Assignment4_SP23_PlateTectonics_ResponseWorksheet
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Dec 6, 2023
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EAS 1180 Assignment 4 Response Sheet
EAS 1180: How to Build a Habitable Planet Spring 2023
Learning Assignment 4: Plate Tectonics
Due:
Wednesday March 22
nd
at 11:59 pm (see late policy in syllabus)
Response Worksheet (39 pts possible)
Please upload only this part of the assignment on Gradescope.
Please indicate here if you are using slip days towards this assignment:
I am using ____ slip days towards this assignment.
Part 1. Seafloor Spreading
1. Study the shades of blue in the South Atlantic Ocean in Figure 1.1. The darker the blue color,
the deeper the water. Why is the depth of the water shallower near the center of the ocean?
(1
pt)
The depth of the water is shallower because of mid-atlantic ridges that ris due to plate
tectonics spreading as new magma pushes up.
2. There is a chain of seamounts, as indicated by high topography and shallow waters, extending
out from Africa near the letter “G” on Figure 1.1. What type of volcanism do you think
formed these seamounts? Explain your reasoning to receive full credit.
(2 pts)
Hotspot volcanism (which forms when an Earth 's tectonic plate moves over an unusually
hot part of the Earth 's mantle). I think it's a hotspot because it can explain why the
shape is the way it is. The plate was over a hotspot, and as it kept moving it formed a
chain of seamounts.
3. Refer back to Figure 1.1. Approximately how many fracture zones intersect the mid-ocean
ridge between points I and J? We will accept a range of answers for this question, so just do
your best to count here.
(1 pt)
Around 6ish Fracture Zones
4. The South Atlantic Ocean formed by expansion along the Mid-Atlantic Ridge, where new
oceanic crust forms. Measure the distance between South America and Africa along the
fracture zone that extends from near point A to near point E on Figure 1.1. Measure along the
curved fracture zone, ending at the continental shelf on each side. Approximately how wide is
the Atlantic Ocean along the fracture zone? Note: you may either print out the map and use a
ruler to measure the distance or measure it digitally using the method of your choice.
(1 pts)
Approximately 3,000km.
1
EAS 1180 Assignment 4 Response Sheet
5. The age of the oldest rocks in the South Atlantic Ocean is 120 Ma. Calculate the average rate
of expansion for the South Atlantic, using the formula rate = distance ÷ age. Express your
answer in units of kilometers per million years (km/Ma).
(1 pts)
About 25km/Ma
6. Convert the rate you just calculated in question 5 to units of centimeters per year (cm/yr),
using 1 km = 100,000 cm. What is the rate in cm/yr? Is this the full spreading rate or the half
spreading rate?
(2 pts)
2.5cm/year. Half-Spreading rate
7. Categorize the spreading rate you calculated in question 6 as fast-, intermediate- or slow
spreading using the values we discussed in class.
(1 pt)
This is categorized as a slow spreading rate.
8. On the profile for the East Pacific Rise (top of Fig. 1.5), an arrow marks the mid-ocean ridge
at the left end of the profile as “0 Ma” (i.e., “today”). Use the scale to determine the distance
from ridge to the oldest anomaly (oldest appearance of the white stripes). Determine the
approximate age of this anomaly and use it to calculate the average spreading rate of the East
Pacific Rise.
(2 pts)
500km/8.4 Ma = 59.5km/Ma (Half)
9. The profile at the bottom of Fig. 1.5 shows magnetic anomalies for the Mid-Atlantic Ridge.
Calculate the average spreading rate of the Mid-Atlantic Ridge using the same strategy you
employed in question 8.
(2 pts)
Distance from ridge to oldest anomaly : 240 km
Approximate age of anomaly : 11.4 Ma
Average spreading rate : 21.05 km / Ma
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10. Compare the two profiles in Fig. 1.5. Does a faster spreading rate produce wider or narrower
anomalies? Explain your reasoning to receive full credit.
(2 pts)
A faster spreading rate produces wider anomalies. This is because in the same time period
a faster spreading rate means that the floor would spread further, and therefore the
width of the anomaly will be wider.
2
EAS 1180 Assignment 4 Response Sheet
Part 2. Plate Tectonics with Maps and Spreadsheets
Use the first page of the Plate Tectonic Map PDF to answer questions 11-16.
11. A plate may consist of oceanic lithosphere or continental lithosphere or both. For
example, the South American Plate includes both types of lithosphere. Name two other
plates also contain both continental and oceanic lithosphere.
