Samantha_Hernandez_Chapter_2_Lab_2
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Johnson County Community College *
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101
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Geology
Date
Dec 6, 2023
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Plate Tectonics (Lab 2)
For this lab you will need the following:
Google Earth
Divergent and Transform Boundaries KMZ file (under the Lab 2 assignment link in Canvas)
Convergent Boundaries KMZ file (under the Lab 2 assignment link in Canvas)
Scientific calculator
– you can use the calculator on your computer and change the setting to
“Scientific”
Divergent and Transform Boundaries
Red Sea rift zone
San Andreas Fault near Wallace Creek, California
Click the link for the Divergent and Transform Boundaries” KMZ file under the Lab 2 assignment
to begin this activity.
Clicking on the link should launch Google Earth™ and open the file in your
“Places” frame.
If not, you can right click on the KMZ file, select “Save Target As…” and save the
file to your desktop.
Open Google Earth™ and under File at the top of the page select “Open” and browse to the KMZ
file you just saved on your desktop.
Divergent Boundaries
Expand the folder “Divergent Boundaries—Location 1” and click the “Divergent Boundaries—Location 1”
placemark. You are looking at the Red Sea between the African and Arabian tectonic plates.
1.
Examine the shape of the shoreline of the Red Sea. What evidence can you cite to support the
notion that the northeast (right-hand) and southwest (left-hand) shores of the Red Sea were
once joined to each other?
The line up with each other kind of like puzzle pieces, with each curve and edge the opposite
side complements it where it would fit together.
2.
Click the box next to the path “Red Sea Rift” to turn it on. If you assume that the shores of the
lake represent the limits of the forming rift valley, how should the ages of the rocks on the
northeastern shore of the Red Sea compare with the ages of the rocks on the southwestern
shore?
The age of the rocks on the northeastern shore of the Red Sea should be the same age of the
rocks on the southwestern shore. This is because if they were once joined then they would have
to be in existence at the same time.
3.
Studies have shown that the rocks rimming the shores of the Red Sea in this area are
approximately 20 million years old. If that is the case, is the Red Sea rift opening at the same rate
throughout its length?
I don’t think that the rift is opening at the same rate throughout its length because it’s narrower
by placemark A than at placemark B. However,
it is opening faster at the southern end by
placemark B.
4.
Use the Ruler tool to determine the width of the rift valley at placemark A and placemark B in
meters, and convert your measurement to mm. Record your measurements below.
Hint: there are 1000 millimeters (mm) in 1 meter (m)
Width of rift valley at placemark A = 197,214_____m =
197,214,000_____mm
Width of rift valley at placemark B = 277,779____m = ____277,779,000_mm
5.
Using the estimated age of the rift of 20 million years, calculate the annual spreading rate in mm
at placemark A and again at placemark B.
Placemark Latitude of Placemark Spreading Rate at Placemark
A _9.8 mm/yr
B
13.9 mm/yr
6.
Radiometric dating of drill core samples taken from the ocean floor just east of North Carolina
revealed an age of about 160 million years (m.y).
If average distance in kilometers from the
coastal shelf of North America to the axis of the Mid-Atlantic Ridge is about 3,200 km, calculate
the spreading rate (velocity) in cm/yr (assuming it is constant) of the Mid-Atlantic Ridge.
Hint:
This is a rate (velocity) question where velocity (v) = distance (d) / time (d)
v
=
d
t
1 kilometer = 1000 meters
1 meter = 100 centimeters
3200x1000x100=320,000,000 cm320,000,000 cm/160,000,000 yrs= 2 cm/yr
Transform Boundaries
Expand the folder “Transform Boundaries—Location 1.” You are looking at a section of the San Andreas
Fault in California. At this location, the beds of several streams and gullies have been offset by
displacement along the fault. The red line represents the approximate location of the fault in this area,
and placemarks A, B, and C mark the location of offset valleys.
Click placemark A in the sidebar to zoom in to Wallace Creek. Turn on the “Wallace Creek Map” layer by
clicking the box next to it. You may want to make it more or less opaque using the slider at the bottom of
the Places sidebar.
7.
Use the Ruler tool to determine the current offset, in feet, of Wallace Creek where it crosses the
San Andreas Fault, and enter that measurement on Figure 11.1, below:
637.2 feet
8.
Imagine you are standing at placemark A and looking NE across the fault. Is the land on the
opposite side of the fault moving to your right? Or is it moving to your left? How would the
motion appear to someone standing on the opposite side of the fault looking back toward you?
If I was looking northeast from the placemark. A the other side of the fault would be moving
towards the right. If I was standing on the opposite side of the fault looking back towards
placemarker A, the fault would look as if it was moving left.
9.
