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For example, the mid-ocean ridges, where new crust is thought to be produced, total 80,000 km in collective length.However, only about 40,000 km of ocean trenches and "collision zones" exist.The reason for this is because the plates themselves are not internally strong enough for forces along their edges to be transmitted across the entire plate.
However, seismic tomography has actually provided strong evidence against the existence of convection currents that are large enough and strong enough to move continental plates.
In fact, many geologists now think that the small upper layer mant Currently, the favored mechanisms used to explain plate movements are the "ridge-push" and the "slab-pull" methods.
This might seem like an obvious expectation, except for the fact that it is not observed in real life.
The ocean trenches do not have enough sediment in them if subduction has truly occurred in these areas over the course of millions of years.
The existence of deep continental roots and the absence of a continuous, global asthenosphere to "lubricate" plate motions, have rendered the classical model of plate movements untenable.
There is no consensus on the thickness of the "plates" and no certainty as to the forces responsible for their supposed movement.It is somewhat like trying to pull a train engine with dental floss.The theory of subduction has other problems as well.The slab-pull is thought to be the "dominant" mechanism.It refers to the gravitational sinking of the subducted slabs as they slide under the edges of continental shelves.It seems as though the lithosphere, which makes up the solid plates, averages 70 km thick beneath the oceans and at least 125 to 250 km thick beneath the continents.