The upper mantle extends from the crust-mantle boundary down to a depth of about 660 kilometers (410 miles) The upper mantle can be divided into two different parts. The top portion of the upper mantle is part of the stiff lithosphere, and beneath that is the weaker asthenosphere. The lithosphere (sphere of rock) consists of the entire crust and uppermost mantle and forms Earth's relatively cool, rigid outer shell. Averaging about 100 kilo- meters (62 miles) in thickness, the lithosphere is more than 250 kilometers (155 miles) thick below the oldest portions of the continents (Figure 8.30). Beneath this stiff layer to a depth of about 350 kilometers (217 miles) lies a soft, comparatively weak layer known as the asthenosphere (weak sphere). The top portion of the asthenosphere has a temperature/pressure regime that results in a small amount of melting. Within this very weak zone, the lithosphere is mechanically detached from the layer below. The result is that the lithosphere is able to move independently of the asthenosphere, a fact we consider in the next chapter.