Lab 11_Crustal Deformation (1) (1)
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GEOS 130 | General Geology
Lab 11: Stress and Strain
Name: _______________________________________________
Overview
In this lab you will investigate the three basic forces that cause crustal deformation, types of
deformation, and the geologic settings where crustal deformation occurs. Reference Chapter
10.1 to formulate your responses, particularly pages 338, 340, and 341.
Part I: Stress
Follow the directions below to explore different types of
stress
that deform rocks.
Use terms
from the Figure 10.2 in your textbook to help answer your questions.
1.
Shape your putty into a circle. Using one hand, pinch the putty, like this
→○←
.
What type of stress is this?
This type of stress shown up above where the putty is pinched to form a circular shape, is
called compressional stress.
Does this type of stress cause crustal thickening, thinning, or sliding?
Compressional stress seems to cause crustal thickening or deformation, because it
pushes rocks together and compresses them. This results in folding or faulting of the Earth’s
crust.
Name the type of geologic setting associated with this type of stress.
The type of geologic setting associated with compressional stress can vary, but it is
commonly found in convergent plate boundaries where two tectonic plates collide. Examples of
such setting include subduction zones, where one plate is forced beneath another, or collision
zones, where two continental plates collide.
2.
Shape your putty into a circle. Now draw a large plus on the surface. Using both hands,
pinch the middle
on both sides and
slowly
pull your hands apart, like this
←○→
.
What type of stress is this?
The type of stress depicted in your description is called tensional stress.
Does this type of stress cause crustal thickening, thinning, or sliding?
This type of stress causes crustal thinning.
Name the type of geologic setting associated with this type of stress.
The type of geologic setting associated with this type of stress is known as a divergent
boundary, where two plates are moving away from each other.
3.
Shape your putty into a circle. Draw a large plus on the surface again. Using both hands,
pinch the top
of the putty on one side and the bottom
of the putty on the other side, then
slowly
pull your hands apart, like this
←
○
→
.
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GEOS 130 | General Geology
Lab 11: Stress and Strain
What type of stress is this?
The type of stress described, as shear stress.
Does this type of stress cause crustal thickening, thinning, or sliding?
Shear stress causes crustal thinning.
Name the type of geologic setting associated with this type of stress.
The type of geologic setting associated with this type of stress is a divergent boundary.
Part II: Deformation
Explore different types of
deformation
that occur in response to stress.
4.
In steps 1, 2, and 3, did the silly putty show elastic, brittle, or ductile deformation?
In steps 1, 2, and 3 of the experiment, the silly putty showed elastic deformation.
Explain your response.
Elastic deformation is when a material can temporarily change its shape under stress but
returns to its original shape once the stress is removed. Silly putty is known for its elastic
properties, so when it was stretched and compressed in these steps, it easily returned to its
original form.
5.
What type of deformation would occur if you continued to pull apart the putty until it broke?
If I continued to pull apart the putty until it broke, I believe it would experience a type of
deformation called plastic deformation.
Explain your response.
Plastic deformation occurs when a material is subjected to stress beyond its elastic limit
and it permanently changes its shape. If I kept pulling the putty apart, it would eventually reach
to a point where it would no longer go back to its original shape.
6.
Describe what happens to atomic bonds during elastic, brittle, and ductile deformation.
a.
Elastic
During elastic deformation, the atomic bonds in a material stretch and deform, but
they do not break. When the stress is removed, the atoms return to their original
positions and the material returns to its original shape.
b.
Brittle
In brittle deformation, the atomic bonds break, causing the material to fracture. This
happens when the applied stress exceeds the material’s strength and the atoms are
unable to rearrange themselves to accommodate the stress.
c.
Ductile
In ductile deformation, the atomic bonds also stretch and deform, but they do not
break immediately. The atoms are able to rearrange themselves through the
movement of dislocations, allowing the material to undergo plastic deformation
without fracturing. This results in the material being able to be shaped or elongated
without breaking.
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GEOS 130 | General Geology
Lab 11: Stress and Strain
Part III: Factors that affect deformation
Follow the directions below to review the four factors that affect how rocks deform.
7.
