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Joined 2026-09-21 Message
Incoming links: What causes earthquakes? by British Geological Survey
Past exam of the plate tectonics course of the University of Oxford 2023 Question 1 b Solution 2026-10-25
Slab pull is generally the largest direct driving contribution for plates attached to substantial cold descending slabs. Cooling makes oceanic lithosphere negatively buoyant; sinking slabs transmit a pulling force to the rest of the tectonic plate. The comparison is a force balance, so the following driving and resisting processes both matter.
- Ridge push is gravitational sliding away from an elevated spreading ridge. Cooling, contraction and deepening of oceanic lithosphere establish the lateral difference in integrated pressure; ridge push is not simply magma squirting sideways into a plate.
- Basal traction on a tectonic plate transfers shear stress between the lithosphere and circulating mantle. It can help or oppose plate motion, depending on their relative velocities. Slab suction is the contribution of slab-induced mantle flow and its associated pressure and traction to nearby plate motion.
- Frictional sliding along faults, shearing along the plate interface, bending of the incoming slab and viscous dissipation in the surrounding mantle resist plate motion. Continental collision introduces additional resistance through buoyancy and deformation of thick continental crust.
- Mantle plumes and continental rifting can locally promote uplift, weakening and separation, but do not provide a universal explanation for all plate motion.
The first two gravitational contributions ultimately belong to the same coupled mantle convection system: the plates are its cold upper thermal boundary layer. They are not passive rafts on a prescribed conveyor belt. A plate with little attached descending slab need not have slab pull as its largest local driver, and a large gross slab pull can be partly consumed by bending and interface resistance. Geological observations and force comparisons nonetheless support the general dominance of slab pull for rapidly moving subducting plates.[ref]
What causes earthquakes? by British Geological Survey. Section explaining mantle circulation, ridge push and slab pull, including the association between subducting plate edges and faster motion.