Questions: Plate Tectonics: Driving Forces and Mechanisms

Question 1

The Pacific Plate moves at 7–10 cm/year while the African Plate moves at only 2–3 cm/year. What best explains this velocity difference?

• A. The Pacific Plate is closer to major mid-ocean ridges, so ridge-push is stronger
• B. Mantle convection currents flow faster beneath the Pacific Ocean
• C. The Pacific Plate has large subducting slabs that exert dominant slab-pull forces; the African Plate lacks significant subduction zones
• D. The African Plate is larger, so total drag forces decelerate it more

Answer: (C)

Slab-pull is the dominant driving force for plate motion. The Pacific Plate is attached to large, cold, dense subducting slabs (e.g., at the Mariana, Japan, and Tonga trenches) that pull the plate forward like a tablecloth sliding off a table. The African Plate has little active subduction, so slab-pull barely acts on it — explaining its much slower velocity despite being surrounded by spreading ridges.

Question 2

A geology student claims that 'mantle convection acts like a conveyor belt that drags plates along its surface, making convection the primary driver of plate motion.' What is wrong with this model?

• A. Nothing — mantle convection is widely accepted as the primary driver
• B. Convection only occurs in the outer core, not the mantle
• C. It overstates mantle drag's role; modern understanding treats slab-pull as dominant, with plates as active participants in convection rather than passive riders
• D. The model is correct for oceanic plates but not for continental plates

Answer: (C)

The classic 'conveyor belt' model is misleading. Modern plate tectonics treats the sinking slab as the downgoing limb of convection — the plate drives convection rather than being carried by it. Slab-pull (gravity acting on the dense, cold oceanic slab) is the dominant force for plates with active subduction, roughly an order of magnitude stronger than ridge-push, while mantle drag can act as either a driving or resisting force depending on relative velocities.

Question 3

Mantle convection currents act as a conveyor belt that passively carries plates along, making convection the primary driver of plate motion.

• T. True
• F. False

Answer: False

This is the most common misconception about plate tectonics. Modern understanding inverts the relationship: the sinking slab *is* the downgoing limb of convection — plates are active participants, not passive passengers. Slab-pull (the gravitational force of cold, dense oceanic lithosphere sinking into the mantle) is the dominant force for plates with active subduction. Mantle drag plays a secondary and variable role.

Question 4

Ridge-push is driven by gravity acting on the elevated topography of mid-ocean ridges, not by heat injection from below.

• T. True
• F. False

Answer: True

Ridge-push is indeed gravity-driven. Hot, buoyant mantle rises beneath ridges, creating elevated topography (ridges sit several kilometers above abyssal plains). This elevation creates a lateral pressure gradient that pushes the lithosphere outward and downhill — like a ball rolling down a slope. The heat is what creates the elevation, but the force itself is gravitational. Ridge-push is weaker than slab-pull (roughly an order of magnitude smaller) but acts over the full length of ridge systems.

Question 5

Why do plates attached to large subducting slabs move faster than plates without significant subduction? Explain the mechanism.

Model answer: As oceanic lithosphere ages and moves away from the mid-ocean ridge where it formed, it cools, thickens, and becomes denser than the underlying asthenosphere. When this cold, dense slab reaches a subduction zone, it sinks into the mantle under its own weight — pulling the entire attached plate behind it. This is slab-pull, and it is the dominant plate-driving force. Plates with large subducting slabs (like the Pacific Plate) are essentially dragged forward by the weight of their own sinking edges. Plates without active subduction (like the African Plate) lack this pulling force and move slowly, driven mainly by the weaker ridge-push force.

The slab-pull mechanism is why plates behave actively in convection rather than passively. The slab sinks because it is denser than surrounding mantle — a direct consequence of cooling and aging. Understanding this mechanism also explains why old oceanic lithosphere sinks while young oceanic and continental lithosphere generally does not.