the right.
That rock layers are undisturbed.
18.) What are the two ways that the Law of Superposition appears to be broken?
Overturned folds and faults
19.) Why is the Law of Superposition really not broken in these two situations?
Because in these two cases, overturned folds and faults, they layers HAVE been disturbed.
Model 14 EXTRA CREDIT
Unconformity R
(X) Tilting/folding
layers H,I,J,K
(Y) Tilting/folding
layers A,B,C,D,E
Unconformity M
Data Table 11
Evidence Inference about the past
17
16
15
Order/
Event
L
N
R
14
13
12
11
10
9
X
K
J
I
H
M
8
7
6
5
4
3
2
1
Y
F
G
E
D
C
B
A
Reason (law)
Crosscutting
Superposition
Crosscutting OR
Superposition
Original horizontality
Superposition
Superposition
Superposition
Superposition
Superposition OR
Crosscutting
Original horizontality
Crosscutting
Crosscutting
Superposition
Superposition
Superposition
Superposition
Superposition
Evidence From
Model 9
Intrusion F
Fault F
The folding and
tilting
Inference we can make about the
past
Subducting plates and possibly
volcanoes if the magma reached the
surface.
An earthquake happened in the
past in this area.
There was great pressure and
maybe there existed an ancient
mountain system
HEY HEY HEY!!!
This is only one possible order. See
the teacher if you have questions
about others that may work. For
instance F could be the last thing or it
could be where it is placed in this key.
Unconformity M