Causes of Plate MotionsThe drivers of plates are not completely understood.  Originally it was believed that the plates moved passively on moving asthenosphere; new evidence suggests that the plates contribute to their own movement.A.  Plate-generated movement:1.

  Slab pull.  Hypothesis is that the weight of the cold slab descending into the asthenosphere pulls the whole plate along.  Normal faulting in the ocean crust provides evidence for these plates experiencing tensional stresses.2.  Ridge push.  Hypothesis is that topography of the elevated mid-ocean spreading ridges drives the plates by gravity away from the ridge.  Calculations suggest that plates could move down the gentle slope formed by the thermal profile of the lithosphere-asthenosphere boundary.  However, during initial rifting, plates begin to move without the help of an elevated ridge.

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B.  Mantle-generated movement:1.  Mantle convection cells.  Rising warm rock from the mantle spreads laterally upon encountering the lithosphere, applying shear stress and causing it to move.a. Shallow convection model:  Only the asthenosphere convects, down to a depth of 700 km.

b. Deep convection model:  the whole mantle down to the core convects.2.  Thermal plumes.  Vertical columns of upwelling mantle 100-250 km in diameter lift the overlying lithosphere and spread laterally, which applies drag to the lithosphere.  Many thermal plumes lie at divergent plate margins.

  Iceland is an example.C.  Plates probably move as a result of a combination of the above mechanisms. Plate separation may be initiated by mantle convection or a plume, but the subsequent formation of a topographically high spreading ridge may then drive the plate in a certain direction.