Copy of Lab 07 Glaciers and Deserts-Szabo
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106
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Dec 6, 2023
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GEOSCI/ENVIR ST-106: Environmental Geology
Lab 7: Glaciers and Deserts - Shaping the Earth’s Surface
Assignment Overview:
The main processes shaping the land surface take place because of
(unfrozen) water. In extreme environments, ice and wind represent other fluids that can shape
landforms. This lab will investigate how the movement of air and ice, contribute to erosional and
depositional processes that shape Earth’s surface.
Submission:
To submit the assignment on Canvas, use the following steps:
1.
In Google Docs, generate a PDF: File → Download as → PDF Document
2.
In Google Docs, use Share → Get Shareable Link, and copy the link address
3.
In Canvas, upload your PDF to the assignment, and paste the link address to your
Google Doc in the assignment comments.
1
Assignment Questions and Responses
Instructions:
Fill out each red highlighted field (_________)
Part 1: Movement of Sediment by Fluids (Air and Water)
Background:
The maximum speed of a particle falling through a fluid depends on when drag
forces and gravitational forces are balanced. For instance, when a skydiver jumps out of a
plane, they can reach a “terminal velocity” of about 120 miles per hour. This is the speed at
which the force of the wind is exactly equal to the force of gravity on their body, and so the
skydiver doesn’t accelerate any more.
What skydivers call “terminal velocity” is the same thing as a “settling velocity” for a grain grain
of sediment. As you can probably guess, settling velocity depends on a variety of factors
including properties of the fluid (like fluid density and fluid viscosity) and properties of the falling
body (like size, shape and density). Settling velocity is also a great way to get an estimate of
what sediment a fluid will carry. If the settling velocity of a grain falling through fluid is 1 cm/s,
then this implies that it will take a fluid moving at least 1 cm/s to pick up the grain from a resting
position.
For spherical grains, Stokes’ Law
gives a good estimate of the settling
velocity for any particle. The formula
for Stokes’ Law is:
? = (?
𝑝𝑎??𝑖?𝑙?
− ?
?𝑙?𝑖?
)??
2
/18µ
?𝑙?𝑖?
where:
v is the settling velocity (m/s)
d is the density of the particle / fluid
(kg/m
3
)
g is gravitational acceleration (9.8
m/s
2
)
s is a particle’s size (diameter in m)
is the fluid viscosity (kg/(ms))
µ
The graph to the left shows curves of
Stokes’ law for a spherical quartz
grain (
= 2.65 g/cm^3) in
?
𝑝𝑎??𝑖?𝑙?
different fluids. A larger version can be
downloaded from the lab page.
2
Using the Stokes’ Law plots as well as the grain-size
classification on this page, answer the following questions:
1. (2 points) An average wind speed for Wisconsin is about
10 miles per hour. What types of sediment could be carried
by these winds, based on the Stokes’ equation? Show how
you arrived at this answer.
(Hint: Make sure to convert wind velocity to m/s first!)
10mph=4.5m/s
10^x=4.5
x=.65
d=10^-4.5*1000=0.032mm
This means that the largest sediment this wind could move
is silt.
2. (2 points) In Wisconsin, the average peak gust speed for
wind is roughly 50 miles per hour. Based on the Stokes’
Law plot, what types of sediment could be carried by these
peak wind gusts?
50mph=22.4m/s
10^x=22.4
x=1.35
d=10^-3*1000=1mm
This means that the largest sediment that could be moved by the wind is very coarse and
coarse sand. But it could also transport medium sand, fine sand, silt and clay.
3. (2 points) Based on the Stokes’ law plot, how fast would wind have to be moving (in miles per
hour) to pick up small gravel particles? How likely is it to see winds of this speed on earth? You
can provide a reference to answer the second part of this question.
d=2 mm=0.002m=2*10^-3 in diameter which is about 10^2.5 m/s which is about 707.4mph. This
velocity is not likely to be seen on Earth.
4. (2 points) How fast would water have to be moving (in miles per hour) to carry a small gravel
particle? Briefly discuss how plausible these velocities are relative to your response in question
3.
d=2 mm=0.002m=2*10^-3m which is 10^0.7m/s which is about 11.2 mph. This can be seen on
Earth, unlike in 3.
3
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5. (2 points) The Stokes’ Law plot above assumes that spherical particles of quartz are being
transported in a fluid. In a few sentences, describe factors that may cause actual sediment
settling velocities to deviate from the ideal Stokes’ Law plots above..
The actual sediment velocities can deviate because of the different types of fluid that they are in,
the size and shape of each particle,
Part 2: Movement of Sediment by Ice
Background:
Ice consists of water linked in chains via hydrogen bonds that are relatively easily
broken, allowing ice to flow over long periods of time. In this lab we will use an elastomer, “gak”
which has similar properties – it contains long chain polymers which are linked by weak
hydrogen bonds to borate ions, allowing layers of polymers to flow past each other (see
animation at:
http://www.dynamicscience.com.au/tester/solutions1/chemistry/gakistuff/gak.htm
).
2.1 Observing Glacier Movement
Your TA will demonstrate movement of a “gak glacier” using two different scenarios: 1) A dry
scenario, in which the base of the glacier touches the surrounding “rock”, and 2) a wet scenario
in which glacial meltwater has penetrated to the base of the glacier. After observing and timing
the process of gak movement, answer the following questions (using complete sentences) and
consider their relation to glacier processes.
6. (3 points) Fill in the table below based on your observations from the video
Experiment
“Sediment” (Marker
dots) Location
Time [minutes]
Distance
[meters]
Gak Velocity
[m/min]
Dry
Center
____5_____
___0.06___
__0.012____
Top Edge
____5_____
___0.03___
___0.006___
Bottom Edge
____5_____
___0.045__
__0.009___
With water
Center
____5_____
____0.05___
__0.01_____
Top Edge
____5_____
____0.015__
___0.003___
4
Bottom Edge
____5_____
____0.03___
__0.006____
2.2 Synthesis Questions
7. (2 points) Why is gak a good proxy for glacial ice flow? What are some shortcomings of this
proxy in simulating glacial processes?
____Glaciers flow plastically, and so does gak so it demonstrates a small scale version of how
glaciers move._____
8. (1 point) What causes the gak glacier to flow?
____Gak flows because of gravity, they flow downhill under their own weight. _____
9. (1 point) What part of a glacier flows the fastest, and why do the other parts of the glacier flow
more slowly?
____The center flows the fastest. The other parts move more slowly due to friction between the
rocks. _____
10. (2 points) Do glaciers flow faster with or without meltwater at the base? Why is this?
___Glaciers move faster with water because the water acts as a lubricant allowing the glacier to
slide over the rocks. _____
11. (2 points) Describe two processes by which a glacier moves rock (i.e., erosional or
depositional processes), and the visible evidence for these processes on real glaciers.
____Erosional process is when the glacier scrapes along rocks and produces till, this creates a
u-shaped valley or can be seen through scrapes in rocks. Depositional processes are when
glaciers start to melt and as it moves down the mountain it deposits minerals, or the water that is
melted off the glacier carries these minerals away down the mountain. _____
5