Plate Tectonics: Boundaries, Motions, Magnetic Records, Plumes and Numericals

Practice questions (17)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

1. At which type of plate boundary is lithosphere neither created nor destroyed, the plates sliding past each other?

   1. A divergent boundary such as the Mid-Atlantic Ridge
   2. A convergent boundary such as the Andean trench
   3. A transform boundary such as the San Andreas Fault
   4. A collisional boundary such as the Himalaya
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   Answer: C — A transform boundary such as the San Andreas Fault

   At a transform boundary the plates slide horizontally past each other along a strike-slip fault, so no lithosphere is made or consumed. Ridges make it, and subduction and collision consume or thicken it.

2. Which pairing of a feature with its plate-boundary setting is correct?

   1. The Himalaya: ocean–ocean convergence producing an island arc
   2. The East African Rift: an incipient divergent boundary within a continent
   3. The Mid-Atlantic Ridge: a transform boundary along which plates slide past
   4. The Andes: continent–continent collision with no subduction
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   Answer: B — The East African Rift: an incipient divergent boundary within a continent

   The East African Rift is continental extension that may lead to a new ocean. The Himalaya is a continent–continent collision, the Mid-Atlantic Ridge is divergent (offset by transforms), and the Andes are an ocean–continent subduction margin.

3. Plates attached to long subducting slabs (Pacific, Nazca, Indo-Australian) move fastest, while plates with little subducting margin (African, Antarctic) move slowest. Which driving force does this pattern most strongly support?

   1. Ridge push, since the plates with the longest ridge margins are the ones that move fastest
   2. Basal drag proportional to plate area, since the largest plates are the ones that move fastest
   3. Tidal drag from the Moon, which pushes every plate steadily westward at much the same speed
   4. Slab pull, the negative buoyancy of the cold, dense slab pulling the plate along
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   Answer: D — Slab pull, the negative buoyancy of the cold, dense slab pulling the plate along

   Fast plates are those with a long subducted slab, which points to slab pull. Plates with long ridge margins, such as the African and Antarctic plates, are slow, which contradicts ridge push as the main force. Basal drag proportional to area would make the large African, Eurasian and Antarctic plates fast, but they are slow, and a Moon-driven westward push would not distinguish between plates.

4. Which of the following statements about the forces that drive plate motion are correct?

   1. Plate motion is driven mainly by the Coriolis force that arises from the rotation of the Earth
   2. Slab pull is the force exerted on a plate by the cold, dense subducting slab attached to it
   3. A subducting slab sinks because it is hotter, and therefore denser, than the surrounding mantle
   4. Ridge push arises from the raised ridge and the plate sliding down the thickening base of the lithosphere
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   Answer: B — Slab pull is the force exerted on a plate by the cold, dense subducting slab attached to it; D — Ridge push arises from the raised ridge and the plate sliding down the thickening base of the lithosphere

   Slab pull and ridge push are both real driving forces, with slab pull the larger. The Coriolis force is far too weak and is not the driver. A slab sinks because it is colder, and therefore denser, than the mantle around it, since a hotter body would be less dense.

5. A magnetometer towed over the sea floor records a positive magnetic anomaly over a stripe of oceanic crust. What does this indicate?

   1. The crust there was magnetised during a reversed interval, and its field reinforces the present field of the Earth
   2. The crust there is magnetised in the present field’s direction, so its field adds to the present field
   3. The crust there is the youngest crust at the ridge crest, whatever direction it happened to be magnetised in
   4. The crust there has lost its magnetisation, because it was later heated above the Curie temperature of its minerals
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   Answer: B — The crust there is magnetised in the present field’s direction, so its field adds to the present field

   A positive anomaly is the sum of the field of the crust and the regional field, which is larger where the crust is magnetised in the same (normal) direction as the present field. Reversed crust subtracts and gives a negative anomaly. The age of the crust does not fix the sign, and crust that had lost its magnetisation would give no anomaly.

6. A magnetic anomaly of age 20 Ma lies 480 km from the ridge axis on one flank, and spreading is symmetric. The full spreading rate, in mm/yr, is ____.

   Numerical answer — type the value.

