One way to determine the major layers of the Earth (i.e., crust, mantle, core) is by changes in chemical composition (i.e., crust, mantle, core). Another way is to examine how different layers behave as a result of changes in their physical properties. Physical change allows us to define two additional layers.

Comparing the two different ways of examining layers in the Earth.
• Lithosphere: The lithosphere consists of the crust and the top part of the upper mantle, which is fairly cool and rigid. The crust and the top part of the upper mantle thus act in a unified fashion as a zone of cool, rigid rock. The lithosphere is an important zone because it plays a critical role in determining the shapes of continents (see Unit B – Plate Tectonics for further details). The lithosphere has an average thickness of 100 km (approximately 62 miles), although its specific thickness can vary from 15 km (approximately 10 miles) up to 300 km (186 miles).

• Asthenosphere: Directly below the lithosphere lies a section of the upper mantle known as the asthenosphere. The asthenosphere is about 200 km (approximately 124 miles) thick. The enormous heat and pressure at these depths causes the material in the mantle to soften and become flexible. This means that the rock in this section of the upper mantle has the ability to flow, but very, very slowly. The ability of a solid material to flow is called plasticity. This “weak” zone of plastic rock allows the cool, solid lithosphere to “float” above it. This property is important in describing how continents arrived at their present locations (see Unit B – Plate Tectonics for further details).