why are the crust and mantle taken as one layer, the lithosphere??

Answer 1

People often confuse the terms 'lithosphere' and 'crust'. The crust is indeed part of the lithosphere, and the lithosphere [minus the crust] is an extension of the mantle.

The distinguishing factor between lithospheric (rock-sphere) mantle and the asthenospheric (soft-sphere) mantle is not compositional, but mainly related to temperature and density.

At ocean spreading centers, where new lithosphere forms, there is no sharp contrast between the new, warm, thin lithosphere and the upwelling asthenosphere below. In the old, cool, dense lithosphere that makes up ocean basins, the difference between the 'layers' is quite distinguishable.

If you look at it from this perspective, the crust is simply coasting atop the lithosphere, getting a free ride. It is the behavior of the mantle, which comprises both the lithosphere and asthenosphere, that controls how plates form, move, and subduct. So yes, the crust is part of the lithosphere, in that it is made up of rocks.

Answer 2

That is not strictly true...and it depends on the criteria you use...

The lithosphere is the outer relatively rigid part of the earth comprising the crust and topmost part of the mantle. It is underlain by the asthenosphere which is hotter and more plastic, and convects, and comprises MOST of the mantle.

The crust and mantle are distinct in terms of their composition and seismic characteristics.

Answer 3

litho = rock (stone). That should say it all. the atmosphere is gas. hydrosphere is water. The core is metallic. probalby a -sphere word for that too, metallosphere?

Andrew is right, I was a bit too glib in my answer.