<P> The asthenosphere (from Greek ἀσθενής asthenḗs' weak' + "sphere") is the highly viscous, mechanically weak and ductilely deforming region of the upper mantle of the Earth . It lies below the lithosphere, at depths between approximately 80 and 200 km (50 and 120 miles) below the surface . The Lithosphere - Asthenosphere boundary is usually referred to as LAB . The asthenosphere is almost solid, although some of its regions could be molten (e.g., below mid-ocean ridges). The lower boundary of the asthenosphere is not well defined . The thickness of the asthenosphere depends mainly on the temperature . However, the rheology of the asthenosphere also depends on the rate of deformation, which suggests that the asthenosphere could be also formed as a result of a high rate of deformation . In some regions the asthenosphere could extend as deep as 700 km (430 mi). It is considered the source region of mid-ocean ridge basalt (MORB). </P> <P> The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments . The lithosphere - asthenosphere boundary is conventionally taken at the 1300 ° C isotherm, above which the mantle behaves in a rigid fashion and below which it behaves in a ductile fashion . Seismic waves pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle, thus it has been called the low - velocity zone (LVZ), although the two are not exactly the same . This decreasing in seismic waves velocity from lithosphere to asthenosphere could be caused by the presence of a very small percentage of melt in the asthenosphere . The lower boundary of the LVZ lies at a depth of 180--220 km, whereas the base of the asthenosphere lies at a depth of about 700 km . This was the observation that originally alerted seismologists to its presence and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity . </P>

Which of the following statements best describes the internal structure of the earth