Quantifying Plate Motion
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Santa Monica College *
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Course
400
Subject
Geology
Date
Apr 3, 2024
Type
Pages
3
Uploaded by SargentTitaniumKangaroo28
Part A – Long-term Plate Motion from Seafloor and Volcanic Chain Ages
5.
Now look at the Atlantic Ocean basin with the "Sea Floor" layer turned
on. Did the northern Atlantic Ocean basin start opening at the same
time as the southern Atlantic Ocean basin? If not, how much older or
younger is the northern Atlantic basin than the southern Atlantic basin?
Describe your reasoning.
The North Atlantic started opening 40 million years before the South Atlantic. The
seafloor at the edges of the North Atlantic Ocean basin are older (180 million years old)
than the seafloor at edges of the South Atlantic Ocean basin (140 million years old),
meaning that the South Atlantic Ocean basin is younger than the North Atlantic Ocean
basin.
6.
When looking at different ocean basins with the "Sea Floor" layer
turned on, could you tell if the rate of spreading at mid-ocean ridges is
constant over time? Explain your answer with specific examples.
The rate of spreading at ocean ridges is not constant over time. If we look at the width
of seafloor age bands, we can observe that the width is not constant even though each
band represents 5 million years. Thus, a wider band indicates a larger rate of spreading
(=rate of plate motion) compared to a narrower band (in other words, the plate moved a
longer distance in the same amount of time).
7.
We have explained the age pattern associated with hot spots by calling
upon stationary mantle plumes under moving plates. But, you could
also explain the pattern with stationary plates over moving mantle
plumes. Settle the debate by referring to Google Earth data sets.
We know that mantle plumes are stationary while plates are moving because of the
seafloor ages. As the seafloor gets older away from the mid-ocean ridge, we know that
the plates are moving. About hot spots, the rate and direction of plate movement we can
calculate based on a volcanic chain is the same as the rate and direction of plate
movement calculated from seafloor ages. Thus, there is no way that the plates are
stationary while the mantle plumes are moving.
Part B – Short-term Plate Motion from Global Position Satellite (GPS)
Data
2.
What type of motion does your plate boundary represent (convergent,
divergent, transform)?
○
Boundary between the Pacific plate and the Nazca plate:
DIVERGENT
○
Boundary between the Pacific plate and the Tonga plate:
CONVERGENT
○
Boundary between the Pacific plate and the Mariana plate:
CONVERGENT
○
Boundary between the Pacific plate and the North American plate
(Aleutian Islands area): CONVERGENT
○
Boundary between the Pacific plate and the North American plate
(Southern California): TRANSFORM
3.
Using GPS time series data, analyze the motion across your assigned
plate boundary.
Google Sheet Short-Term
Links to an external site.
4.
When your analysis is complete, describe the GPS-documented motion
across your assigned plate boundary in a few sentences. Your
description should include: (a) the sense of motion (convergent,
divergent, left-lateral transform or right-lateral transform), and (b) the
relative rate of motion across the boundary (how fast the two plates are
converging, diverging or sliding past one another).
○
Boundary between the Pacific plate and the Nazca plate: the plates
are diverging at a rate of 132.83 mm/yr
○
Boundary between the Pacific plate and the Tonga plate: the plates
are converging at a rate of 160.53 mm/yr
○
Boundary between the Pacific plate and the Mariana plate: the
plates are converging at a rate of 59.37 mm/yr (note that the plates
are moving in the same direction)
○
Boundary between the Pacific plate and the North American plate
(Aleutian Islands area): the plates are converging at a rate of 96.31
mm/yr
○
Boundary between the Pacific plate and the North American plate
(Southern California): the plates are sliding past one another in a
right-lateral motion at a rate of 24.45 mm/yr (note that the plates
are moving in the same direction)
6.
Using the rate of motion across the boundary between the Pacific plate
and the North America (California) plate (5.5 cm/yr), how long will it take
for San Francisco and Los Angeles to become suburbs of one another?
Show your math.
San Francisco and LA will become suburbs of one another in 10 million years.
Distance between LA and San Francisco* = 560 km = 560,000 m = 56,000,000 cm
Rate of plate motion (based on the map) = 5.5 cm/yr
Rate = Distance/Time Time = Distance/Rate
Time = 56,000,000 cm/5.5 cm/yr = 10,181,818 yr = ~10 Myr
*measured on Google Earth along the plate boundary
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