# GEOPHYSICS 210 – Relative Plate Motion Questions A Plate margins by sdaferv

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```									                GEOPHYSICS 210 – Relative Plate Motion Questions
A. Plate margins and triple junction questions
Use the given relative plate velocities at two plate margins to:
• calculate the relative velocity at the third plate margin (magnitude and geographic
direction)
• fill in all plate margins on the map with the appropriate symbols, arrows and labels
• determine the type of triple junction
Assume that all plates move rigidly, without rotation. All divergent margins are spreading
symmetrically, such that the half-spreading rate is the same on both sides of the ridge.
A1                                               A2

AVB   = 2 cm/yr (East)                           AVC   = 3 cm/yr (North)

BVC   = 4 cm/yr (East)                           BVC   = 2 cm/yr (East)
Plates A and B are continental plates;           Plate A is a continental plate; Plates B
Plate C is an oceanic plate                      and C are oceanic plates

AVC   = ____________ direction: _______          AVB   = _____________ direction: ______
Type of triple junction: _______                 Type of triple junction: _______
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A3                                            A4

AVB   = 6 cm/yr (East)                        AVB   = 3 cm/yr (N45°W)

AVC   = 6 cm/yr (South)                       AVC   = 3 cm/yr (S45°W)
All plates are oceanic plates.                All plates are oceanic plates.

BVC   = ____________ direction: ______        BVC   = _____________ direction: _____
Type of triple junction: _______              Type of triple junction: _______
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B. Plate evolution questions
In the following questions, assume that all plates move rigidly, without rotation. All divergent
margins are spreading symmetrically (half-spreading rate is the same on both sides of the ridge).
Question B1. Plate B is currently 100 km wide. Use the following relative plate velocities to
determine the width of Plate B in 2 Ma (1 Ma = 1 million years). On the map, fill in the dashed
plate margins with appropriate symbols, arrows and labels.

AVB   = 2 cm/yr (East)         BVC     = 4 cm/yr (East)
All three plates are oceanic plates.

Question B2. Observer X is standing on Plate C near the triple junction of Plates A, B and C.

• on the map, fill in the dashed plate margins with appropriate symbols, arrows and labels.

• in 25 years, where will the triple junction be located with respect to X?

AVB   = 2 cm/yr (East)         BVC     = 4 cm/yr (East)
Plates A and C are continental plates; Plate B is an oceanic plate.
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Question B3. Plate B is currently 120 km wide. Use the following plate velocities to determine
the width of Plate B in 1 Ma.

• on the map, fill in the dashed plate margins with appropriate symbols, arrows and labels.

• what type of triple junction is at T?

AVB   = 4 cm/yr (West)         AVC   = 2 cm/yr (East)
Plate A is a continental plate; Plates B and C are oceanic plates.

Question B4. A and B are oceanic plates located west of continental Plate C. Use the
following relative plate velocities to determine the subduction rate of Plate A (CVA) and add the
appropriate symbols, arrows and labels to the plate boundaries on the map.

AVB   = 2 cm/yr (West)         CVB   = 3 cm/yr (East)
A seamount on Plate A is currently 80 km west of point X on the western edge of Plate C.
When will the seamount be subducted?
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C. Euler pole questions
Question C1. Planet Tecton has a radius of 3500 km and is made up of two tectonic plates. The
plate boundary between Plate A and Plate B lies exactly on the geographic equator. The Euler
pole for relative plate motion is located at the geographic North Pole (N, 90°N), with an angular
rotation velocity of BωA of 3.0 × 10-7 degrees per year.

Point X is located at 0°N, 90°E. Determine the relative plate motion (BVA) at X (magnitude in
cm/yr and geographic direction). What type of plate boundary is this?

Question C2. Now assume that the Euler pole is located at 0°N, 0°E. If BωA is 2.0 × 10-7
degrees per year, what is the relative plate velocity (BVA) at X (magnitude in cm/yr and
geographic direction)? What type of plate margin will be found at X?
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Question C3. On Earth (radius 6371 km), the Mid-Atlantic Ridge can be approximated as a
north-south plate boundary along 30°W longitude, with the Euler pole for relative plate motion
located at the geographic north pole (90°N), as shown below. Seafloor magnetic anomalies at
30°N indicate a full spreading rate of 2.0 cm/yr, with Europe moving east relative to North
America. What is the angular rotation velocity (in degrees per year) for the Euler pole for
Europe with respect to North America (NAωE)?

Question C4. Same as C3, but give the angular rotation velocity for North America with
respect to Europe (EωNA) if the Euler pole is located at the geographic north pole (90°N).

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