(2 pts)
The North American plate and the African plate.
12. The Philippine Plate contains only oceanic lithosphere. Name two other plates that are
strictly oceanic (entire plate is covered by oceans).
(2 pts)
Juan de Fuca Plate and Cocos Plate
13. Name one plate that consists mostly of continental lithosphere.
(1 pt)
Eurasian Plate
14. Fill in the table below for the following plate boundaries, giving the type of boundary, its
geographic name and the rate of relative movement across the boundary
(3 pts):
1. Between the Scotia and Antarctic Plates south of South America 2. Between
the Indian and Somali Plates south of the Arabian Peninsula
3. Between the
Philippine and Pacific Plates north of Australia
Plate
Boundary
Type
(circle one)
1 (south of South
America)
2 (south of the
Arabian Peninsula)
3 (north of
Australia)
Convergent
Divergent
Transform
Convergent
Divergent
Transform
Convergent
Divergent
Transform
Name
Shackleton Fault
Carlsberg Ridge
Iza-Mariana Trench
Rate
1cm/year
3cm/year
4cm/year
15. Which mid-ocean ridge labeled on the map is spreading the slowest? At what rate is the
slowest ridge spreading?
(2 pts)
The Gakkel Ridge is spreading the slowest with a rate of 0.6cm/year
16. Which mid-ocean ridge is spreading the fastest? At what rate is the fastest ridge spreading?
(2 pts)
The East Pacific Rise is spreading the fastest with a aret of 16cm/year
3
EAS 1180 Assignment 4 Response Sheet
Use the second page of the Plate Tectonic Map PDF to answer questions 17-19.
17. Now look at the East Pacific Rise map. This map shows more detail about plate tectonic
features than the world map does. What is the spreading rate of the East Pacific Rise (EPR)
south of the Chile Rift?
(1 pt)
10cm/year
18. Note that the spreading rate for the EPR seems to differ as you traverse from North to South
along the plate boundary. This seems contradictory – the EPR appears to be spreading at two
different rates! Note the EPR transects across several different plates, forming the boundary
between the Pacific and Nazca plates; the Pacific and Cocos plates and the Pacific and
Antarctic plates. Can you speculate as to why the spreading rates for the EPR are different as
you traverse from N to S?
(1 pt)
I think it 's because the Cocos Plate, Nazca Plate, and Antarctic Plate move at different
rates, hence the difference in spreading rates.
19. Go to
http://www.unavco.org/software/geodetic-utilities/plate-motion-calculator/plate
motion-calculator.html/
. Enter latitude 41°S (-41 in the first box of the Latitude block) and
longitude 90°W (-90 in the first box of the Longitude block). Scroll down past all the other
blocks and submit. What is the spreading rate for the Chile Rift here? Express your answer in
cm/yr and round to the nearest whole number.
(1 pt)
67.95mm/year -> about 7 cm per year
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4
EAS 1180 Assignment 4 Response Sheet
Part 3: The Hawaiian Hotspot Track
20. Copy and paste your graph from your Hawaii Excel workbook here. Your x-axis should be
ages of the islands in the Emperor Seamounts chain in millions of years, and your y-axis
should be the distance of the islands starting from the active volcano Kilauea. Make sure you
have a trend line of best fit plotted through your data.
(2 pts)
Slope Equation: Y=71.876X + 238.79
21. The slope of your trendline represents the velocity of the Pacific Plate. What is the
approximate rate of plate movement from your equation? Include the proper units.
(2
pts)
71.9km/Ma every 1 million years (7.19cm/yr)
22. Plate velocities are often given in units of centimeters per year. Change your velocity in
question #8 to these commonly used units. (1 km = 100,000 cm).
(1 pt)
5 km / Ma = 5.95 cm/yr
23
.
Look at the points and line on your graph again. The fit of the line to all of the data would
actually be better if you used two line segments instead of a single line. You could break your
data set into two line segments that have different slopes, which means that either the average
rate of motion for the Pacific Plate has changed or the hotspot itself has moved. If the Pacific
Plate changed direction, approximately when did the change occur (at what time do the two line
segments intersect)?
(2 pts)
14 Ma
24
.
Did the average rate of movement of the Pacific Plate increase or decrease at this time?
(2
pts)
The average rate of movement of the Pacific Plate increased at this time
5
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