Folds, Faults and the Deformation of Earth

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Folds, Faults and the Deformation of Earths
Crust
Cockscomb structure in Utah
2
Pre-crash
What Happened to this car?
1984
Post-crash
It was deformed (folded) because of the collision
3
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.
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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.

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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.

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The Slinky

• The Slinky is Elastic i.e. it can be stretched
  and stretched but it snaps back to its original
  length.

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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

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The 2nd Type of Stress

• Tension the force that pulls rocks apart
• Rocks that are pulled apart by tension tend to
  become thinner

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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.

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The Deformed Earth

• Plate tectonics and the movement of lithospheric
  plates causes the Earths crust to DEFORM.
• No, no, dont look at me!!! Im hideous!!

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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 earths crust.

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What happened to these rocks?
What evidence do you see, which caused your
interpretation?
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Which of the following types of tectonic forces
tends to squeezeand shorten a rock body?
A. compressive forces B. shearing forces C.
tensional forces D. all of the above
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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
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What type of forces dominate at convergent plate
margins?

• compressive forces
• B. shearing forces
• C. tensional forces
• D. torsional forces

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What type of forces dominate at convergent plate
margins?

• A. compressive forces
• B. shearing forces
• C. tensional forces
• D. torsional forces

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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

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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

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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.

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Which of the following statements best describes
the behavior of rocks during deformation?
ductile materials deform by folding
T- High or Low? P- High or Low?
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?
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These are Sedimentary Rocks
They
must
been
have
The principles of Original Horizontality and
Superposition apply-
folded
But these layers are not any longer Horizontal
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These Rocks were Brittle so they were fractured
to form a Fault
a
a
b
b
The faults were not caused by Compression Forces
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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
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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?
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Measuring the orientationof a surface
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Rocks also have a Strike and Dip
Strike- the direction to which the horizontal
line points
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Geological Map shows the rocks exposed on surface
over which you are walking
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3- D View of Geological Structures
Plan view i.e. the surface that one looks down to
see
The same formation
Outcrop the view where the rocks have been
vertically cut through for a new highway or
natural erosion
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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

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Combinations of Stresses
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A Rift Valley formed by Extension
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Fresh Scarp Faceshowing surface exposed by a
very recent earthquake


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Symmetrical Folds
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