In the upper part of the earth, in the first 400 miles or so, this simple layer model is slightly complicated by the earth's cooling history. The three basic layers (crust, mantle, core) developed early in the earth's history. Originally, the earth was much hotter, meaning that the outermost layers were either liquid or in a highly plastic state. Later, as the planet began to cool from the outside inward, the outermost layers solidified and steadily became thicker. 

In the early stages of this cooling, only the uppermost crust was solid. As this solid, rigid layer became thicker over time, this cooled, rigid zone crossed the crust/mantle boundary into the upper mantle. Today, this cooling line, (roughly associated with the 1400oC temperature line), lies about 60 mi (100 km) below the surface. This solid, mostly rigid, rock layer that envelops the earth is called the lithosphere, and today, includes both the crust and the uppermost mantle. Some of this lithosphere is crustal lithosphere, and some, mantle lithosphere. Functionally, the two types of lithosphere act together as one single rigid unit. 

Because of the continued thermal activity in the deeper earth, this lithosphere is not continuous like the solid shell of an egg, but is broken into pieces called plates. Some plates are enormous, thousands of kilometers in size, others are much smaller. Because the earth is denser as we go deeper, the lithosphere is less dense than the lower layers, and the lithospheric plates float on top of the deeper layers of the mantle like gigantic rafts. 

Below the lithosphere lies a zone where the body waves slow down significantly. This Low Velocity Zone (LVZ) is not due to compositional changes but indicates an area of the upper mantle where the materials are either highly plastic or semi-molten. This semi-molten or highly plastic zone is called the asthenosphere. This asthenosphere, perhaps some 200 miles (300 or so km) thick is subject to deformation and, being less rigid than the remainder of the mantle, it can flow. The boundary between the lithosphere and the asthenosphere is therefore temperature dependent, and corresponds to the 1400^o  isotherm.