Dynamic Earth Practice Test

Basal drag is the frictional force that the flowing mantle exerts on the base of the lithospheric plate. Mantle convection sets up large, slow currents in the asthenosphere, and the portion of the plate resting on top is coupled to those flows. The shear stress from the moving mantle at the lithosphere–asthenosphere boundary effectively drags the plate along in the direction of the underlying mantle flow. This explains why plates move even away from subduction zones. Slab pull involves the sinking subducting slab pulling the rest of the plate, which is a different mechanism acting at subduction zones. Mantle convection is the driving circulation, while basal drag is the direct transfer of that flow’s motion to the plate’s base. Normal polarity has no role in this mechanical interaction.