Use the Ruler tool to determine the offsets on the smaller river at placemark B, and on the
gullies on either side of placemark C. Fill in the chart below:
Wallace Creek offset = _______45.25___ft.
Placemark B offset = _____147________ft.
Placemark C offset = _____45________ft.
10.
Studies indicate that at the time the offset of Wallace Creek began 13,000 years ago, it flowed
through the valley indicated at placemark D, and only more recently cut its present channel into
the land on the western side of the fault. Determine the average annual offset along this section
of the San Andreas Fault (in inches) during the last 13,000 years.
590
11.
Visit the link to the Wallace Creek field trip guide in placemark A (or here:
http://www.scec.org/wallacecreek/guides/gsa-wc.pdf
), and compare your measured offsets and
calculated annual offset rate with those reported in the opening pages of the field trip guide.
Convergent Boundaries
Cascadia Margin of the U.S. Pacific Northwest Pacific Coast of Central America
Aleutian Islands, Alaska
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Click on the link for the Convergent Boundaries KMZ file to begin this activity.
Convergent Boundaries—Location 1: The convergent plate boundary along the Cascadia Margin
of the U.S. Pacific Northwest
You are looking roughly north along the coast of Oregon and Washington states. Take a few minutes to
fly northward from placemark A to placemark B, and then fly back along the spine of the Cascades from
Mt. Rainier to Mt. Shasta by clicking on Rainier in the Places sidebar and then clicking the Play button at
the bottom of the Places sidebar. After the flight is done, zoom in to the individual volcanoes, and click
their icons for pictures and background information.
Study the diagrams and information at placemark A or B, and do the following:
12.
Use the Ruler tool to determine the direct distance from the trench to the volcanic peaks
immediately to the east. Make several measurements, and record the approximate latitude of
your measurement and the trench-volcano distance in the spaces provided below.
Distance
Latitude of Observation Trench-Volcano Distance
267 km
47 degrees N
268 km
45 degrees N
269 km
44 degrees N
273 km
43 degrees N
Research has shown that melting begins to occur as the subducted plate arrives at a depth somewhere
between 100 and 120 km below Earth’s surface. If the less dense molten rock rises to the surface, it may
erupt as a volcano.
13.
Refer to the diagram and formula described in placemark A or B, and the average trench-to-
volcano distance you recorded above, to determine the descent angle of the Juan de Fuca plate
beneath the North American continent.
Descent angle of the Juan de Fuca plate = 100/263 = 38
Convergent Boundaries—Location 2: The convergent plate boundary along the coast of Central
America
Click each of the volcanoes in the Places sidebar to fly from one to the next. Click their icons for pictures
and background information. Answer/do the following:
14.
Assume that melting along the subducted plate begins between 100 and 120 km below the
surface of Earth, and use the diagram and formula in placemark C or D to determine the descent
angle of the Cocos Plate beneath Central America.
Descent angle of the Cocos Plate = _______32.5 degrees
Convergent Boundaries—Location 3:
The convergent plate boundary along the Aleutian Islands
Click each of the volcanoes in the Places sidebar to fly from one to the next. Click their icons for pictures
and background information. Answer/do the following:
15.
Once again, assume that melting along the subducted plate begins between 100 and 120 km
below the surface of Earth, and use the diagram and formula in placemark E or F to determine
the descent angle of the Pacific Plate beneath the Aleutian Islands.
Descent angle of the Pacific Plate = ___39 degrees_
Putting It All Together
16.
What factors might influence the descent angle of a subducted plate?
The thickness and strength of the plate If it's thick, that'll increase the descent angle.
17.
How does the temperature of a given slab of seafloor affect its density?
The cooler the temp the denser it's likely to be, and the warmer it is the less dense it is going to
be.
18.
How does the thickness of a given slab of seafloor affect its weight?
Geologists believe sea floor spreading results from convection in the mantle and lower crust that
brings hotter, less dense, and more plastic material up toward the surface; the colder, more
dense rock and sediment, such as subducted crustal material, sinks toward the mantle. These
convective forces tear the ocean crust apart at the midoceanic ridge.
19.
What happens to the density and thickness of a given slab of seafloor as it moves away from the
rift zone where it originated? Why?
The plates become thicker and denser. The plates cool as they move away over time, causing
them to contract in the process and become more dense.
Location Distance from rift/origin
Cascadia Margin __50 miles_
Coast of Central America _1,700 miles
Aleutian Islands _6,821 miles
*If you are having trouble identifying the origin region, you can turn on the Hint at each location by
checking the box next to it in the Places sidebar.
20.
Describe and explain the relationship you’ve observed between the age of a subducted plate,
and the descent angle of the subduction of that plate.
One plate of oceanic lithosphere converges with another plate. The
subducting plate is
overridden by the leading edge of the other plate. Without subduction, plate tectonics could not
exist.
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