The authors of the theory of plate-tectonics have proposed several different causes for the plate-motions. Some of the important causes are as follows:
1. Formation of oceanic crust
As it has already been explained new crust is formed in zones of spreading. Since this crustal for mation is a continuous process, the new crust pushes the earlier ones from the ridge crest and causes spreading of the lithospheric plate. This process is believed to cause plate motion.
2. Oceanic-topography
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As the mid-oceanic ridges, from where new lithospheric plates originate, are normally 2 to 4 kms above the level of the ocean floor and slope away more or less symmetrically from the crest, it is thought to be responsible for plate motion.
3. Rate of Motion
The formation of new crust at the mid-oceanic ridge is accompanied with an absorption of the lithospheric plate at the deep oceanic trench.
It is believed that since spreading of plates at ridge-crest occurs at rates ranging from 1 to 6 cms per year but are consumed at the rate of 5 to 15 cms per year at the oceanic trenches the plates must move to maintain a balance between the two.
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4. Temperature difference
The plate being subducted is characterized by low heat flow indicating that the oceanic plate cools, continuously with increasing distance from the ridge-crest as they move from the mid-oceanic ridge to oceanic-trenches.
The cooling of the plate increases its density and it becomes heavy enough to sink back into the mantle . Thus, the difference in the heat flow values seem to be responsible for plate motion.
5. Gravity difference
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Mid-oceanic ridges are almost close to isostatic equilibrium but trenches are marked by very strong negative gravity anomalies. This gravitational difference is believed to cause plate motion.
6. Mantle Convection Convection
Current condition in the mande zone seems to cause plate motion. This is thought to be the slow displacement or convection of mantle-material under the action of the temperature difference between its floor and roof. The velocity of the convectional currents, according to D. Tozfer (1965) is within the range of 1-3 cm per year.
The flow of mantle material acts as a kind of fluid conveyor. The diverging current drags the lithospheric plates along the direction of their flow. It is believed that between the rising convection currents (i.e. the mid-oceanic ridges) and the sinking currents (i.e. the subduction zone) the horizontal motion of the convection current exerts a dragging force on the lithospheric plates and causes the plate motion.