Which layer of the Earth includes both the crust and uppermost mantle?

The lithosphere is the layer of the Earth that encompasses both the crust and the uppermost part of the mantle. This layer is characterized by its rigid and solid nature, which is essential for forming tectonic plates. The lithosphere is relatively cool compared to the underlying layers and is integral in geological processes such as plate tectonics, earthquakes, and volcanism.

The crust, which is the surface layer, varies in thickness and composition, while the uppermost mantle beneath it is semi-solid but behaves elastically. The lithosphere, therefore, acts as a distinct layer on which geological activity occurs, and its rigidity differentiates it from the more ductile asthenosphere located beneath it. This understanding of the lithosphere is crucial, as it highlights the interaction between the surface and the internal processes of the Earth.

Other layers like the hydrosphere and biosphere pertain to water and life on Earth, respectively, and do not involve the geological structure. The asthenosphere, while closely related, refers specifically to the more plastic portion of the upper mantle lying just below the lithosphere, and it does not include the crust.