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Recent geochemical and isotopic data support the hypothesis that the earth's mantle convects in two separate layers with an interface at the 650-km seismic discontinuity. This paper studies the implications of this hypothesis with regard to the thermal boundary layers that are expected to develop adjacent to the interface between the convecting layers. We are particularly concerned with a possible decrease in the mantle viscosity across this interface. In order to study the structure of the thermal boundary layers in a fluid with a strongly temperature-dependent viscosity we utilize an approximate method developed by Howard (1966). If the upper and lower mantle have the same viscosity law, we find that stratified convection requires a lower mantle viscosity which is several orders of magnitude lower than the upper mantle viscosity. This is not consistent with studies of postglacial rebound. It is possible to obtain a viscosity law for the lower mantle that gives a viscosity nearly equal to that of the upper mantle and reasonable temperatures. However, the required law is not consistent with the currently available experimental data on lower mantle materials. Therefore we conclude that thermal arguments favor whole mantle convection. INTRODUCTION It is generally accepted that mantle convection is the drivingmechanism for plate tectonics and continental drift; in fact, the lithospheric plates are the thermal boundary layers of mantle convection cells [Turcotte and Schubert, 1982, pp. 163-167]. However, two major uncertainties concerning mantle convection remain. These are (1) whether there are separate upper and lower mantle convection systems and (2) whether and in what form a second scale of mantle convection other than the plate motions occurs in the upper mantle. Arguments regarding upper versus whole mantle convection center on the role of the 650-km seismic discontinuity. If this discontinuity is caused by a change in mantle composition, it is unlikely that mantle convective flows could penetrate it. The higher density lower mantle would be stable with respect to the less dense upper mantle. The alternative is that the density change associated with the seismic discontinuity is due to a solid-solid phase change. A phase change may enhance convection, or hinder convection, depending upon its thermodynamic properties [Schubert and Turcotte, 1971]. Laboratory studies indicate that the spinel-basic oxide phase change may have the properties needed to account for the discontinuity [Ming and Bassett, 1975; Watt and O'Connell, 1978]. However, other authors disagree [Liu, 1980; Jeanloz and Thompson, 1981]. The fate of the subducted lithosphere is directly related to this problem. If an upper mantle convection system exists, the subducted lithosphere would be expected to lie on the 650-km compositional boundary. The termination of Benioff zones at this depth favors upper mantle convection. However, some seismologists believe that they can observe the propagation of seismic waves through subducted lithosphere at depths...