Why do lithospheric plates move?
The theory of plate tectonics suggests that the lithospheric plates move because of convection currents in the semi molten mantle.
Convection currents can be observed on the surface of a hot liquid. If there are minerals in the liquid that can solidify as they cool, the minerals are observed to form plate like structures. In the middle of the liquid there is an up current as the heated liquid rises to the surface. On the edges there are down currents where the cooled liquid sinks back down to the bottom.
It is theorized that something analogous happens in the semi molten mantle. There are hot spots where the mantle is heated by the liquid outer core. These hot spots cause the material in the mantel to come to the surface as magma creating new crust. New magma comes to the surface pushing the old magnetized crust away from the hot spot. This pressure from the rising material from the hot spot causes the crust in the form of lithospheric plates to move.
The deep ocean trenches found off the coast of South America and other places are thought to subduction zones where the cooled crust sinks and is forced back down into the mantle. This actions idestroys the ocean crust and its fossilized sedimentary layers. The subsections destroy ocean plates allow the formation of new ocean crust at the mid ocean ridges.
The argument by analogy of convection currents in the mantle seems to explain the observed motion of the lithospheric or tectonic plates.