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The lithosphere
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. 

The asthenosphere

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 1400o  isotherm.

Internal Pressures and Temperatures

Along with density, both pressures and temperatures rise with depth throughout the earth, reaching in excess of 5000o C and millions of atmospheres, near the center. Thus the earth is not only density zoned, but also temperature- and pressure-zoned. These temperature and pressure gradients are important because it is the interaction of these two parameters with the type of material that determines the state (phase) of the various layers.

Although this model may, at first glance, appear highly hypothetical, several of its components, such as phase and compositional changes, have been cross-checked under laboratory conditions. Moreover, mathematical predictions based on this model. have matched the actually observed earthquake wave behavior. 

The earth is made of the same materials as the remainder of our solar system. Therefore, we can expect that the materials from which the inner planets formed should have a composition similar to that of the earth. They are accessible to us in the form of occasional debris that fall from space to the surface of our planet. And, when we analyze these heavenly stones, they fall into two categories: the stony meteorites, made up of silicates and oxides similar to those we could expect in the crust and the mantle; and nickel/iron meteorites similar in composition to the materials we would expect to find in the core. This external line of evidence represents a splendid confirmation of the human ability to extrapolate from what is known and to explore even the unseen and unreachable, as long as there are data and observations available.