LITHOSPHERIC plate motions at the Earth’s surface result from thermal convection in the mantle(1). Understanding mantle convection is made difficult by variations in the material properties of rocks as pressure and temperature increase from the surface to the core. The plates themselves result from high rock strength and brittle failure at low temperature near the surface. In the deeper mantle, elevated pressure may increase the effective viscosity by orders of magnitude(2-5). The influence of depth-dependent viscosity on convection has been explored in two-dimensional numerical experiments(6-8), but planforms must be studied in three dimensions. Although three-dimensional planforms can be elucidated by laboratory fluid dynamic experiments(9,10), such experiments cannot simulate depth-dependent rheology. Here we use a three-dimensional spherical convection model(11,12) to show that a modest increase in mantle viscosity dth depth has a marked effect on the planform of convection, resulting in long, linear downwellings from the upper surface boundary layer and a surprisingly ’red’ thermal heterogeneity spectrum, as observed for the Earth’s mantle(13). These effects of depth-dependent viscosity may be comparable to the effects of the plates themselves.