The lithosphere is the rigid outermost shell of a terrestrial planet or celestial body, comprising the crust and the underlying rigid portion of the upper mantle. It is fundamentally linked to the theory of plate tectonics, where the lithosphere is broken into several major and minor plates that move relative to one another over the underlying, more ductile layer of the asthenosphere. Variations in the lithospheric thickness and composition dictate global geological processes, including mountain building, volcanism, and the formation of deep-sea trenches. The lithospheric plates are subjected to immense mechanical stresses due to forces such as slab pull and ridge push, leading to continuous restructuring and the dissipation of tectonic energy across the globe.