Differentiation led to changes in the composition of rocks as we go deeper into the Earth. A second type of layering is revealed by seismic studies. The layers are not distinguished by their composition, but instead by how the rock moves in response to a force. In general, the outer layers of the Earth and Moon have cooled off enough to become almost completely solid. Therefore, regardless of composition, these layers are relatively rigid and brittle, while the hotter, interior layers are more plastic and deformable. The brittle surface layer is called the lithosphere, from the Greek word "lithos," or rock. The Earth’s lithosphere extends about 100 kilometers down through the crust into the upper mantle.
Diagram of the internal structure of the Earth. Click here for original source URL.
The regions below the lithosphere, however, especially from about 100 to 350 kilometers deep in the Earth, are warmer and may include pockets of molten lava. This region, the asthenosphere, can deform plastically. Putty is an example of a material that behaves plastically. Pull on it slowly, and it will stretch and flow, but pull on it sharply, and it will break. You can think of most of the Earth's mantle as something like putty, or a glacier: a piece of a glacier can be broken off with a hammer. It seems to be solid and brittle, yet, over long time periods, the glacier flows sluggishly. In the same way, the mantle seems rigid, but it can flow slowly in massive currents.
The presence of a cool, rigid lithosphere over a warm, plastic asthenosphere is typical of many planetary bodies. The thickness of the lithosphere is an important factor in explaining many surface features. For example, the Moon’s lithosphere is about 1,000 kilometers deep — much thicker than the Earth’s lithosphere. This is because the Moon is smaller than the Earth, and so it cooled more quickly and completely. The Martian lithosphere is thicker than Earth’s for the same reason. This allows Martian mountains to grow higher than on Earth, because they have more support. Thick lithospheres also partially explain the lack of plate tectonics in the rest of the Solar System — a planet’s lithosphere must be thin enough to break into separate plates.
Cutaway diagram of Earth's internal structure (to scale) with inset showing detailed breakdown of structure (not to scale). Click here for original source URL.