1 Folds, Faults and the Deformation of Earth’s Crust Cockscomb structure in Utah

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Folds, Faults and the Deformation of Earth’s Crust

Cockscomb structure in Utah

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Pre-crash

Post-crash

What Happened to this car?

It was deformed (folded) because of the collision

1984

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Coincidentally, note how this car is so much more safe

The car deformed like an accordionwhile the passenger compartment did not suffer.That is possible because the front part of thecar 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 Earth’s crust

to DEFORM….

“No, no, don’t look at me!!! I’m 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 earth’s crust.

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What happened to these rocks?

What evidence do you see, which caused your interpretation?

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A. compressive forcesB. shearing forcesC. tensional forcesD. all of the above

Which of the following types of tectonic forces tends to squeezeand shorten a rock body?

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A. compressive forcesB. shearing forcesC. tensional forcesD. all of the above

Which of the following types of tectonic forces tends to squeeze and shorten a rock body?

This looks like the safe carbecause of how it folded

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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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A. compressive forcesB. shearing forces

C. tensional forces

D. torsional forces

What type of forces dominate at convergent plate margins?

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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 forcesC. 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?

Marble is specialbecause it can beBrittle or Ductile

depending onPressure and Temperature

Brittle materials deform by faulting

T- High or Low?P- High or Low?

T- High or Low?P- High or Low?

ductile materialsdeform by folding

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The principles ofOriginal Horizontality and Superpositionapply-

These are Sedimentary Rocks

But these layers are not any longer Horizontal

folded

must

have beenThey

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These Rocks were Brittle so they were fractured to form a Fault

The faults were not causedby Compression Forces

a

a

b

b

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These Rocks were Brittle so they were fractured to form a Fault

The faults were not caused by Compression Forces

aa

b

b

The length of the rock formationis greater than the original lengthbecause of the type of fault

The faults were causedby Tension

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More FOLDED Sedimentary Rocks

But HOW MUCH have they been folded?

or

How can one measure the angle that theyhave 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

Outcro

p the view w

here th

e rocks h

ave been verti

cally cut

thro

ugh for a

new h

ighway o

r natu

ral e

rosio

n

Plan view i.e. the surface that one looks down to see

The sameformation

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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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