The theory of plate tectonics explains the movement of the plates by convection cells in which layers?

upper mantle

Explanation:

The tectonic plates are fragments of the lithosphere, composed of the upper part of the upper mantle and the earth's crust, which behave like a strong, relatively cold and rigid layer. The plates of the lithosphere are thinner in the oceans, where their thickness varies from a few kilometers in the ocean ridges to 100 kilometers in the deep ocean basins.

On the contrary, the continental lithosphere usually has a thickness between 100 and 150 kilometers, although it can reach 250 kilometers in older portions of the continents. Below the lithosphere, there is a very ductile mantle region, known as the asthenosphere, where the temperature and pressure are so high that the rocks are in a state of fusion (molten rocks). It is on this asthenosphere that the tectonic plates "slide". It is currently held that the plates are displaced as a result of a convective flow in the mantle. This flow, driven by temperature differences (hence the density of the material) drives the lithospheric plates, indirectly generating the formation of mountain ranges as well as volcanic activity (directly or indirectly) and seismic throughout the planet. It is thought that the superheated rock feathers that make up the upward flow of the convective movement are generated at the core-mantle boundary (where the temperature is maximum and the minimum density accordingly) and slowly rise to the surface. The horizontal part of the flow is the one that drags the plates causing their collision or their separation and finally it is added that possibly the edges of convergent plates (where the cold and dense plates of the lithosphere are subducted) coincide with a downward flow in the mantle.