Once again, apply tension to your putty BUT, this time, quickly
pull your hands apart. Did the
silly putty show brittle or ductile deformation?
If you quickly pull your hands apart. After applying tension to the putty, the silly putty
would most likely show brittle deformation.
8.
Describe how time (pulling the putty quickly or slowly) affects the type of deformation.
When time at which the putty is pulled can affect the type of deformation. if the putty is
pulled quickly, it is more likely to show brittle deformation. On the other hand, if the putty is
pulled quickly, it is likely to show brittle deformation. This means that it will break or fracture
when subjected to tension. On the other hand, if it is more likely to show ductile deformation.
This means that it will stretch and deform without breaking.
9.
Imagine the silly putty was frozen. Describe how frozen versus warm silly putty responds to
stress. State your response in terms of brittle, ductile, or elastic deformation.
The behavior of Silly Putty in response to stress depends on its temperature. When frozen, it
behaves in a brittle and non-ductile manner, while at warmer temperatures, it becomes
ductile and exhibits some elasticity. These differences in response to stress are a result of
the changes in Silly Putty’s mechanical properties as its temperature varies.
10. Look at Textbook Figure 1.20 showing Earth’s layers.
Is cold silly putty an analogue for the lithosphere or the asthenosphere?
If cold silly putty is rigid and behaves more like a solid, it is a better analogue for the
lithosphere. Warm, softened silly putty might serve as a rough analogue for the as
asthenosphere due to its ductile and flowable characteristics when warmed.
Explain your response.
This analogy helps in visualizing and understanding the different behaviors and properties of
Earth’s lithosphere and asthenosphere, even though silly putty is a simplified representation
of these complex geological layers.
Explain how the different confining pressures in the lithosphere and the asthenosphere
affects the way rocks deform in these Earth layers.
These variations in rock deformation behaviors are fundamental to our understanding of the
mechanical properties and behavior of Earth’s layers and play a central role in geological
processes, including the creation and modification of Earth’s crust and the generation of
geological hazards like earthquakes and volcanic eruptions.
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GEOS 130 | General Geology
Lab 11: Stress and Strain
11. We just completed the Rock Cycle unit. Name a rock that would have a tendency for brittle
deformation.
One example of a rock with a tendency for brittle deformation is granite. Granite is an
igneous rock composed primarily of minerals like quartz, feldspar, and mica. It typically has
a coarse-grained texture and is known for its hardness and strength.
Explain your response
Granite tends to deform in a brittle manner because it is relatively rigid and lacks the
internal ductility required for significant plastic deformation. When subjected to stress,
granite is more likely to fracture or break along pre-existing plans of weakness or new
fractures that form in response to the applied stress.
12. Name a rock that would have a tendency for ductile deformation. \
A rock with a tendency for ductile deformation is shale. Shale is a fine-grained sedimentary
rock is composed primarily of clay minerals, silt-sized particles, and organic matter. It is
known for its ability to deform plastically over time.
Explain your response.
Shale tends to deform in a ductile manner because of its composition and the relatively
low confining pressures and temperatures found at moderate depths withing the Earth’s
crust. When subjected to stress over extended periods, shale can slowly flow and deform
without breaking or fracturing significantly.
13. Summarize the four factors that affect how rocks deform by completing the table below.
Factor
Brittle
Ductile
Temperature
Brittle deformation occurs at
relatively low temperatures,
where rocks are less likely to
deform plastically.
Ductile deformation is favored
at higher temperatures, allowing
rocks to flow and deform
plastically.
Confining
Pressure
High confining pressure in the
brittle regime prevents
significant plastic deformation
and promotes fractures and
faulting.
Lower confining pressure in the
ductile regime allows rocks to
deform plastically and flow over
time.
Rock type and
composition
Brittle behavior is typical in rigid
rocks like granite, quartzite, and
basalt.
Ductile behavior is often
observed in less rigid rocks like
shale, schist, and certain types
of sedimentary rocks.
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GEOS 130 | General Geology
Lab 11: Stress and Strain
Brittle deformation is associated
with high strain rates and
sudden stress release, leading
to fractures and earthquakes.
Ductile deformation occurs at
lower strain rates over longer
periods, resulting in slow, plastic
flow and folding.
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