Tectonic processes mostly take place at the plate edges. A plate moves as a single entity along the surface of the Earth over a plastic mantle. ... Oceanic plates form at the mid-ocean ridges. They thicken as they move away (at about 1 km for every million years) and have a basaltic composition. Compressive stress happens at convergent plate boundaries where two plates move toward each other. Tensional stress happens at divergent plate boundaries where two plates are moving away from each other. Shear stress is experienced at transform boundaries where two plates are sliding past each other.