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   Answer: 48

   Half rate = 480 km/20 Myr = 24 km/Myr = 24 mm/yr, because 1 km/Myr is 1 mm/yr. The full rate is twice the half rate, 48 mm/yr. Quoting 24 gives the half rate.

7. A basalt formed under a geocentric axial dipole field has a well-determined mean inclination and declination. Which statement about what it constrains is correct?

   1. Inclination gives paleolatitude, but paleolongitude is not constrained, as the dipole field is the same at every longitude
   2. The declination gives the paleolatitude of the site, while the inclination gives its paleolongitude once the polarity is known
   3. Both the paleolatitude and the paleolongitude follow from the inclination alone, since the field differs between longitudes
   4. The inclination fixes the absolute speed of the plate over the deep mantle, independent of any reference frame
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   Answer: A — Inclination gives paleolatitude, but paleolongitude is not constrained, as the dipole field is the same at every longitude

   Under an axial dipole the field depends only on latitude, so inclination (tan I = 2 tan λ) gives latitude, while longitude leaves no trace. Declination records the rotation of the site about a vertical axis, not latitude, and a single inclination says nothing about the plate’s absolute speed, which needs a reference frame.

8. A lava flow 60 Myr old now lies at 20° N. It carries a stable magnetisation of normal polarity with inclination −30°, and the field was a geocentric axial dipole. Take the Earth’s radius as 6,371 km and assume steady northward motion only. The mean northward plate speed since the flow formed, in cm/yr, to one decimal place, is ____.

   Numerical answer — type the value.

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   Answer: 6.7

   tan λ = ½ tan(−30°) = −0.2887, so λ = −16.1°, which is southern because the polarity is normal and I is negative. The flow moved from 16.1° S to 20° N, 36.1° of latitude = 36.1 × 111.19 = 4,014 km. Speed = 4,014 km/60 Myr = 66.9 km/Myr = 6.7 cm/yr. Taking λ as +16.1° gives only 3.9° of motion, a wrong result.

9. The relative motion of two plates is a rotation about an Euler pole with angular velocity 0.6°/Myr. Take the Earth’s radius as 6,371 km. The relative speed at a point on the plate boundary that is 40° of arc from the pole, in cm/yr, to one decimal place, is ____.

   Numerical answer — type the value.

   Show answer

   Answer: 4.3

   v = ω R sin θ with ω = 0.6° = 0.6 × π/180 = 0.010472 rad/Myr: v = 0.010472 × 6,371 × sin 40° = 66.71 × 0.6428 = 42.9 km/Myr = 4.3 cm/yr. Using sin 90°, the maximum, would give 6.7 cm/yr.

10. A seamount 1,200 km from an active hotspot volcano is 15 Myr old. Assume the plate has moved at a constant speed over a fixed plume along the line of the chain. The age of a seamount 3,000 km from the active volcano, in Myr, to one decimal place, is ____.

    Numerical answer — type the value.

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    Answer: 37.5

    Speed = 1,200 km/15 Myr = 80 km/Myr (8 cm/yr). Age = 3,000/80 = 37.5 Myr. Scaling the age by 3,000/1,200 gives the same result, whereas adding the distances is a slip.

11. A slab dips at 45° and descends along its dip at the convergence rate of 8 cm/yr. The time for a point on the slab to travel from the trench to a depth of 660 km, in Myr, to one decimal place, is ____.

    Numerical answer — type the value.

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    Answer: 11.7

    Distance along the slab = 660/sin 45° = 933.4 km. Rate = 8 cm/yr = 80 km/Myr. Time = 933.4/80 = 11.67 Myr, which is 11.7 Myr. Dividing the depth 660 km by 80 gives 8.25 Myr, which ignores the dip.

12. Which process best explains the formation of an extensional back-arc basin such as the Mariana Trough?

    1. Collision of two continents, which thickens and uplifts the crust in the region behind the volcanic arc
    2. Strong compression from ridge push, which buckles the plate behind the volcanic arc downward into a basin
    3. Slab rollback, in which an old, steep slab sinks faster than the plates converge, so the trench retreats
    4. Flat subduction of a young, buoyant slab that locks the two plates together and compresses the overriding plate
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    Answer: C — Slab rollback, in which an old, steep slab sinks faster than the plates converge, so the trench retreats

    When the slab sinks faster than the plates converge, the trench retreats toward the ocean and the overriding plate is stretched, so back-arc crust forms by spreading. Collision and flat, coupled subduction produce compression and uplift, not extension, and ridge push is too weak to buckle a plate into a basin.

