Folds,
Faults
and the
Deformation
of Earth’s
Crust
Cockscomb structure in Utah
1
What Happened to this car?
Pre-crash
1984
Post-crash
It was deformed (folded) because of the collision
2
Coincidentally, note how this car is
so much more safe
The car deformed like an accordion
while the passenger compartment did not suffer.
That is possible because the front part of the
car absorbs the stress by folding.
3
Stress vs. Strain
The force applied to some body is the stress.
The changes to that body are the strain that is
recorded.
You work very hard lifting things.
Your back is strained.
4
Silly Putty
Silly Putty absorbs the Stress that is
applied to it when it hits the floor. However,
the Strain is released when it bounces
away from that Stress.
Silly Putty is elastic, it bounces if it were
dropped on the floor.
5
The Slinky
The Slinky is Elastic i.e. it can be stretched
and stretched but it snaps back to its
original length.
6
The 1st Type of Stress:

Compression: occurs when crustal rocks
are squeezed together

Can reduce the volume of rocks, can push
the rocks higher up or deeper down into the
crust
7
The 2nd Type of Stress

Tension: the force that pulls rocks apart

Rocks that are pulled apart by tension tend
to become thinner
8
The Final Type of Stress

Shearing: pushes rocks in opposite
horizontal directions

These rocks bend, twist, or break apart as
they slide past each other.
9
The Deformed Earth

Plate tectonics and the movement of
lithospheric plates causes the Earth’s crust
to
DEFORM….
“No, no, don’t look at me!!! I’m hideous!!”
10
Isostasy and the Mattress Test

Have you ever seen the commercial for
foam mattresses where they place a glass
of red wine on one side and a man jumps
up and down on the other side?

Isostasy: the balancing of up-and-down
movements of the earth’s crust.
11
What happened to these rocks?
What evidence do you see, which caused your interpretation?
12
Which of the following types of
tectonic forces tends to squeeze
and shorten a rock body?
A.
B.
C.
D.
compressive forces
shearing forces
tensional forces
all of the above
13
Which of the following types of tectonic
forces tends to squeeze and shorten a rock
body?
A. compressive forces
B. shearing forces
C. tensional forces
D. all of the above
This looks like the safe car
because of how it folded
14
What type of forces dominate at
convergent plate margins?
compressive forces
B. shearing forces
C. tensional forces
D. torsional forces
A.
15
What type of forces dominate
at convergent plate margins?
A. compressive forces
B. shearing forces
C. tensional forces
D. torsional forces
16
The San Andreas Fault is a result
of what type of forces?
A.
B.
C.
D.
compressive forces
shearing forces
tensional forces
all of the above
17
The San Andreas Fault is a result
of what type of forces?
A. compressive forces
B. shearing forces
C. tensional forces
D. all of the above
18
Which of the following statements
best describes the behavior of rocks
during deformation?
A. Brittle materials deform by faulting, whereas
ductile materials deform by folding.
B. Brittle materials deform by folding, whereas
ductile materials deform by faulting.
C. Both brittle and ductile materials deform by
faulting.
D. Both brittle and ductile materials deform by
folding.
19
Which of the following statements best
describes the behavior of rocks during
deformation?
T- High or Low?
P- High or Low?
ductile materials
deform by folding
Marble is special
because it can be
Brittle or Ductile
depending on
Pressure and
Temperature
Brittle materials
deform by faulting
T- High or Low?
P- High or Low?
20
These are Sedimentary
Rocks
The principles of
Original Horizontality
and Superposition
apply-
But these layers are not any longer Horizontal
21
These Rocks were Brittle so they were
fractured to form a Fault
a
a
b
b
The faults were not caused
by Compression Forces
22
These Rocks were Brittle so they were
fractured to form a Fault
The faults were not caused by Compression Forces
The length of the rock formation
is greater than the original length
because of the type of fault
The faults were caused
by Tension
a
a
b
b
23
More FOLDED Sedimentary Rocks
But HOW MUCH have they been folded?
or
How can one measure the angle that they
have rotated from the original horizontal?
24
Measuring the orientation
of a surface
25
Rocks also have a Strike and Dip
26
Geological Map shows the rocks exposed
on surface over which you are walking
27
3- D View of Geological
Structures
Plan view i.e. the surface that
one looks down to see
The same
formation
28
Ductile Materials and
Deformation
They undergo Plastic Deformation i.e.
they change shape and do not break but
do not return to their original form
29
Combinations of Stresses
30
A Rift Valley formed by
Extension
31
Fresh Scarp Face
showing surface exposed by a
very recent earthquake
*
*
32
Symmetrical Folds
33
34