Ridge push
[3] Ridge push is mostly active in lithosphere younger than 90 Ma, after which it has cooled enough to reach thermal equilibrium with older material and the slope of the lithosphere-asthenosphere boundary becomes effectively zero.
[11][12] Early models of plate tectonics, such as Harry Hess's seafloor spreading model, assumed that the motions of plates and the activity of mid-ocean ridges and subduction zones were primarily the result of convection currents in the mantle dragging on the crust and supplying fresh, hot magma at mid-ocean ridges.
[4][7] Further developments of the theory suggested that some form of ridge push helped supplement convection in order to keep the plates moving, but in the 1990s, calculations indicated that slab pull, the force that a subducted section of plate exerts on the attached crust on the surface, was an order of magnitude stronger than ridge push.
[4][6][12] Modern research, however, indicates that the effects of slab pull are mostly negated by resisting forces in the mantle, limiting it to only 2-3 times the effective strength of ridge push forces in most plates, and that mantle convection is probably much too slow for drag between the lithosphere and the asthenosphere to account for the observed motion of the plates.
[14][15] Slab pull is similarly opposed by resistance to the subduction of the lithosphere into the mantle at convergent plate boundaries.
[3][14] Research by Rezene Mahatsente indicates that the driving stresses caused by ridge push would be dissipated by faulting and earthquakes in plate material containing large quantities of unbound water, but they conclude that ridge push is still a significant driving force in existing plates because of the rarity of intraplate earthquakes in the ocean.