13. A volcanic chain on the Pacific plate shows ages that rise steadily with distance from one active volcano. What is the most direct inference?

    1. The plate has moved over a fixed hot source in the mantle, and the chain trends along the plate’s motion
    2. The chain lies on a transform fault, and each volcano erupted in turn when the fault last slipped at that point
    3. A subduction zone beneath the chain has fed each volcano in turn, as the slab rolled back beneath the plate
    4. The chain follows a divergent boundary that has propagated slowly in one direction along the plate
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    Answer: A — The plate has moved over a fixed hot source in the mantle, and the chain trends along the plate’s motion

    Age progression along a line is the signature of a plate moving over a fixed melting anomaly, as in the Hawaiian chain, and the line records the direction of motion. Transform faults, subduction rollback and propagating ridges do not give a steady age increase along a chain away from a single active volcano in the middle of a plate.

14. The Deccan Traps of India are generally attributed to which hotspot, whose later track includes the Chagos–Laccadive Ridge?

    1. Hawaii
    2. Iceland
    3. Yellowstone
    4. Réunion
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    Answer: D — Réunion

    The Deccan flood basalts are linked to the Réunion plume. As India moved north the plume left the Chagos–Laccadive Ridge and the Mascarene Plateau, ending at the active volcano on Réunion. Hawaii, Iceland and Yellowstone lie far from India’s track.

15. Which statement about earthquakes at a subduction zone is correct?

    1. Earthquakes occur only in the shallow crust, above about 100 km, because the slab is too warm below that
    2. Hypocentres define a Benioff–Wadati zone dipping beneath the overriding plate to about 700 km
    3. Hypocentres deepen toward the ocean, away from the volcanic arc, following the dip of the trench wall
    4. Earthquakes are common throughout the lower mantle, all the way down to the core–mantle boundary
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    Answer: B — Hypocentres define a Benioff–Wadati zone dipping beneath the overriding plate to about 700 km

    Earthquakes in the cold slab trace a plane dipping from the trench beneath the arc, and the deepest are about 700 km. They deepen toward the continent or arc side, not toward the ocean, they are not confined to the shallow crust, and none occur in the deep lower mantle.

16. Which of the following statements about magma generation at plate boundaries are correct?

    1. Arc volcanoes stand directly above the trench axis, where the slab first bends down
    2. Basalt at mid-ocean ridges forms by decompression melting of upwelling mantle peridotite
    3. Arc magmas form where water released from the slab lowers the melting point of the mantle wedge
    4. Melting at mid-ocean ridges is triggered mainly by water added from a subducting slab
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    Answer: B — Basalt at mid-ocean ridges forms by decompression melting of upwelling mantle peridotite; C — Arc magmas form where water released from the slab lowers the melting point of the mantle wedge

    Ridges melt by decompression as the mantle rises, and arcs melt by flux melting, since slab-derived water lowers the solidus of the wedge. Arc volcanoes lie where the slab is about 100 to 120 km deep, inland of the trench, and ridges have no subducting slab to supply water.

17. Which of the following statements about continental rifting are correct?

    1. Passive rifting is driven by far-field tension, with volcanism following lithospheric thinning
    2. Rift valleys are bounded by steep reverse faults that shorten and thicken the crust
    3. Rift volcanism is always andesitic and calc-alkaline, as it is in volcanic island arcs
    4. Active rifting begins with doming of the crust over a hot, buoyant mantle upwelling
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    Answer: A — Passive rifting is driven by far-field tension, with volcanism following lithospheric thinning; D — Active rifting begins with doming of the crust over a hot, buoyant mantle upwelling

    Passive rifting is stress-driven and active rifting starts with plume-related doming. Rift valleys are bounded by normal faults because the crust is extending, and rift magmas are bimodal and often alkaline, not arc-like andesites.